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Editorial Office LING Xiaoliang ZHAO Weiquan HUANG Jing HUANG Yiqin Executive Editor LING Xiaoliang Advances in Polar Science

Contents Vol.22 No. 3 September 2011

Articles

Potential application of biogenic silica as an indicator of paleo-primary productivity in East Antarctic lakes · · · 131 JIANG Shan, LIU XiaoDong, XU LiQiang & SUN LiGuang

Thermodynamic processes of lake ice and landfast ice around Zhongshan Station, Antarctica· · · · · · · · · · · · · · · · · · 143 LEI RuiBo, LI ZhiJun, ZHANG ZhanHai & CHENG YanFeng

Summer freshwater content variability of the upper ocean in the Canada Basin during recent sea ice rapid retreat GUO GuiJun, SHI JiuXin, ZHAO JinPing, JIAO YuTian & XU Dong· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 153

Chemical composition of marine aerosols of the 26th Chinese National Antarctic Research Expedition· · · · · · · · ·· 165 XU GuoJie, CHEN LiQi, ZHANG YuanHui, WANG JianJun, LI Wei & LIN Qi

A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ magnetic latitude for solar minimum HE Fang, ZHANG BeiChen, Joran Moen & HUANG DeHong· · ··· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ··· 175

Extraction of auroral oval boundaries from UVI images: A new FLICM clustering-based method and its evaluation WANG Qian, MENG QingHu, HU ZeJun, XING ZanYang, LIANG JiMin & HU HongQiao· · · · · · · · · · · · · · 184

Improved significance testing of wavelet power spectrum near data boundaries as applied to polar research ZHANG ZhiHua & John C Moore· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ··· · · · · · · · · · · · · · · · · · · · · · · · · · · · · ··· 192

Elevation determination of nunataks in the Grove Mountains· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 199 WANG ZeMin, AI SongTao, ZHANG ShengKai & DU YuJun · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00131 September 2011 Vol.22 No.3 131–142

Potential application of biogenic silica as an indicator of paleo-primary productivity in East Antarctic lakes

JIANG Shan, LIU XiaoDong∗, XU LiQiang & SUN LiGuang

Institute of Polar Environment, University of Science and Technology of China, Hefei 230026, China

Received February 26, 2011, accepted August 14, 2011

Abstract We collected two lake sediment cores (MC and DM) from the East Antarctic region for analysis of biogenic silica and other biogeochemical parameters (e.g., organic matter, C, N, S, H). Based on synthetically comparative research, we focused on the potential application of biogenic silica (BSi) for the reconstruction of paleo-primary productivity in the East Antarctic lakes. Analytical results showed that a large number of diatoms were well preserved in the freshwater lake sediments, and that concentrations of biogenic silica displayed notable fluctuations over different water depths. The content of biogenic silica had a consistent profile over water depth, and this pattern changed with organic matter, reflecting their potential as eco-environmental proxies. Low lev- els of BSi and organic matter indicated reduction of lake algal production, and corresponded to decreased lake primary productivity. Due to the fragile ecosystem state and limited contribution of terrestrial organic matter in the East Antarctic lakes, the contents of biogenic silica in the lacustrine sediments can sensitively indicate the evolutionary history of paleo-primary productivity. Overall, BSi is an ideal proxy for the reconstruction of past eco-environmental change recorded in the lacustrine sediments on East Antarctica.

Keywords East Antarctica, lacustrine sediments, BSi, paleo-primary productivity, organic mat- ter

Citation: Jiang S, Liu X D, Xu L Q, et al. Potential application of biogenic silica as an indicator of paleo-primary productivity in East Antarctic lakes. Adv Polar Sci, 2011, 22: 131–142, doi: 10.3724/SP.J.1085.2011.00131

0 Introduction sediments can be used to reconstruct paleo-primary Lake sediments are good materials to infer paleo-climatic productivity[5−6]. In recent years, studies on biogenic and paleo-environmental change. These sediments can be silica (BSi) in lake sediments have drawn more attention used to record abundant information about climate, veg- from researchers in this field[7]. Biogenic silica (com- etation and human activity over time. At present, lake pared to silica crystals), is a type of amorphous silicon. sediments have been used widely to study past global It mainly derived from diatom frustules in water, and changes[1−2]. a small amount also originated from plant silicate phy- Due to the fragile characteristics of lake ecosystems toliths, sponge spicules, radiolaria and chrysophyta[8]. in Antarctica, even slight changes in environmental con- Silicon is the second most abundant element. Compared ditions would result in great fluctuations of lacustrine with oxygen, nitrogen and carbon, the biogeochemical ecology and biogenic production. These variations are cycle of silicon is slow, while its sedimentary rate is rapid. likely to be recorded in the lacustrine sediments[3−4]. Thus, silicon can be preserved in the form of biogenic Multiple biogeochemical proxies (e.g., pigments, silica in sediments[9]. BSi is a measurement of silica pro- nutrient elements, amino acid, isotopes) from lake duced by diatoms and chrysophytes, and is a reasonably

Jiang Shan (female, Doctor student, polar paleoecological research, email: [email protected]) ∗Corresponding author(email: [email protected]) journal.polar.gov.cn 132 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3 good proxy for the production of those algal species. might encourage resuspension. In addition, because change in lake productivity has a The two sediment cores investigated in this study close relationship with nutrient concentration and sur- were recovered from Mochou Lake and Daming Lake face temperature in the water, BSi in lake sediments may around Zhongshan Station on the Mirror Peninsula (Fig- reflect change in historical climate conditions[10]. Nu- ure 1). These two lakes were formed by glacial erosion, merous studies from high latitudes, such as Lake Baikal and predominantly are fed by snow-melt water and local in Russia[11−12], mid-latitudes, such as Lake Pipa in precipitation. Japan[13], and low latitudes, such as Huguangyan Maar Based on the physico-chemical data of Mochou Lake lake in the Leizhou peninsula in China[14], as well as reported by Li et al.[4], the altitude is 8 m (a.s.l.). The Arctic lake sediments[15], have shown that high BSi val- maximum depth is about 4 m, the surface area is 0.005 ues correspond to warm and humid climate conditions. km2, and the catchment area is 0.127 km2. Lake wa- These values strongly contrast those of low level BSi gen- ter is lost mainly through evaporation and outflow in erated from cold and dry climates. The above results the summer. Although Mochou Lake is still considered suggest that variation in diatom productivity is closely to be oligotrophic, the concentrations of nutrient ele- related to climate change. Thus, the BSi recorded in la- ments are much higher than other lakes in the Larse- custrine sediments can be used to indicate paleoclimatic mann Hills. In Mochou Lake, the concentrations of nutri- changes. Even with the possibly negative influence of ent elements, species and biomass are much higher than post-depositional dissolution processes and sedimentary those of other lakes in the Larsemann Hills, and inor- [12] rates , BSi continues to be an ideal proxy for recon- ganic matter mainly exists in the form of NH4-N and [10] struction of past eco-environmental changes in lakes . PO4-P. The dissolved oxygen is saturated, and the wa- A multi-proxy analysis, combined with chronology ter body is weak acid to weak alkaline. Na+ and Cl− are results, is helpful for high-resolution reconstruction of the dominant species of cation and anion, respectively[17]. paleo-climate, paleo-precipitation and paleo-salinity With respect to Daming Lake it is a shallow lake with a changes, as well as glacial advance and retreat maximum water depth of 4 m and an altitude of 30 m patterns[16]. However, as an important proxy for primary above sea level (a.s.l.). The lake is weakly brackish and productivity, the application of BSi is still limited for sed- remains ice covered for 9–10 months of the year. Ac- iments from high latitude Antarctic lakes. In this study, cording to limnological data[18], weak seasonal thermal, we investigated biogenic silica in two lake sediment cores salinity and nutrient stratification occur under the ice from the Larsemann Hills. Combined with other biogeo- cover when the upper part of the water column is cooled chemical proxies, we discuss the potential application of and diluted by melting ice. In winter, under the ice, biogenic silica to paleo-environmental reconstruction, es- oxygen is consumed by the biota and weak anoxia can pecially on historic changes of lake primary productivity persist. in East Antarctica. 2 Sampling and methods 1 Study area The MC (85 cm) and DM (63 cm) cores were recovered The Larsemann Hills constitute an ice-free area of ap- from Mochou Lake and Daming Lake, respectively (Fig- proximately 40 km2, located on the Ingrid Christensen ure 1). In order to avoid the influence of human distur- Coast of Princess Elizabeth Land in Eastern Antarctica bance on the sedimentary environment, the coring site at (69◦120S–69◦280S, 76◦E–76◦300E) (Figure 1). The Larse- MC was far away from the pumping region, where is an mann Hills have a continental Antarctic climate, which is important source of cooling water for Zhongshan Station. colder and drier than the maritime Antarctic. More than The detailed lithological profile of MC has been described 150 freshwater lakes are located in this area. The sedi- by Liu et al.[19]. Based on lithological characteristics of ments are particularly well suited for paleolimnological the MC core, the bottom sediment unit (between 85 and studies, since there is no significant bioturbation and a 74 cm) was distinctly different from the top 70 cm limited season of open water when wind-induced mixing sediment layer. The former mainly consists of black Potential application of biogenic silica as an indicator of Antarctic paleo-primary productivity 133

Figure 1 Study area showing the sites at Mochou and Daming lakes. organic mud, which has a strong and unpleasant odour, modified alkali solvent extraction methods[26−28]. The most likely from hydrogen sulphide accumulated in the detailed procedures for these methods are described as sediments, as a result of anoxia. However, the latter follows. We transferred 100 mg of each freeze-dried is dominated by relatively fresh and greenish organic sediment sample into a 10 mL polyethylene centrifuge matter, which mainly originates from crudely laminated tube, and then added H2O2 (10%). After letting the algal and microbial mats. The middle sediment unit, samples stand undisturbed for 30 min, we added HCl between 74 and 70 cm, shows transitional lithological (1 mol·L−1), and then oscillated them for half an hour. characteristics. This is the result of the sedimentary Then, 5 mL deionized water were added and then cen- marine-freshwater transition that is evident in all re- trifuged for 8 min at a speed of 4 000 r·min−1. The gional lakes below an altitude of 8 m (e.g. [20]). Addi- supernatant was removed and then dried overnight at tionally, in the bottom 11 cm of sediments, some gravels 60◦C in the oven. This procedure was followed in or- were found to be rounded. However, a few clasts present der to briefly dry the samples. Then, BSi was dis- in the top 70 cm sediment layer were subrounded or solved by treating the samples with 8 mL Na2CO3 weakly rounded, indicating relatively short transporta- (2 mol·L−1) for 5 h at 85◦C in a water bath, with tion processes and/or hydrodynamically weak sedimen- complete sample agitation every hour. Then, the sam- tary environments. The sedimentary change was con- ples were centrifuged. Aliquots of 2.0 mL supernatant sistent with the history of the Holocene sea-level fluc- were removed, to which 5 mL HCl (1:1) and deionized [21−23] tuation in the study area . In this study, we fo- H2O were added. BSi was measured on these aliquots cused on the fresh water sediments above 70 cm. The by Inductively Coupled Plasma Optical Emission Spec- lithology of the DM core was consistent with the de- trometer (ICP-OES), and the relative standard deviation scription by Hodgson et al.[18]. The surface 13 cm of (RSD) was controlled within 2%. sediments were not compressed, and mainly were com- Total nitrogen (TN), total carbon (TC), total sul- posed of lake algae. The sediments between 13 and 17 fur (TS) and total hydrogen (H) were determined by cm became increasingly compressed, and their main com- a C/N/S/H elemental analyzer (vario EL III) with ponents were still lake algae. However, gravel abun- a RSD less than 1%. The potassium dichromate dance increased gradually, and sediments from 17 to oxidation-chemical volumetric method was used to mea- 63 cm were predominantly composed of gravels with sure total organic carbon (TOC) with a RSD of a small amount of lake algae. Unlike the sedimen- 0.5%. The SEM analysis for diatoms was conducted tary conditions of low altitude Mochou Lake, Daming on a FEI Sirion 200 schottky field emission scan- Lake experienced no submersion since the Last Glacial ning electron microscope at the University of Science Period. and Technology of China. The samples were broken, At present, there are many approaches to measur- mixed, and fixed on a sample holder, and coated with ing the content of BSi[20,24−25]. In our study, we used gold. 134 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3

3 Results and discussion clearly (e.g., MC-5, MC-25, MC-59), suggesting that the diatoms were well preserved in the sediments. In fact, 3.1 Preservation of diatoms the diatom species preserved in lakes are well studied, In general, the majority of the biogenic silica originated and they typically are used to discuss historical records from diatoms, as well as sponge spicules and the sili- of climate variations in Antarctic lakes. For example, con minerals[29]. Some sedimentary samples at differ- Hodgson et al.[18] suggested that there were abundant ent depths were scanned using the electron microscope. diatoms in Daming Lake, and the species and abundance As seen in Figure 2, under the condition of relatively displayed clear change over water depths. The well pre- low magnification (less than 5 000 fold), we could ob- served diatoms in the sediments of Daming and Mochou serve a great proportion of algal residue (e.g., MC-43, lakes provide the potential to measure pigment and BSi MC-69). When the microscope was magnified more than contents. 10 000 times, the structures of the diatom were observed

Figure 2 SEM photos of diatom samples from Mochou Lake sediments. The sample number in the diagram stands for the sampling depth.

3.2 Biogeochemical proxies in Mochou Lake limited due to low biological activity and poor chemical sediments weathering[4]. Thus, input of organic matter from ter- The changes in TN, TC, TS, H, TOC and BSi are plot- restrial plants and nutrients washed out from terrestrial ted in Figure 3 versus water depth and age. All the soils are insignificant. In Mochou Lake, the organic mat- curves show the same pattern of change, indicating a ter in the sediments is mainly composed of relatively fresh [6] [30] common source. Correlation analyses suggest that TN, brown-green algae . Bird et al. suggested that post- TC, H have strong positive correlations with TOC (Ta- depositional decomposition and microbial degradation of ble 1). The values of TOC and TC are approximate, sedimentary organic matter were very weak, perhaps due indicating low content of inorganic carbon, such as car- to the extremely low temperatures in East Antarctica. bonate. Thus, total C, N and H were mainly derived Combined with observations on lithology and field study from organic matter. Although TS and TOC showed the (exposed bedrock was found around the lake, where there positive correlation at the 0.01 level (Table 1), the low was almost no vegetation cover), we presumed that the correlation coefficient suggests that the sulfur content in organic matter in the lake sediments mainly originated [16] the sediments may have been susceptible to influence of from endogenous material, namely dead algae . This post-depositional oxidation conditions, except for the sig- hypothesis is supported by the following analyses on car- nificant effect of organic matter inputs. This is supported bon and nitrogen isotopes, and organic geochemical pa- by the fact that the catchment of the Mochou Lake lacks rameters in the sediments. The vertical depth profile land vegetation, and the process of soil formation is very curves of TN and TOC are very similar, implying that Potential application of biogenic silica as an indicator of Antarctic paleo-primary productivity 135

Figure 3 Profiles of biogeochemical proxies versus water depth and chronology in Mochou Lake lacustrine sediments. The age data on the right are redrawn from the reference 19.

Table 1 Correlations between TN, TC, TS, H, TOC and BSi in Mochou Lake lacustrine sediments TN TC TS H TOC BSi S2 TN 1.000 TC 0.927 1.000 TS 0.581 0.441 1.000 H 0.923 0.977 0.441 1.000 TOC 0.916 0.982 0.425 0.957 1.000 BSi 0.742 0.768 0.282 0.749 0.776 1.000 S2 0.916 0.866 0.353 0.846 0.871 0.755 1.000

Note: All the correlations are significant at the level of 0.01, with the exception of the correlations between BSi, S2 and TS, which are at the 0.05 level. these two proxies can be used to indicate the change of MC was far away from the pumping site (Figure 1). In- lake algal abundance in Mochou Lake. Thus, changes of deed, according to studies of human impacts on freshwa- TOC and TN have clear eco-environmental implications, ter lakes in the Larsemann Hills, prior to 1986, local hu- and their high values are consistent with the high algal man impact was virtually nil[31]. Thus, the environmen- growth and high primary productivity of the lake. tal changes associated with human activities have been very sudden. Based on the result of chronology of the 3.3 Source of sedimentary organic matter in MC core[19], the adjacent 1 cm samples represent approx- lacustrine sediments imately 40 years intervals. Since the Zhongshan Station The direct influence of modern human activity on sedi- was built only 20 years ago, the anthropogenic pollution mentary organic matter should not be ignored. Several may not have caused serious influence on the chemical stations have been built around the study area since 1986 composition of lacustrine sediments below the surface. (e.g., Zhongshan Station, Progress 1 and 2, Law Base). In order to further confirm this result, we identified the Thus, the study samples are likely to be subject to fre- source of organic composition and assessed the possibility quent anthropogenic influences. For instance, as an im- of anthropogenic pollution and post-depositional degra- portant source of cooling water for Zhongshan Station, dation on sedimentary organic matter, based on analyses the outlet water into Mochou Lake might contain some of carbon, nitrogen isotope compositions and organic geo- human-derived organic components, and this could pol- chemical parameters. lute the lake water and the deposited sediments in Mo- Stable carbon and nitrogen isotopes, and elemen- chou Lake, especially in the surface sediment samples. tal C/N ratios have been used effectively to evaluate In order to avoid human impacts of outwater on sedi- the sources of organic compositions in lake sediments[32]. mentary composition in the MC core, the coring site of The values of δ13C and δ15N in Mochou sediments varied 136 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3 from –11.0‰ to –9.1‰, and from 2.9‰ to 4.2‰, re- ple two end-member mixture model (algae and anthro- spectively. In addition, both isotopes showed small fluc- pogenic sources) for our analysis. The calculated results tuations (Figure 4). In comparison with data of high- showed that less than 10% of the organic component was 13 latitude plants and other lake sediments, the δ Corg from anthropogenic sources. In fact, the variation of sed- 13 values of Mochou Lake sediments are extremely high. imentary δ Corg in the Mochou Lake was related closely In fact, algae in the moats, shallow water or lakes to the high algal productivity condition, and the ∼2‰ 13 13 with seasonal ice-cover commonly have very high δ Corg difference throughout the δ Corg profile was largely due (∼–10‰) in East Antarctica[33−34]. This is likely caused to the change of historical primary productivity over the by the CO2-diffusion limited environment that results past 3 000 years (to be addressed in detail in a future 13 from high primary productivity in summer. Never- paper). The extremely high δ Corg values of Mochou theless, Antarctic phytoplankton growing in such envi- Lake sediments appear to be related to the intensified 15 13 ronmental conditions have low δ N values, generally photosynthesis of algae, and the fluctuation of δ Corg varying from –5‰ to 6‰[35], and the fluctuations of down the depth profile is likely caused by change of his- δ15N can reflect the variation of the organic matter in torical primary productivity of the lake. With increasing both N2-fixing plants and non-N2 fixing algae. If an- algal biomass and sufficient solar radiation, intensified thropogenic organic components have significantly in- algal photosynthesis allows dissolved inorganic carbon to fluenced Mochou Lake sediments, the values of δ13C be diffused into algal cells at maximum speed and then and δ15N can change sensitively in the subsurface sed- assimilated. This could lead to a significant weakening of iment samples. According to data previously reported, the isotopic fractionation between the carbon substrates sewage generally has significantly high δ15N, but low and the organic products, which would result in more 13 [36] 13 13 13 δ C values . For example, the δ C of surface sed- C assimilated into plants and higher δ Corg values in iments adjacent to the sewage outfall was as below as lacustrine sediments. Conversely, natural lake algae gen- –22‰ at the McMurdo Station in Antarctica, and this erally are depleted in 15N relative to human-derived or- result also was found at other sewage discharge points[36] ganic matter[39]. Waste water typically has a δ15N of or in the sedimentary organic composition impacted by 10‰–22‰, and even treated sewage also has relatively human activities[37−38]. However, the δ13C values in high δ15N (∼10‰)[40]. As shown in Figure 4, there was the upper MC sediments remained at high levels (about no significant change between δ15N values in the upper –10‰), suggesting that the predominant factor control- and lower samples. The δ15N values in the uppermost ling δ13C variations is likely to be algal production, samples did not reach peak levels throughout the MC and the effect of the anthropogenic organic input was sediment core, and the values remained very low (∼4‰). negligible. Assuming that the anthropogenic organic This finding is consistent with the results of δ13C. Thus, component could contribute to the entire variation of we suggested that the input of the anthropogenic organic 13 δ Corg in the MC sediments, we employed the sim- component should have weak effects on MC sediments,

Figure 4 Changes of organic matter proxies including S1, S2, TOC, C/N, δ13C and δ15N in Mochou lacustrine sediments. Potential application of biogenic silica as an indicator of Antarctic paleo-primary productivity 137 and that variations of δ13C and δ13N reflect characteris- BSi in this shallow freshwater lake[47]. In future studies, tics of organic components from natural sources. measurements of absolute diatom contents and stomato- In the MC lacustrine sediments, the C/N ratios ex- cyst in sediment samples using organic pigment methods hibited a small range, varying from 11.6 to 14.5, with may allow for comparisons with BSi values, and may pro- a mean of 13.2. The C/N ratios indicated that the vide more detailed information on the sources of BSi in organic matter was predominantly characteristic of an Mochou Lake. autochthonous source[41]. The lower C/N in the near- Correlation analysis showed that BSi had a strongly surface samples indicated enhanced contributions of or- positive correlation with some geochemical proxies for ganic matter derived from lake aquatic sources. Follow- algal abundance (Table 1), indicating that the content ing the procedure reported by Sanei and Goodarzi[42], of BSi in Mochou Lake was mainly controlled by the organic matter parameters (e.g., S1, S2) were obtained amount of lake algae. Thus, BSi could effectively re- using Rock-Eval six analyses (Vinci Technologies, Rueil- flect the historical change of lake primary productivity. Malmaison, France). S1 represented the quantity of free Furthermore, the BSi only originated from lake algae, “volatile” hydrocarbons; and S2 indicated the quantity and it was buried in the sediments after the algae died. of the higher molecule, kerogen-derived hydrocarbons re- The close relationships between BSi and other geochem- leased by the thermal cracking of organic matter. Ac- ical parameters could further confirm that sedimentary cording to the Rock-Eval analysis, S1 is a good indica- organic matter was predominantly derived from natu- tor of the labile portion of algal-derived organic matter, ral products in the lake, and that anthropogenic or- which is known to decompose easily. However, the S2 ganic input did not significantly influence organic matter compounds originated from lake algae are more resis- compositions in MC sediments. The results of biogenic tant to degradation during early diagenesis. Thus, the silica measurements in surface sediments of Prydz Bay quantity of S2 may serve as a more suitable indicator showed that spatial variation of the biogenic silica was of algal productivity in aquatic sediments than that of related closely to nutrients, chlorophyll a and primary S1[42−44]. Correlation analyses showed that S2 and TOC productivity[45]. Thus, the BSi was capable of reflect- in MC sediments were highly correlated (r=0.86, n=60), ing change of primary productivity in the upper water and this correlation did not change significantly in the column. As illustrated in Figure 3, changes of BSi and upper 6 cm of the samples (r=0.84, n=4). These re- organic matter contents in the sediments revealed that sults suggested that the organic matter in the MC sedi- there were at least three stages with low lake primary ments was mainly derived from lake algae, and the effects productivity (marked by shaded areas), corresponding of both post-depositional microbial degradation and an- to 64–57 cm, 46–32 cm and 22–10 cm, respectively. The thropogenic source were minor. change of algal abundance in the lake was interpreted to be related to climate conditions, solar variations and nu- 3.4 BSi in Mochou Lake sediments and pale- trient levels. With a warming climate, lake water tended oenvironmental implications to become warmer and more productive. The lake could For the MC sediment core, the content of BSi varied create more suitable conditions for the growth of phyto- between 5% and 8%, and the average value was 6.9% plankton and zooplankton, finally leading to an overall (n=69), much lower than that in the surface sediments increase in lake productivity from benthos to plankton. in Prydz Bay (4.89%–85.41%, averaging 30.90%)[45]. A However, during climatic deterioration, the catchment 56-lake dataset in east Antarctica suggests that diatom area of Mochou Lake could be covered with more snow flora and species diversity are lower in Mochou Lake in and ice, making it more difficult for plants to receive the Larsemann Hills, and three taxa (C. cf. molesta, solar radiation and nutrient inputs. This would result P . abundans and S. inermis) are the most abundant in a decline of algal production, as well as limiting di- diatoms. These species usually appear in both freshwa- atom propagation[48]. Thus, the trough stages of BSi ter and slightly brackish lakes of the Larsemann Hills in the sediments of Mochou Lake likely corresponded to [46−47]. In addition to diatom species, the stomatocysts at least three periods of historically deteriorated climate (i.e., chrysophytes) may be another important source of conditions in the study area. 138 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3

3.5 Change of biogenic silica in Daming Lake sistently less than 2%, and the top 10 cm sediments had Changes of TN, TC, TS, H and BSi contents versus depth BSi contents of up to 12%–13%. Comparing the profiles in the DM sediment core are plotted in Figure 5. Like of BSi and TN, TC, TS, H contents over depth in the Mochou Lake, Figure 5 shows that the profiles of TN, Daming lacustrine sediments, they showed very similar TC, TS and H contents over depth had consistent trends. change trends. Furthermore, Figure 6 shows that BSi Their contents were relatively low in the sediments be- had highly positive correlations with TN, TC, TS and H low 15 cm. They increased rapidly from 15 cm, and then in the sediments, suggesting that all these proxies can be remained higher in the upper 10 cm of sediments. The used to commonly indicate paleo-limnological changes in content of BSi also displayed a notable change versus Daming Lake. depth. The sediments below 15 cm had BSi levels con-

Figure 5 Changes of TN, TC, TS, H and BSi contents in the DM sediment core against depth. The LOI, total bacteri- ochlorophylls and total chlorophyll on the right are redrawn from the analytical data of reference 18 for a sediment core taken from the center of Daming Lake.

Based on a 115-cm sediment core taken from the site at DM in our study was closer to the lake shore. As center of Daming Lake, Hodgson et al.[18] used multi- mentioned above, the difference in light intensity, tem- ple proxies, including TOC, carbon stable isotopes, fossil perature and nutrient inputs may have affected the algae pigments and diatoms to reconstruct continuous paleo- biomass and lake productivity. Based on field observa- environmental records. These records included histori- tions, algae grow more in the lake center, and thus the cal climatic changes, salinity and lake water level over deposition of lake algae may be more rapid in the middle the past 40 000 years. Here, we have redrawn the data of the lake. Conversely, the clastic deposition rate was of loss-on-ignition (LOI), total bacteriochlorophyll and likely to have been faster near the lake shore. In addition, total chlorophyll in Figure 5. These three parameters the different sediment compactions also may have caused are considered to be related to changes in lake primary the depth difference of environmental proxy changes in productivity. As shown in Figure 5, the pigment con- the different sediment cores. In spite of that, the litho- tents and the LOI values, indicative of the change in logy throughout these two cores still displayed consistent organic matter, increased rapidly in the sediment layer change characteristics. Thus, we suggest that the critical between 55–40 cm, and they remained high in the up- depths of both sediment cores should correspond to the per 40 cm. The BSi, TN and other proxies analyzed in same eco-environmental event. According to Hodgson et our DM core had the same trends. However, a sudden al.[18], pigments increased within 50–40 cm, yielding date increase in these environmental proxies occurred at 15– ranges between 17 000 and 11 000 a BP. During this pe- 10 cm in the DM core, which was very different from riod, seasonal meltwater input to the lake increased and the critical depth of 40–55 cm reported by Hodgson et the lake received more light intensity, leading to rapid al.[18]. The depth difference may be related to the cor- enhancement of lake productivity. Meanwhile, favorable ing position. The sediment core studied by Hodgson et climatic conditions may have promoted algal growth in al.[18] was collected in the lake center, whereas the coring Daming Lake, and the increased productivity of the dia- Potential application of biogenic silica as an indicator of Antarctic paleo-primary productivity 139 toms may have resulted in the high value of BSi. The matic conditions occurred since ∼7 400 a BP. This would relatively high content of BSi in the upper 10 cm of the have resulted in a significant enhancement of lake produc- sediments in the DM core was consistent with high levels tivity. The notably high values of BSi, TC, TN and TS in of pigments obtained from Hodgson et al.[18], suggest- the top sediment layer of the DM core likely revealed the ing that the sediments in the upper 10 cm of the DM increased lake primary productivity due to the warming core were likely deposited during the period of Holocene climatic conditions. This provides further evidence that glacial retreat. According to Hodgson et al.[18], Daming biogenic silica is a good proxy for reconstructing change Lake became seasonally ice-free during summer at the of paleoproductivity in Antarctic lakes. beginning of the Holocene deglaciation, and warmer cli-

Figure 6 Correlations between TN, TC, TS, H and BSi in the Daming Lake sediments.

3.6 Advantages of BSi as a proxy for paleo- Distribution of different diatom species in lacustrine sedi- productivity in East Antarctic Lakes ments provide an important proxy for the reconstruction In many lakes of East Antarctica, including Larse- of past climatic variations, and up to now they have been mann Hills, Vestfold Hills and McMurdo Dry Val- successfully applied to the reconstruction of lake salin- ley, diatoms are well preserved in the lacustrine ity, ice thickness, sea level and other paleo-environment sediments[46,49−51], and they are important components information[46]. However, the diatom species can not of microbial communities in Antarctic and sub-Antarctic characterize the total primary productivity in lakes. lakes[52]. Many diatom species only exist in the extremely We analyzed productivity proxies, such as organic cold Antarctic[53]. This is the case for some species matter, pigments, and BSi in lake sediments from Ny- present in the Larsemann Hills lakes[46]. In high lati- Alesund,˚ Arctic [54]. Those proxies exhibited good corre- tude regions, especially in the Antarctic, diatoms provide lations in the sediments with relatively low organic mat- unique biogenic indicators for paleolimnological study. ter content (e.g., the period of Little Ice Age [LIA]). 140 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3

The deteriorating climatic conditions could have caused change of historical diatom productivity had occurred in the decrease in lake primary productivity and other bio- the lakes. Historically, the primary productivity experi- geochemical indices. However, at the beginning of the enced at least three troughs during the past 3 000 years in 20th century, lake productivity increased significantly Mochou Lake. However, in Daming Lake, the relatively due to climate warming, and thus organic matter and pig- high concentration of BSi in the upper 10 cm likely in- ments displayed higher contents in the upper sediments. dicates increased lake primary productivity due to the However, at that time the BSi showed the opposite trend Holocene climatic warming and glacial retreat. in these sediment samples[54]. Axford et al.[55] also found (3) The content of BSi in Arctic lacustrine sediments similar BSi variations in surface sediments from Arctic is likely susceptible to the input of terrestrial organic lakes, and suggested that the dilution effect brought by matter, whereas the influencing factors on the variation the large input of terrestrial organic matter could not be of sedimentary BSi in the East Antarctic lakes are rela- ignored. The above results show that BSi contents may tively simple. Thus, BSi is an ideal proxy for the recon- be influenced by many factors in Arctic lakes. For exam- struction of past eco-environmental changes recorded in ple, the input of terrestrial organic matter derived from the lacustrine sediments of East Antarctica. abundant mosses growing around the lakes may cause significant influence on the change patterns of BSi in the sediments. Under this condition, the profile of BSi con- Acknowledgments We would like to thank the Chinese Arctic tent over depth in the lacustrine sediments could not re- and Antarctic Administration, State Oceanic Administration for flect the true change of lake primary productivity. How- logistic field sampling support . We are appreciative to Dr Huang ever, in the remote East Antarctic continent, the exposed Tao for sampling DM sediment core in the field. This study was supported by the National Natural Science Foundation of China bedrock and sparse vegetation around lakes may have (Grant nos. 40876096, 41076123 and 40606003), the Open Re- limited the input of terrestrial organic matter into lakes, search Funds from SOA Key Laboratory for Polar Science (Grant and thus the effect of exogenous organic matter on the no. KP2007002) and a special fund for excellent PhD thesis of distribution of BSi was likely negligible. Overall, BSi is CAS. Thanks also are extended to several anonymous reviewers for their constructive comments which improved the manuscript. an ideal proxy for the reconstruction of paleoproductivity recorded in the East Antarctic lakes. References

4 Conclusions 1 Wang S M, Dou H S. China Lake. Beijing: Science Press, 1998: 580 Based on comparative research between BSi and other 2 Shen H Y, Li S J, Shu W X. Pigments in the lake sediments: environmental proxies for the sediments of Mochou Lake environmental indicator. Marine Geology & Quaternary Ge- and Daming Lake on East Antarctica, we can draw the ology, 2007, 27(3): 37–42 following conclusions: 3 Hodgson D A, Noon P E, Vyverman W, et al. Were the Larsemann Hills ice-free through the Last Glacial Maxi- (1) In the DM and MC sediment cores, BSi showed mum? Antarctic Science, 2001, 13(4): 440–454 the consistent change patterns with bio-geochemical 4 Li S K, Zhang Q S, Zhu L P. Landform development and proxies, such as organic matter. In addition, they ex- environmental evolution since late Pleistocene in ice-free hibited strong positive correlations. This implied that Antarctica. Beijing: Ocean Press, 1998 the content of BSi, like TOC, pigments, and TN, was re- 5 Kaufman D. An overview of late Holocene climate and en- sponsible for the historical change of lake algal biomass, vironmental change inferred from Arctic lake sediment. Pa- leolimnology, 2009, 41(1): 1–6 and the high value indicated rapid growth of lake algae. 6 Liu X D, Sun L G, Xie Z Q, et al. Geochemical evidence for Thus, the BSi level in the sediments could effectively in- the influence of historical sea bird activities on no worries dicate the fluctuation of historical lake primary produc- lake sediments in the Zhongshan station area, East Antarc- tivity. tica. Journal of Chinese Polar Research, 2004, 16(4): 295– (2) A great number of diatoms were well preserved 309 in the freshwater lake sediments in East Antarctica. 7 Smol J P, Birks H J B, Last W M (eds.). Tracking environ- mental change using lake sediments. Volume 3: Terrestrial, Throughout the depth profile, the BSi content showed algal, and siliceous indicators. Dordrecht: Kluwer Academic notable variations, reflecting the fact that the significant Publishers, 2001: 400 Potential application of biogenic silica as an indicator of Antarctic paleo-primary productivity 141

8 Li J, Chen J A. An effective cleaning method for produc- 25 Liu F, Zhao L B, Huang L F, et al. A methodological study ing pure diatom samples from lake sediments. Earth and on the determination of biogenic silica sourced from newly Environment, 2007, 35(1): 91–96 sedimentated diatom in Xiamen Inner-bay sediment. Jour- 9 Birks H J B, Jones V J, Rose N L. Recent environmen- nal of Xiamen University (Natural Science), 2008, 47: 294– tal change and atmospheric contamination on Svalbard as 299 recorded in lake sediments-an introduction. Paleolimnology, 26 Mortlock R A, Froelich P N. A simple method for the rapid 2004, 31(4): 403-410 determination of biogenic opal in pelagic marine sediments. 10 Hu F S, Kaufman D, Yoneji S, et al. Cyclic variation and Deep-Sea Research, 1989, 36(9): 1415–1462 solar forcing of Holocene climate in the Alaskan subarctic. 27 Liao B, Zhu J, Wu Y Y, et al. Determination method of Science, 2003, 301(5641): 1890–1893 biogenic silica in reservoir sediments. Journal of Neijiang 11 Colman S M, Peck J A, Karabanov E B, et al. Con- tinen- Teachers College, 2006, 21: 226–229 tal climate response to orbital forcing from biogenic silica 28 Zhang L L, Chen M H, Xiang R, et al. Distribution of bio- records in Lake Baikal. Nature,1995, 378(6559): 769–771 genic silica in surface sediments from southern South China 12 Swann G E A, Mackay A W. Potential limitations of bio- Sea and its environmental signif icance. Journal of Tropical genic silica as an indicator of abrupt climate change in Lake Oceanography, 2007, 26(3): 24–29 Baikal, Russia. Paleolimnology, 2006, 36(1): 81–89 29 Conley D J, Schelske C L, Stoermer E F, et al. Modification 13 Xiao J L, Inouchi Y, Kumai H, et al. Biogenic silica record of the biogeochemical cycle of silica with Eu-trophication. in Lake Biwa of central Japan over the past 145,000 years. Marine Ecology-Progress Series, 1993, 101(1-2): 179–192 Quaternary Research, 1997, 47(3): 277–283 14 Wang W Y, Liu J Q, Peng P A. Determination and appli- 30 Bird M I, Chivas A R, Radnell C J, et al. Sedimentologi- cation of biogenic silica in lake sediments: An example from cal and stable-isotope evolution of lakes in the Vestfold Hills, Huguangyan marr lake, southern China. Geochimica, 2000, Antarctica. Palaeogeography Palaeoclimatology Palaeoecol- 29(4): 327-330 ogy, 1991, 84(1–4): 109–130 15 Michelutti N, Douglas M S V, Wolfe A P, et al. Height- 31 Burgess J S, Spate A P, Norman F I. Environmental impacts ened sensitivity of a poorly buffered high arctic lake to of station development in the Larsemann Hills, Princess Eliz- late-Holocene climatic change. Quaternary Research, 2006, abeth Land, Antarctica. Journal of Environmental Manage- 65(3): 421–430 ment, 1992, 36(4): 287–299 16 Sun L G, Xie Z Q, Liu X D, et al. Ecological geology on 32 Gonneea M E, Paytan A, Herrera-Silveira J A. Tracing or- Ice-free Antarctica. Beijing: Science Press. 2006: 306 ganic matter sources and carbon burial in mangrove sedi- 17 Li Z S, Wang J, Lei Z H. Hydrochemical properties of lakes ments over the past 160 years. Esruarine, Coastal and Shelf in Larsemann Hills, Antarctica. Chinese Journal of Polar Science, 2004, 61(2): 211–227 Research, 1997, 9(1): 71–77 33 Burkins M B, Virginia R A, Chamberlain C P, et al. Ori- 18 Hodgson D A, Verleyen E, Sabbe K, et al. Late Quater- gin and distribution of soil organic matter in Taylor Valley, nary climate-driven environmental change in the Larsemann Antarctica. Ecology, 2000, 81(9): 2377–2391 Hills, East Antarctica, multiproxy evidence from a lake sed- 34 Doran P T, Wharton Jr R A, Lyons W B, et al. Sedimen- iment core. Quaternary Research, 2005, 64(1): 83–99 tology and geochemistry of a perennially ice-covered epishelf 19 Liu X D, Sun L G, Xie Z Q, et al. A preliminary record lake in Bunger Hills Oasis, East Antarctica. Antarctic Sci- of the historical seabird population in the Larsemann Hills, ence, 2000, 12 (2): 131–140 East Antarctica, from geochemical analyses of Mochou Lake 35 Dunton K H. δ15N and δ13C measurements of Antarctic sediments. Boreas, 2007, 36(2): 182–197 peninsula fauna: trophic relationships and assimilation of 20 Verleyen E, Hodgson D A, Milne G A, et al. Relative sea- benthic seaweeds. American zoologist, 2001, 41: 99–112 level history from the Lambert Glacier region, East Antarc- 36 Conlan K E, Rau G H, Kvitek R G. δ13C and δ15C shifts tica, and its relation to deglaciation and Holocene glacier in benthic invertebrates exposed to sewage from McMurdo readvance. Quaternary Research, 2005, 63(1): 45–52 Station, Antarctica. Marine Pollution Bulletin, 2006, 52: 21 Verleyen E, Hodgson D A, Sabbe K, et al. Coastal oceano- 1695–1707 graphic conditions in the Prydz Bay region (East Antarc- tica) during the Holocene recorded in an isolation basin. 37 Botello A V, Mandelli E F, Macko S, et al. Organic carbon Holocene, 2004a, 14(2): 246–257 isotope ratios of recent sediments from coastal lagoons of 22 Verleyen E, Hodgson D A, Sabbe K, et al. Late Quater- the Gulf of Mexico, Mexico. Geochimica et Cosmochimica nary deglaciation and climate history of the Larsemann Hills Acta, 1980, 44(3): 557–559 (East Antarctica). Quaternary Science, 2004b, 19(4): 361– 38 Gearing P J, Gearing J N, Maughan J T, et al. Isotopic 375 distribution of carbon from sewage sludge and eutrophica- 23 Liu S M, Zhang J. A study on the measurement of biogenic tion in the sediments and food web of estuarine ecosystems. silica. Marine Sciences, 2002, 26(2): 23–26 Environmental Science and Technology, 1991, 25: 295–301 24 Ye X W, Liu S M, Zhang J. The determination of biogenic 39 Talbot M R. Nitrogen isotopes in palaeolimnology. In Last silica and its biogeochemistry significance. Advance in Earth W M, Smol J P. (eds). Tracking environmental change using Sciences, 2003, 18(3): 420–426 lake sediments. Kluwer Academic Publishers, 2001, 401–439 142 JIANG Shan, et al. Adv Polar Sci September(2011) Vol.22 No.3

40 Costanzo S D, O’donohue M J. Dennison W C, et al. A 48 Wang J Q, Liu J L. Amino acids and stable carbon iso- new approach for detecting and mapping sewage impacts. tope distributions in Taihu Lake, China, over the last 15,000 Marine Pollution Bulletin, 2001, 42(2): 149–156 years, and their paleoecological implications. Quaternary 41 Meyers P A, Teranes J L. Sediment organic matter. In Last Research, 2000, 53(2): 223–228 W M, Smol J P. (eds). Tracking environmental change using 49 Esposito R M M, Horn S L, McKnight D M, et al. Antarctic lake sediments. Kluwer Academic Publishers, 2001: 239–269 climate cooling and response of diatoms in glacial meltwater 42 Sanei H, Goodarzi F. Relationship between organic mat- streams. Geophysical Research Letters, 2006, 33: L07406 ter and mercury in recent lake sediment: The physical- 50 Wagner B, Cremer H. Limnology and sedimentary record of geochemical aspects. Applied Geochemistry, 2006, 21: Radok lake, Amery Oasis, East Antarctic. In Futterer D K, 1900–1912 Damaske D, Kleinschmidt G, et al.(eds). Antarctica: Con- 43 Outridge P M, Sanei H, Stern G A, et al. Evidence for con- tributions to global earth sciences. Springer-Verlag, Berlin, trol of mercury accumulation rates in Canadian High Arctic Heidelberg, New York, 2006: 447–454 lake sediments by variations of aquatic primary productiv- 51 Ohtsuka T, Kudoh S, Imura S, et al. Diatoms composing ity. Environmental Science and Technology, 2007, 41(15): benthic microbial mats in freshwater lakes of Skarvsnes ice- 5259–5265 free area, east Antarctica. Polar Bioscience, 2006, 20: 113– 44 Stern G A, Sanei H, Roach P, et al. Historical interrelated 130 variations of mercury and aquatic organic matter in lake sed- 52 Spaulding S A, Mcknight D M. Diatoms as indicators of en- iment cores from a subarctic lake in Yukon, Canada: Fur- vironmental change in Antarctic freshwaters. In Stoermer ther evidence toward the algal-mercury scavenging hypoth- E F, Smol J P (eds). The diatoms: applications for the esis. Environmental Science and Technology, 2009, 43(20): environmental and earth sciences. Cambridge: Cambridge 7684–7690 University Press, 1999: 245–263 45 Hu C Y, Yao M, Yu P S, et al. Biogenic silica in surfi- 53 Van de Vijver B, Beyens L, Lange-Bertalot H. The genus cial sediments of Prydz Bay of the Southern Ocean. Acta Stauroneis in the Arctic and Antarctic locations. Biblio- Oceanologica Sinica, 2007, 29: 48–54 theca Diatomologica, 2004, 50: 311 46 Sabbe K, Verleyen E, Hodgson D A, et al. Benthic diatom 54 Jiang S, Liu X D, Xu L Q, et al. The changes of pigments flora of freshwater and saline lakes in the Larsemann hills content and their environmental implication in the lake sed- and Rauer islands, east Antarctica. Antarctic science, 2003, iments of Ny-˚Alesund, Svalbard, Arctic. Chinese Journal of 15(2): 227–248 Polar Science, 2010, 21(1): 60-70 47 Sabbe K, Hodgson D A, Verleyen E, et al. Salinity, depth 55 Axford Y, Geirsdo´ttir, Miller G H, et al. Climate of the and the structure and composition of microbial mats in con- Little Ice Age and the past 2000 years in northeast Iceland tinental Antarctic lakes. Freshwater Biology, 2004, 49: 296– inferred from chironomids and other lake sediment proxies. 319 Journal of Paleolimnology, 2009, 41(1): 7–24 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00143 September 2011 Vol.22 No.3 143–152

Thermodynamic processes of lake ice and landfast ice around Zhongshan Station, Antarctica

LEI RuiBo1∗, LI ZhiJun2, ZHANG ZhanHai1 & CHENG YanFeng1

1Polar Research Institute of China, Shanghai 200136, China; 2State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China

Received July 27, 2011; accepted August 16, 2011

Abstract Thermodynamic processes of ice in three lakes and landfast ice around Zhongshan Station, Antarc- tica, were observed in 2006. The mass balance of lake ice was compared with that of landfast ice. The responses of lake ice and sea ice temperatures to the local surface air temperature are explored. Vertical conductive heat fluxes at varying depths of lake ice and sea ice were derived from vertical temperature profiles. The freeze up of lake ice and landfast ice occurred from late February to early March. Maximum lake ice thicknesses occurred from late September to early October, with values of 156–177 cm. The maximum sea ice thicknesses of 167–174 cm occurred relatively later, from late October to late November. Temporal variations of lake ice and landfast ice internal temperatures lagged those of air temperatures. High-frequency variations of air temperature were evidently attenuated by ice cover. The temporal lag and the high-frequency attenuation were greater for sea ice than for lake ice, and more distinct for the deeper ice layer than for the upper ice layer. This induced a smaller conductive heat flux through sea ice than lake ice, at the same depth and under the same atmospheric forcing, and a smoother fluctuation in the conductive heat flux for the deeper ice layer than for the upper ice layer. Enhanced desalination during the melt season increased the melting point temperature within sea ice, making it different from fresh lake ice.

Keywords Sea ice, lake ice, thermodynamics, thickness, temperature, Antarctica

Citation: Lei R B, Li Z J, Zhang Z H, et al. Thermodynamic processes of lake ice and landfast ice around Zhongshan Station, Antarctica. Adv Polar Sci, 2011, 22: 143–152, doi: 10.3724/SP.J.1085.2011.00143

0 Introduction climate system. Lake ice thermodynamic processes can Sea ice that grows attached to the coast, the front of influence the biological, chemical and physical processes a glacier tongue or ice shelf, or a grounded iceberg, is of lake systems. For example, the duration and com- defined as landfast ice. Because of its immovability, it position of lake ice controls the seasonal heat budget has long been considered as an ideal measure of local of lake systems[6]. Lake ice modulates underwater light climate change, similar to lake ice[1−4]. RADARSAT conditions[7], mixing processes[8], and nutrient recycling ScanSAR data has shown that landfast ice constitutes and oxidation[9]. Ice breakup strongly influences plank- about 28% by volume of sea ice in the Antarctic sec- ton dynamics[10] and winter fish kill[11]. tor of 75◦–170◦E, during November[5]. Because of its Lake coverage in the Larsemann Hills region, where longer ice season and larger thickness compared to pack Zhongshan Station is located, is 3.15%, representing a to- ice in the same region, landfast ice cannot be neglected tal area of 6.3 km2. The lakes are mainly fed by snowmelt when evaluating the role of Antarctic sea ice within the runoff[12], and they may generally freeze over completely

∗Corresponding author (email: [email protected]) journal.polar.gov.cn 144 LEI RuiBo, et al. Adv Polar Sci September(2011) Vol.22 No.3 by ice in mid to late February, and become ice free by 1 Field work and data the end of December to mid January of the following year. The ice season of lakes close to the continental ice Meteorological data were obtained from a manned sheet is relatively longer[12]. There are several islands weather station, one kilometer inland from the experi- off Zhongshan, and the nearby bathymetry is greatly mental site, at 14 m above sea level. During 2006 the undulated[13]. Therefore, a large number of icebergs, in- mean air temperature was –9.6◦C representing a posi- cluding those calved from D˚alk Glacier, tend to ground tive anomaly of 0.4◦C compared to the mean from 1989 offshore from the Larsemann Hills. Landfast ice off Larse- to 2006. The lowest monthly mean air temperature oc- mann Hills usually breaks off in early summer, often curred in July (–18.6◦C), and the highest in January because of contemporaneous high wind speeds, monthly (0.4◦C). The lowest annual air temperature was –30.2◦C, peak tides, and/or penetrating ocean swell[14]. Freeze up on 28 July, and the highest was 7.6◦C, on 5 January. Sur- usually takes place in February or March. Occasionally, face winds at Zhongshan were generally from the north a small portion of landfast ice may survive through sum- or east (about 77.1% of the time). mer to form multi-year sea ice within a narrow bay or Lake ice observations were made at Lakes Mochou, fjord, both of which provide protection from ocean swell Tuanjie and Progress (Figure 1). Lake Mochou was the and wind fetch[15]. Zhongshan Station can thus provide site of primary investigation. Morphological properties of an ideal supporting base for year-round synchronous in- these lakes are shown in Table 1. Lake Progress is near vestigations of lake ice and landfast ice thermodynamic the continental ice sheet, whereas Lakes Mochou and processes. Tuanjie are near the coast. A water volume of about 60 Thermodynamic processes of ices on three lakes in m3 per month was supplied from Lake Mochou to Zhong- the Larsemann Hills and landfast ice around Zhongshan shan Station, through a pump fixed on the bottom of the Station were observed through 2006. Snow thermal con- lake, about 10 m from its eastern shore. On 10 March, ductivity and oceanic heat flux under ice cover were three hot wire thickness gauges were deployed in a tri- estimated based on in-situ snow and landfast ice mass angular configuration at the centre of Lake Mochou, for balance data[16]. The contributions of snow cover to monitoring ice and snow thicknesses. The gauges had a the landfast ice thermodynamic processes were explored precision of §0.5 cm. Details on this gauge may be found based on comparison between the data from 2005 and in Lei et al.[18]. Measurements were taken from 17 March 2006[17]. The principal object of the current study is to 30 October, at a frequency of one to three days. On to compare the thermodynamic processes of landfast ice 10 April, a thermistor string was deployed among the with those of lake ice, forced by the same local meteoro- hot wire thickness gauges, to measure the vertical logical conditions. temperature profile from ice to water. The thermistors

Table 1 Morphological properties of the lakes[12]

Lake Altitude/m Max. depth/m Area/m2 Max. length/m Max. width/m Mochou 10 3.8 5 000 50 30 Tuanjie 5 4.5 50 000 255 225 Progress 65 34 105 000 600 300

were assembled with PT100 sensors at level A, with a April to 20 October, at an interval of 30 min. Snow and precision of §0.1◦C. The coherence of these sensors was ice thicknesses at Lakes Tuanjie and Progress were mea- controlled by selecting those that read within §0.05 K sured by making a hole with a drill with a 5-cm diameter of each other during calibration prior to the deployment. stainless steel auger. This was done every 5–10 days at The vertical spacing of the sensors was 6 cm for depths three holes, from 15 March to 17 November. The accu- between 0 to –144 cm, and 12 cm for depths between racy of the drill-hole method was §0.5 cm. –144 to –204 cm. Measurements were taken from 18 Landfast ice measurements were made at three sites, Thermodynamic processes of lake ice and landfast ice around Zhongshan Station 145 as shown in Figure 1. These were located off the north- m·s−1, although the speed would increase rapidly by the east coast of the Mirror peninsula (S1), off its northern end of the snowfall event. Due to the snow blowing, coast (S2), and to the south of Nella Fjord (S3). The snow depth at both Lake Mochou and S2 was very small primary investigated site was S2. The shortest distances throughout the experiment. Furthermore, lake ice sur- from the S1–S3 sites to the coast are 400 m, 280 m and face was relatively slippery, resulting in a smaller snow 150 m, respectively. The ocean depths at S1–S3 are 24.6 depth at Mochou than that at S2. m, 35.7 m and 5.5 m, respectively. On 26 March, three Complete freeze up of Mochou, Tuanjie and Progress hot wire thickness gauges were deployed in a triangu- occurred on 27 February, 1 March and 24 February, re- lar form at S2, for monitoring ice and snow thicknesses. spectively. The Freeze up date of lakes is related to lo- Measurements lasted from 1 April to 20 December, at an cal atmospheric forcing, heat storage in the lake during interval of 1–3 days. On 27 March, a thermistor string, summer-fall, morphological properties of the lake and lo- with specifications identical to the one at Lake Mochou, cal topography[19]. The freeze up of the Lake Mochou was deployed at S2 to measure the vertical temperature occurred relatively early, which may be explained by its profile from ice cover to seawater. These measurements small size, and wind shields by hills and artificial struc- were taken from 1 April to 12 December, at an interval tures around the lake. The freeze up of Lake Tuanjie oc- of 30 min. curred latest, because of its relatively large size and lack of the wind shields. The freeze up of the Lake Progress occurred earliest, which may be attributed to its location near the continental ice sheet. Ice growth rates at these lakes were very similar after the formation of a contin- uous ice sheet, until the end of July. Then the growth rate at Lake Mochou dramatically declined, producing the smallest ice thickness of all lakes. Annual maximal ice thickness appeared earliest at Lake Mochou, latest at Lake Progress. The smallest annual maximum ice thick- ness was at Lake Mochou. This might be attributed to the pump at the lake bottom, which may have produced a larger heat flux from the water to the ice bottom. Continuous ice sheets occurred on 1 March, 5 March and 6 March at S1–S3, respectively. These dates were slightly later than those of the lakes, because of the lower freezing point of seawater. Nella Fjord is shallow and well sheltered from the ocean swell and prevailing winds. The S3 site froze 2–5 days earlier than other ar- eas around Zhongshan Station (S1 and S2). Ice growth Figure 1 Locations of landfast ice and lake ice measure- rates at S1–S3 were very similar after the continuous ice ments around Zhongshan Station in 2006. sheet formed, until the end of October. From then on- ward, the ice growth rate at S3 became smaller and ice 2 Results thickness reached its annual maximum relatively earlier (30 October, 167 cm). This might be attributed to the 2.1 Snow and ice thickness shallow snow cover on the Larsemann Hills, where sand Figure 2 shows time series of ice thickness at multiple and small stones were swept onto the leeward side of the sites and those of snow thickness over Lake Mochou and Mirror Peninsula, and deposited onto the ice surface of S2. Every snow depth peak in the time series may be as- Nella Fjord. Surface deposits of sand and stones reduced sociated with a snowfall event. During a snowfall event, albedo, promoting surface melt and ponding during late wind speed was relatively weak, normally less than 4 spring and summer. The ice growth rate at S1 and S2 146 LEI RuiBo, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 2 Variation in snow depth (a), lake-ice thickness (b), and landfast ice thickness (c) during 2006. remained close during the entire experiment. Annual ation transferred through the lake ice cover after July, maximum ice thicknesses at both sites were reached by accelerating ice thaw. the end of November, with values of 172 cm and 174 cm, 2.2 Ice temperaturevertical profile and its re- respectively. In other words, the landfast ice reached its sponse to surface ice temperature annual maximum thickness about 1–2 months later than the lake ice around Zhongshan Station. Figure 4 shows the temporal evolution of internal ice tem- Figure 3 presents time series of ice growth rates at perature at Lake Mochou and S2. At both sites, the ice Lake Mochou and S2, along with their monthly aver- temperature in the top layer fluctuated greatly, in re- age values. Generally speaking, the ice growth rates at sponse to variations in surface air temperature. There Lake Mochou and S2 were similar throughout the en- was substantial attenuation and lag for the fluctuation tire experiment, and their correlation coefficient was 0.82 at deeper layer, especially at S2. The temperature at ice (P <0.01). Maximum ice growth rates at both sites bottom remained at the freezing point of local water at occurred at the beginning of May, with values of 1.45 both sites. From mid September onward, ice tempera- cm·d−1 and 1.35 cm·d−1, respectively. From then on- ture climbed as summer approached, which reduced the ward, ice growth rates at both sites decreased with in- ice-temperature gradient at both sites. However, the in- creasing ice thickness. From the end of July onward, the crease in landfast ice temperature was greater than that ice growth rate at Mochou decreased more rapidly than of the lake ice. Interior melt of sea ice may result in de- at S2, causing a greater deviation of rate between the salination, which makes sea-ice melt to be different from sites. This resulted an earlier ice melting at Mochou. that of lake ice. The desalination within sea ice may This could be related to a smaller extinction coefficient increase the ice melting point, elevating the ice temper- for lake ice[20−21], which would allow more solar radi- ature above the freezing point of local seawater. Thermodynamic processes of lake ice and landfast ice around Zhongshan Station 147

Figure 3 Growth rates of lake ice and landfast ice (a) and their monthly average values (b).

To quantify ice temperature variations at Lake Mo- tween 10-day mean values of the daily variation of lake chou and S2 in response to the daily changes in air tem- ice temperatures and those of landfast ice were 2.3, 2.8, perature, we calculated the daily ice temperature changes 3.4, 5.0 and 8.8, for depths of –18 cm, –48 cm, –78 cm, at depths of –18 cm, –48 cm, –78 cm, –108 cm and –108 cm and –138 cm, respectively. The differences can –138 cm, for both Lake Mochou and S2. The 10- be explained by the difference in composition between day mean calculated values are presented in Fig- lake ice and sea ice. Sea ice is composed of pure ice crys- ure 5. Daily changes in ice temperatures decreased tals, brine, solid salt, air bubbles and other impurities. with increasing depth for both lake ice and land- The outstanding difference between the compositions of fast ice. At the same depth, however, the mag- lake fresh ice and sea ice is that there is no liquid or solid nitudes of daily changes in lake ice temperatures salt within the ice matrix. The phase fractions between were greater than those in landfast ice. This diffe- liquid salt and solid salt within sea ice depend on in-situ rence became more distinct with depth. The ratios be- ice temperatures. The changes in the phase fractions 148 LEI RuiBo, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 4 Evolution of internal temperature of lake ice at Lake Mochou (top), and of landfast ice at S2 (bottom); blue solid lines represent ice-water interfaces, and gaps represent intervals when no data were available. may release or absorb some heat. Thus, daily changes chou, which was 0.64 times that at S2, 0.98§0.71 d. in sea-ice temperatures were relatively weaker compared This temporal lag at –66 cm depth was 1.2 d and with those in lake ice, despite forcing by the same at- 2.1 d for Mochou and S2, respectively. In period 2, the mospheric conditions. Furthermore, the relatively heavy mean maximum lag correlation coefficient from the sur- snow cover over S2 may have attenuated the daily face to a depth of –102 cm was 0.89§0.04 at Mochou, changes in ice temperature there. which was 1.2 times that at S2, 0.77§0.08. The temporal Figure 6 shows the vertical profiles of daily minimum lag according to the maximum lag correlation coefficient and maximum ice temperature at Lake Mochou and S2, was 1.20§0.80 d at Mochou, which was 0.49 times that from 20 April through 20 September. The profiles again at S2, 2.43§1.70 d. The temporal lags at –102 cm depth illustrated that lake ice temperature had larger daily vari- were 2.5 d and 5.4 d for Mochou and S2, respectively. ation than that of landfast ice. Because of a skeletal layer The results from both periods indicate that the maxi- at the bottom of landfast ice, which was composed of a mum lag correlation coefficients decreased linearly with large fraction of brine[22], there was a 5–10 cm isothermal depth (P <0.01), and the corresponding temporal lags layer at the bottom of landfast ice. This layer did not increased linearly with depth (P <0.01) for both lake ice appear in the lake ice. and landfast ice. We now quantify the temporal lag of ice temper- 2.3 Conductive heat flux through the ice cover ature relative to those in surface air temperature. We calculated lag correlation coefficients between ice tem- Conductive heat flux through ice cover is the primary peratures and surface air temperature, for two time win- forcing for ice growth at the bottom. Vertical discon- dows: from 1 June to 15 July, the period during the polar tinuity of the conductive heat flux through ice cover is night (period 1), and from 17 August to 30 September, the crucial factor for interior ice melt. To quantify the the period after the polar night (period 2). vertical discontinuity of the conductive heat flux, we cal- The calculations are shown in Figure 7. In pe- culated the flux in the layers of –12 to –30 cm, –54 to –72 riod 1, the mean maximum lag correlation coeffi- cm and –96 to –114 cm, for both Mochou and S2. The cient from the surface to a depth of –66 cm was flux was calculated by § 0.79 0.07 at Mochou, which was 1.3 times as that at ∂Ti Fc = ki (1) S2, 0.60§0.13. The temporal lag according to the max- ∂zi imum lag correlation coefficient was 0.62§0.40 d at Mo- where Fc is conductive heat flux, ki is ice thermal Thermodynamic processes of lake ice and landfast ice around Zhongshan Station 149

Figure 5 Daily variation of lake ice and landfast ice temperatures, at varying depths. conductivity, which is a function of ice temperature and ture, there was a temporal lag between the low-frequency [23−24] ice salinity , and (∂Ti/∂zi) is vertical ice temper- change of Fc at the middle/low layers and that at the top ature gradient. Figure 8 shows that the Fc through layer, especially for sea ice. The downward Fc (negative ice cover was primarily related to surface air temper- value) may occur at the top layer when the surface air ature and ice thickness. Under the same surface air temperature increases rapidly, which causes conductive temperature, the Fc decreased with increasing ice thick- heat to be concentrated in the subsurface layer of the ice ness. There was a notable high-frequency oscillation in column. Conductive heat flux slowed distinctly decrease

Fc through the top layer (–12 to –30 cm), especially for when ice melt commenced, especially in the middle/low the sea ice at S2, similar to the time series of ice temper- layer, because of the weak ice-temperature gradient. For ature. This high-frequency oscillation was compressed in the skeleton layer at the sea ice bottom, with substantial the deeper layer. Under the same surface air tempera- brine fraction and small ice-temperature gradient, the Fc ture, the Fc through both the middle (–54 to –72 cm) and at layers of –54 to –72 cm and –96 to –114 cm was low (–96 to –114 cm) layers was smaller for sea ice than small for sea ice, during which time the ice bottom for lake ice. Also similar to the time series of ice tempera- immediately grew downward past the investigated layer. 150 LEI RuiBo, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 6 Vertical profiles of daily maximum and minimum temperatures of lake ice and sea ice; also shown are ∆Ta for daily variation of air temperature, and T hs1 and T hs2 for snow depths above lake ice and landfast ice.

Figure 7 Lag correlation coefficients of ice temperatures at varying depths relative to air temperature: a and b for lake ice during periods 1 and 2, respectively, c and d for landfast ice during periods 1 and 2, respectively. Thermodynamic processes of lake ice and landfast ice around Zhongshan Station 151

Figure 8 Variations in conductive heat flux at varying depths of lake ice (a) and landfast ice (b).

This reflects a sea-ice thermodynamic process that is dif- Under the same atmospheric forcing, the lake/sea ferent from lake ice. ice thermodynamic processes also may be influenced by the solar radiations absorbed by the ice column, and by 3 Conclusions the heat flux from the bottom water. Thus, optical and standard oceanographic measurements (e. g., turbulence A continuous ice sheet formed over the lakes and coastal measurements under the ice) still need to be made, in regions around Zhongshan Station from later February to association with mass balance measurements for a thor- the beginning of March. The freeze date of the landfast ough understanding of the thermodynamic processes of ice was slightly later (about 0–6 d) than lake ice. The lake ice and landfast ice around Zhongshan Station. lake ice attained its annual maximum ice thickness from the end of September to the beginning of October, 1–2 months earlier than landfast ice. The annual maximum ice thickness was very similar for lake ice and landfast ice, Acknowledgments This work was supported by the National Basic Research Program of China(Grant no. 2010 CB950301), the ranging from 167 to 177 cm, except at Lake Mochou. The China Postdoctoral Science Foundation (Grant no.20100470400), annual maximum ice thickness at Mochou was smallest, and the Shanghai Postdoctoral Sustentation Fund(Grant no. which might be attributable to the influence of a pump 11R21421800). We are grateful to Mr. Dong Li, Mr. Liu Xiufeng, at the lake bottom. Mr. Jin Wei, and Mr. Du Shuangzhi for their field support. We would like to acknowledge the meteorological station of Zhongshan Due to enwrapped brine and heavy snow over the for providing the atmospheric data. surface, temporal variations in landfast ice temperature had a weaker response and a stronger temporal lag to References temporal variations in surface air temperature, compared to lake ice. During the melt season, the sea ice tempera- 1 Livingstone D M. Break-up dates of alpine lakes as proxy ture increased to warmer than the freezing point of local data for local and regional mean surface air temperatures. Climatic Change, 1997, 37: 407–439 seawater, because of the interior desalination. This re- 2 Magnuson J J, Robertson D M, Benson B J, et al. His- sulted in a weaker ice-temperature gradient and conduc- torical trends in lake and river ice cover in the Northern tive heat flux than in lake ice. Hemisphere, Science. 2000, 289(5485): 1743–1746 152 LEI RuiBo, et al. Adv Polar Sci September(2011) Vol.22 No.3

3 Shirasawa K, Leppa¨ranta M, Saloranta T, et al. The thick- off Zhongshan Station, Antarctica. Chinese Journal of Polar ness of coastal fast ice in the Sea of Okhotsk.Cold Regions Science, 2008, 19 (1): 54–62 Science and Technology, 2005, 42(1): 25–40 15 Tang S L, Qin D H, Ren J W, et al. Structure, salinity and 4 Heil P. Atmospheric conditions and fast ice at Davis, East isotopic composition of multi-year landfast sea ice in Nella Antarctica: A case study. Journal Geophysical Research, Fjord, Antarctica. Cold Regions Science and Technology, 2006, 111, C05009, doi:10.1029/2005JC002904 2007, 49(2): 170–177 5 Giles A B, Massom R A, Lytle V I. Fast-ice distribution 16 Lei R B, Li Z J, Cheng B, et al. Annual cycle of landfast in East Antarctica during 1997 and 1999 determined using sea ice in PrydzBay, east Antarctica. Journal of Geophysical RADARSAT data. Journal of Geophysical Research, 2008, Research, 2010, doi:10.1029/2008JC005223 113, C02S14, doi:10. 1029/2007JC004139 17 Lei R B, Li Z J, Dou Y K, et al. Observations of the growth 6 Qin B, Huang Q. Evaluation of the climatic change impacts and decay processes of fast ice around Zhongshan Station in on the inland lake—a case study of LakeQinghai, China. Antarctica. Advances in Water Science, 2010, 21(5): 708– Climatic Change, 1998, 39: 695–714 712 (In Chinese) 7 Leppa¨ranta M, Reinart A, Erm A, et al. Investigation of 18 Lei R B, Li Z J, Qin J M, et al. Investigation of new tech- ice and water properties and under-ice light fields in fresh nologies for in-situ ice thickness observation. Advances in and brackish water bodies. Nordic Hydrology, 2003, 34(3): Water Science, 2009, 20(2): 287–292 (In Chinese) 245–266 19 Palecki M A, Barry R G. Freeze-up and break-up of lakes 8 Mironov D, Terzhevik A, Kirillin G, et al. Radiatively driven as an index of temperature changes during the transition convection in ice-covered lakes: Observations, scaling, and a seasons: A case study for Finland. Journal of Climate and mixed layer model. Journal of Geophysical Research, 2002, Applied Meteorology, 1986, 25: 893–902 107(C4), 3032, doi: 10.1029/2001JC000892 20 Arst H, Erm A, Leppa¨ranta M, et al. Radiativecharacteris- 9 Livingstone D M. Lake oxygenation: application of a tics of ice-covered fresh- and brackish-waterbodies. Proceed- one-box model with ice cover. Internationale Revue der ings of the Estonian Academy of Sciences-Geology, 2006, gesamten Hydrobiologie und Hydrographie, 1993, 78(4): 55(1): 3–23 465–480 21 Lei R B, Leppa¨ranta M, Erm A, et al. Field investigations 10 Gerten D, Adrian R. Climate-driven changes in spring of apparent optical properties of ice cover in Finnishand Es- plankton dynamics and the sensitivity of shallow polymic- tonian lakes in winter 2009. Estonian Journal of Earth Sci- tic lakes to the North Atlantic Oscillation. Limnology and ences, 2011, 60(1): 50–64 Oceanography, 2000, 45(5): 1058–1066 22 Eicken H. From the microscopic to the macroscopic to the 11 Barica J, Mathias J A. Oxygen depletion and winterkill risk regional scale: Growth, microstructure and properties of sea insmall prairie lakes under extended ice cover. Journal of ice. In Thomas D, Dieckmann G S. Sea ice-an introduction the Fisheries Research Board of Canada, 1979, 36: 980–986 to itsphysics, biology, chemistry and geology, Blackwell Sci- 12 Li S K. Lakes of the Larsemann Hills, East Antarctica. Jour- ence, London, 2003: 22–81 nal of Lake Sciences, 1995, 7(3): 193–202 (In Chinese) 23 Untersteiner N. On the mass and heat budget of arctic sea 13 Feng S Z, Xue Z, Chi W Q. Topographic features around ice. Meteorology and Atmospheric Physics, 1961, 12(2): Zhongshan Station, Southeast of Prydz Bay. Chinese Jour- 150–182 nal of Oceanology and Limnology, 2008, 26(4): 467–472, 24 Yen Y C. Review of thermal properties of snow, ice and doi:10.1007/s00343-008-0467-8 seaice.Cold Regions Research and Engineering Laboratory 14 Lei R B, Li Z J, Zhang Z H, et al. Summer fast ice evolution Report 81–10, Hanover, New Hampshire, 1981: 1–27 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00153 September 2011 Vol.22 No.3 153–164

Summer freshwater content variability of the upper ocean in the Canada Basin during recent sea ice rapid retreat

GUO GuiJun1,2, SHI JiuXin1,2∗, ZHAO JinPing1,2, JIAO YuTian1,2 & XU Dong1,2

1 College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China; 2 Key Laboratory of Physical Oceanography, Ministry of Education, Qingdao 266003, China

Received August 2, 2011; accepted August 24, 2011

Abstract Freshwater content (F W C) in the Arctic Ocean has changed rapidly in recent years, in response to significant decreases in sea ice extent. Research on freshwater content variability in the Canada Basin, the main storage area of fresh water is very important to understand the input-output freshwater in the Arctic Ocean. The F W C in the Canada Basin was calculated using data from the Chinese National Arctic Research Expeditions of 2003 and 2008, and from expeditions of the Canadian icebreaker Louis S. St-Laurent (LSSL) from 2004 to 2007. Results show that the upper ocean in the Canada Basin became continuously fresher from 2003 to 2008, −1 except during 2006. The F W C increased at a rate of more than 1 m·a , and the maximum increase, 7 m, was in the central basin compared between 2003 and 2008. Variability of the F W C was almost entirely limited to the layer above the winter Bering Sea Water (wBSW), below which the F W C remained around 3 m during the study period. Contributors to the F W C increase are generally considered to be net precipitation, runoff changes, Pacific water inflow through the Bering Strait, sea ice extent, and the Arctic Oscillation(AO). However, we determined that the first three contributors did not have apparent impact on the F W C changes. Therefore, this paper focuses on analysis of the latter two factors and the results indicate that they were the major contributors to the F W C variability in the basin.

Keywords Freshwater content, Canada Basin, sea ice, Arctic Oscillation(AO)

Citation: Guo G J, Shi J X, Zhao J P, et al. Summer freshwater content variability of the upper ocean in the Canada Basin during recent sea ice rapid retreat. Adv Polar Sci, 2011, 22: 153–164, doi: 10.3724/SP.J.1085.2011.00153

0 Introduction tent (F W C) variability has therefore aroused great at- The Arctic Ocean (Figure 1) is one of the major tention. Previous research has shown that among river source regions for surface waters of the subpolar At- runoff, net precipitation (precipitation minus evapora- lantic Ocean, where deep convection caused by weak tion), sea ice melting/freezing and wind-driven circula- stratification is the main force of ocean circulation and tion each has some influence on the F W C changes in the thermodynamics[1]. Sea ice export is one of the primary Arctic Ocean[3−8]. Using a vast collection of observa- ways by which the Arctic Ocean provides freshwater to tional data, Polyakov et al.[9] studied the F W C changes the Atlantic Ocean. Mauritzen and H¨akkine[2] stated in the Arctic Ocean over the past 100 years. They ob- that the thermohaline circulation in the North Atlantic tained F W C spatial distributions by using linear inter- might be strengthened by 10%–20% in response to a polation and analyzed the influencing factors with their decline of the ice outflow of 800 km3. Freshwater con- respective contributions to the F W C variability. They

∗Corresponding author(email: [email protected]) journal.polar.gov.cn 154 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3

forms the Arctic halocline below the fresher surface water (SW), which is crucial for maintaining the low tempera- ture character of the SW[13]. Most Pacific water is stored in the Canada Basin and forms three distinct water masses with varying temperature and salinity after enter- ing the Arctic Ocean. These three are Alaskan Coastal Water (ACW), summer Bering Sea Water (sBSW) and winter Bering Sea Water (wBSW). These water masses have two types of vertical structure in the Canada Basin: along the exterior of the Transpolar Drift Stream in- fluenced by the Pacific water, the wBSW underlies the sBSW; in the southern Canada Basin, the wBSW un- derlies the ACW[14]. A typical temperature and salinity profile in the southern Canada Basin is shown in Figure 2. In this figure, the top 50 m layer in the Arctic Ocean is Figure 1 Map of the Arctic Ocean and study area (inset). defined as SW, where the temperature is nearly constant concluded that the central Arctic Ocean became increas- and salinity is less than 30; below the SW is the Pacific ingly saltier over the twentieth century, with a rate of halocline water, defined by a temperature near the freez- freshwater loss of 239§270 km3 per decade[9]. Since the ing point within the salinity range 30< S <34, where the late twentieth century, however, dramatic changes of the ACW is above the wBSW. Under the Pacific halocline Arctic atmosphere-ice-ocean system, including rapid sea water lies intermediate Atlantic Water (AW), which has ice retreat, has occurred because of the global warming, a salinity higher than 34[15]. Freshwater changes take resulting in a fact that the Arctic Ocean became cor- place mainly in the upper Arctic Ocean water masses respondingly fresher[10]. Potential causes for the F W C above the AW, and the depth changes of the AW do not anomalies include precipitation larger than evaporation, affect the F W C. Thus, we take the mean salinity of in- runoff changes, Pacific water inflow through the Bering termediate AW, 34.8, as the reference salinity, on which Strait, sea ice melting/freezing and freshwater export. the F W C is calculated[9]. The water masses above the The first three mechanisms are considered too weak to AW, influenced by the Pacific water, can be divided into cause significant F W C variability, leaving the last two two parts by the wBSW. First, the water in the lower factors as probable causes[3−9]. Aagaard and Carmack’s layer below the wBSW originates from the Bering Sea estimates suggested that the freshwater storage in the and the Chukchi Sea shelves, which has a constant tem- Arctic Ocean was approximately 80 000 km3 (relative to perature and salinity in winter[16]. In contrast, the waters the salinity 34.8)[3]. Of this, the Canada Basin (Figure in the layer above the wBSW containing summer Pacific 1) contained 45 800 km3, representing about 60% of the water and sea ice melt water is the most variable region total Arctic Ocean storage[3]. This large proportion gives within the Arctic Ocean. Therefore, we define the upper import to research on the F W C variability in the Canada ocean as the layer above the wBSW by a temperature Basin, toward an understanding of freshwater changes in minimum with salinity less than 33.1, upon which we the Arctic Ocean. paid considerable emphasis in our paper. The Bering Strait is the only ocean gateway between In the late 1980s, the upper part of the Arctic Ocean the Pacific and Arctic Oceans. Pacific water flows into began to become fresher in response to decreasing sea − the Chukchi Sea through the Bering Strait, then gath- ice volume[17 18]. Meanwhile, export of multiyear ice − ers into a polar basin across the slope[11]. Although from the ocean began to accelerate[19 20]. Since the mid- the Bering Strait inflow is the major freshwater source 1990s, sea ice extent has drastically decreased and con- − for the Arctic Ocean, previous studies have shown that tinued to reach new record lows[21 23]. Satellite data its annual variability was not sufficient to significantly indicates that sea ice extent in the summer Arctic Ocean influence F W C changes[12]. The Pacific water inflow has declined by 11.5% per decade since 1979 and, in Summer freshwater content variability of the upper ocean in the Canada Basin 155

Figure 2 Temperature and salinity profiles in southern Canada Basin (based on Station LSS042 of the Canadian LSSL cruise of 2005, marked with yellow symbol in Figure 3).

August, 2007, it broke all the previous records. In 2008 inant, the Canada Basin releases freshwater under cy- and 2010, the September minimum extents were the sec- clonic wind forcing. Water released from the basin flows ond and the third lowest in the satellite record[24]. Com- into the Atlantic Ocean through the Fram Strait, re- bined with decreases in extent, the age of sea ice in the ducing freshwater in the upper ocean of the basin[26]. central Arctic basin has also dramatically changed since Proshutinsky et al.[26] investigated the F W C storage in the mid-1980s. In 1987, 57% of the ice pack was at least the BG and found a fact that interannual changes in the 5 years old, and a quarter of that ice was at least 9 years F W C during 2003–2007 were characterized by a strong old. By 2007, however, only 7 percent of the ice pack positive trend in this region, with a maximum of approx- was at least 5 years old, and virtually none was at least 9 imately 1.5 m·a−1 in the central basin. One of the major years old[25]. Sea ice melt in the Arctic Ocean occurred causes for this was the anticyclonic atmosphere circu- mainly in the Canada Basin, lowering salinity and in- lation, which aggravated the mixing due to the Ekman creasing freshwater in the upper ocean. pumping. In response, the F W C in the Canada Basin Changes of the F W C in the Arctic Ocean have been increased significantly[28]. Considering that wind forcing left untouched till the discussion provided by Proshutin- is limited and its influence on sea water would be weak- sky et al.[26]. They suggested that changes of F W C stor- ened by ice cover, water mixing should be confined to the age in the anticyclonic Beaufort Gyre (BG) could po- upper ocean. We thus emphasized our analysis mainly on tentially be much larger than those due to variations of the F W C of the upper ocean, i.e., the layer with salinity river runoff and ice export events, and were tied to the lower than 33.1, to investigate its variability in the Arctic decadal mode of ocean circulation variations, the Arc- Ocean. tic Oscillation (AO)[26]. The BG is a wind-driven gyre In the new century, more accessible observational bounded by latitudes 70.5◦N and 80.5◦N, and longitudes data in the Canada Basin facilitates study of F W C 170◦W and 130◦W[27]; it is a basic surface circulation changes in the upper ocean. We analyzed CTD data from pattern and is well within our study area (Figure 1). the Chinese and Canadian expeditions, to investigate the During the negative phase of the AO, an anticyclonic F W C variability and its influences for 2003–2008 in the atmosphere circulation with high pressure induces sub- Canada Basin. stantial mass transport in the ocean toward the center of the anticyclonic gyre. Such continuous volume trans- 1 Data sources and methods port generates a downward velocity (Ekman pumping). In this case, the Canada Basin accumulates freshwater. 1.1 Data sources In contrast, when the positive phase of the AO is dom- We calculated the F W C in the water column of the up- 156 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3 per ocean in the Canada Basin by comparing salinity is calculated as vs. depth with reference salinity. The hydrographic data z2 S − S z F W C ref ( )dz that we analyzed came from CTD data obtained during = S (1) Zz1 ref the 2nd and 3rd Chinese National Arctic Research Expe- S z S ditions in 2003 and 2008 (the 2nd CHINARE–Artic, the where is water salinity at depth and ref is the ref- [29] 3rd CHINARE–Artic), and expeditions of the Canada erence salinity . Here we took the mean salinity of the S S z z icebreaker Louis S. St-Laurent (LSSL) in the Canada AW as ref , namely ref =34.8. 1 and 2 are the upper Basin, from 2004 to 2007. Hydrographic stations used in and lower boundaries of the layer, respectively, within F W C F W C this study are shown in Figure 3. Data from the Chinese which the is calculated. Thus, the is a expeditions were collected mainly in the western basin, measure of how much freshwater volume is in the ocean z z whereas stations of LSSL’s cruises covered most of the column bounded by 1 and 2. While the upper ocean F W C z z basin. Although the number of stations was limited and was calculated, 1=0 and 2 was the depth of the distribution of those exhibited large gaps owing to ob- upper ocean’s bottom boundary, i.e., the depth of the F W C servational conditions in the Arctic Ocean, those profile 33.1 isohaline. When we calculated the below the z data were adequate to analyze upper ocean F W C vari- wBSW, we took the depth of the 33.1 isohaline as 1 and z ability in the basin. the depth of reference salinity (34.8) isohaline as 2. Based on observational data from the Chinese ex- peditions and the Canadian LSSL cruises, we calculated the F W C of the two layers bounded by the wBSW. Fig- ures 4 and 5 show its distribution and variability in the Canada Basin during 2003–2008. To aid understanding of F W C changes in the upper ocean, the depth of the 33.1 isohaline is shown in Figure 6.

2 Results and discussion

2.1 Freshwater content variability Although an accurate total freshwater volume in the Canada Basin was unavailable for lack of observational Figure 3 Hydrographic stations of Chinese National Arctic stations, the profiles we obtained are satisfactory for Research Expeditions in 2003 and 2008 and Canadian LSSL showing the tendency of the F W C variability in the cruises in the Canada Basin for 2003–2007. basin. Calculations on the observational data show that for We also analyzed monthly mean sea ice concentra- the period 2003–2008, the F W C below the wBSW re- tion obtained from the National Snow and Ice Data Cen- mained roughly constant, 3 m in the southern basin and ter (NSIDC). This product was created with resolution 2.5 m in the north (Figures 4a–4e). There was an excep- 25×25 km2 using satellite passive microwave data from tion in 2008, however, when the F W C below the wBSW the Special Sensor Microwave/Imager (SSM/I). Monthly exhibited a mild increase (Figure 4f). Our conclusion is mean AO index data since January 1950 used in our anal- that the F W C changes were generally limited to the up- ysis were from NOAA/CPC, constructed by projecting ◦ per ocean, because liquid freshwater obtained from the daily 1 000 mb height anomalies poleward of 20 N onto sea ice melt and BG accumulation do not usually influ- the loading pattern of the AO, which is defined as the ence water masses to depth below the wBSW. We be- leading mode of Empirical Orthogonal Function (EOF) lieve that the mild F W C increase below the wBSW in analysis of monthly mean 1 000 mb height during 1979– the southern basin in 2008 was related to a combination 2000. of rapid decline of sea ice extent (Figure 7f) and a strong 1.2 Freshwater content calculation dominance of the AO negative phase. We will discuss The FWC (m) within a horizontal unit area in the ocean this in detail in section 2.2.2. Summer freshwater content variability of the upper ocean in the Canada Basin 157

Figure 4 Freshwater content for 2003–2008 in the layer below wBSW in the Canada Basin (m). a. 2003; b. 2004; c. 2005; d. 2006; e. 2007; f. 2008.

McPhee et al.[10] confirmed that the F W C in the increased rapidly from less than 17 m in 2003 to 22 m in southeast Canada Basin in 2008 had increased by as 2008, showing that the upper ocean had become much much as 11 m, 60% greater than the climatological mean fresher. In the early part of this period, the F W C in- from 1950 to 2000. Our calculations suggested that the crease was relatively mild (Figures 5a, 5b). However, the F W C variability took place primarily in the upper ocean. F W C in 2005 increased by approximately 2 m in most Figure 5 shows the magnitude of freshwater at each sta- areas, except for the southeast basin, where the decline tion in the basin. During 2003–2008, the F W C in the of sea ice extent temporarily halted (Figure 5c). There basin exhibited an apparent increasing trend, 1 m·a−1, was an overall decrease in F W C in 2006 (Figure 5d), but except for the year 2006. In the central basin, the F W C thereafter, rapid growth in F W C broke out and in 2008, 158 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 5 Freshwater content for 2003–2008 in the upper layer of the Canada Basin (m). a. 2003; b. 2004; c. 2005; d. 2006; e. 2007; f. 2008. the absolute maximum F W C increase, 3 m, was reached ing the F W C in the south much higher than that in the in the south central region (Figure 5f). Another conclu- north. sion from Figure 5 is that freshwater stored in the south- Salinity in the upper ocean clearly changed in re- ern Canada Basin, especially in the southeast, was much sponse to the F W C variability. Thus we showed the greater than that in the northern area. The Bering Strait spatial distribution of the bottom depth of the upper inflow was the key contributor to the imbalanced distri- ocean in the Canada Basin (the depth of 33.1 isohaline) bution because oceanic heat flux through the strait would in Figure 6, to reveal indirectly the changes in F W C of melt large amounts of sea ice in the southern basin[12]. the upper ocean. This figure demonstrates that, coinci- Sea ice melt produced a great deal of freshwater, mak- dent with the increasing tendency of F W C, base depth Summer freshwater content variability of the upper ocean in the Canada Basin 159 of the upper ocean also showed large changes with sig- ter. As stated previously, the exceptional case of 2006 nificant positive trends during 2003–2008. In 2003, the was related to the pause in the decrease of sea ice extent base depth of the upper ocean was generally less than 160 (Figure 7d), which caused salinity of the upper ocean to m. Thereafter, thickness of the upper ocean increased by increase slightly. approximately 10 m·a−1 in the south except for the year 2.2 Causes of the FWC variability in 2003–2008 2006, and it reached 200 m in the central basin in 2008. The reason for this rapid increase was that the freshening What were the key factors contributing to the strong upper ocean in the Canada Basin transported substan- positive trend in the F W C observed in 2003–2008? Con- tial freshwater downward through Ekman pumping[10], sidering freshwater sources, there are such contributors which lowered sea water salinity below the surface wa- having an impact on F W C as net precipitation, runoff

Figure 6 The depth of the interface of S=33.1 in the Canada Basin for 2003–2008. a. 2003; b. 2004; c. 2005; d. 2006; e. 2007; f. 2008. 160 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3 changes, Bering Strait inflow and sea ice melt- F W C would require nearly 17 m of net sea ice melt. We ing/freezing. When freshwater transport and distribu- therefore conclude that sea ice melt alone could not bring tion are taken into account, oceanic circulation also has about such a large F W C increase (almost 5 m during an influence on the F W C variability. Due to lack of cur- 2003–2008). There must be other factors carrying im- rent observational data, we explored the impact of the portant effects on the F W C variability. Proshutinsky AO index, which is closely related to oceanic circulation. et al.[28] showed that the F W C variability in the BG The Bering Strait inflow is the major source of fresh- region was closely linked to the atmospheric pressure. water in the Arctic Ocean. The annual mean transport During a dominant negative AO index, high pressure in through the strait is 0.8 Sv, equivalent to 25 000 km3 the atmosphere above the sea surface would induce in- of the Pacific water flowing into the Arctic[12]. As long- crease in F W C, and vice versa[28]. Therefore, the at- term mean salinity of the inflow is generally near 32.5[3], mospheric conditions and the corresponding oceanic cir- the freshwater volume contained in the imported Pacific culation variability were also important contributors to water is around 1 600 km3. No statistically significant F W C changes in the upper ocean of the Canada Basin change has been found in the annual mean volume trans- during 2003–2008. port, in spite of its great magnitude. During 2003–2008, 2.2.1 Sea ice retreat the difference between the maximum and minimum of the inflow transport was less than 0.2 Sv[12], therefore the We explored the influence of changes in sea ice extent on maximum magnitude of annual change in imported fresh- the upper ocean F W C in the Canada Basin. With ac- water was approximately 400 km3, which was too small celerated global warming, sea ice in the Arctic Ocean has to account for F W C variability in the Canada Basin. retreated dramatically since the 1990s, frequently break- According to NCEP/NCAR reanalysis data, long-term ing records of minimum extents. The changes in sea ice variation of the net precipitation over the Arctic Ocean volume ranges from –2 500 km3 to +2 500 km3, which was around 10%, ranging from –200 km3 to +200 km3. closely parallel the F W C changes in the Arctic Ocean[9]. These values are so small compared with the magnitude The increasing Bering Strait oceanic heat flux is one of the F W C changes, measured in thousands of cubic of the main causes of the ice retreat in the Arctic Ocean. kilometers[9], that the net precipitation should be ne- Consequently, the ice retreat occurs predominantly in glected in our F W C anomaly analysis. In addition, we the region influenced by this inflow. Heat flux increased also neglected the influence of river runoff changes on the from 2001 to a maximum in 2007, 5×1020–6×1020 J·a−1, F W C variability. This decision is supported by the fig- which was twice the 2001 heat flux and enough to melt a ures of Aagaard and Carmack[3], who demonstrated that third of the total seasonal Arctic sea ice melt of 2007[12]. the total runoff from the major rivers entering the Arctic In response to increasing heat transport, sea ice extent di- Ocean was about 3 800 km3·a−1, with an annual change minished significantly in 2003–2008. This accelerated sea varying from –200 km3 to +200 km3. Furthermore, the ice retreat enhanced consequently the upper ocean F W C Mackenzie is the largest river emptying into the Canada in the Canada Basin. This process was especially pro- Basin, and its annual mean runoff is only about 340 km3, nounced in 2007 and 2008, when sea ice extent reached with an interannual variability 5%–20%[3]. a record low for summer (Figures 7e, 7f), causing the Thus we can determine that the impacts of Bering upper ocean F W C to be substantially higher than that Strait inflow, net precipitation and runoff are too small to in previous years. The anomalous increase in the sea ice cause the observed F W C variability. The only remain- extent (Figure 7d) is a plausible explanation for the over- ing candidate responsible for the rapid increase in F W C all F W C decrease of 2006. However, Figures 7a and 7c seems to be sea ice melt. Indeed, the sea ice volume demonstrate that the upper ocean freshwater anomalies anomalies range from –2 500 km3 to +2 500 km3, which and sea ice volume anomalies were not in phase in 2003 would surely have a significant impact on changes in up- and 2005. We attribute this lack of uniformity to the fact per ocean freshwater storage[5]. However, as McPhee et that the AO projected differently on the upper ocean in al.[10] suggested, an 11 m change of the Arctic Ocean these two years, as explained in the next section. Summer freshwater content variability of the upper ocean in the Canada Basin 161

Figure 7 Sea ice concentration for 2003–2008. a. 2003; b. 2004; c. 2005; d. 2006; e. 2007; f. 2008.

2.2.2 Arctic oscillation changes Arctic Ocean, and at least twice larger than the amount of fresh water stored in the sea ice[3]. With Proshutin- Proshutinsky et al.[4] proposed a mechanism for the ac- sky’s mechanism, the F W C annual changes are closely cumulation and release of freshwater in the Arctic Ocean associated with freshwater transport caused by atmos- that is centered on the BG, and is related to the tem- pheric conditions. poral variability of its freshwater content. The amount The AO has alternated between positive and neg- of freshwater calculated relative to the salinity 34.8 in ative phases, with no particular periodicity. Starting in the Canada Basin was about 45 000 km3, which was 10– the 1970s, the oscillation tended to remain in the positive 15 times larger than the total annual river runoff to the phase, causing lower than normal Arctic air pressure. 162 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3

Polyakov et al.[9] found that over the twentieth century In 2007, the sea ice cover reached a record low, and the the central Arctic Ocean became increasingly saltier with impact of sea ice melt was too great to be offset by the a freshwater loss rate of 239§270 km3 per decade. In positive phase of the AO. As a result, the F W C exhib- the new century, however, the negative phase of the AO ited a significant increase in the basin. A strong nega- started to dominate (Figure 8) and inducing Arctic at- tive phase of the AO in 2008 generated substantial mass mospheric conditions opposite to those of the positive transport of fresher surface water toward the center of phase. According to the hypothesis of Proshutinsky et the anticyclonic gyre, resulting in downward freshwater al, the Canada Basin releases freshwater during the pos- motion via Ekman pumping. In the same year, the sea ice itive phase and hence the upper ocean F W C in 2003 extent was the second lowest ever. The combination of was less than normal, despite the relatively heavy sea ice these two factors brought about the maximum F W C in melt. Alternatively, in 2005, a dominant negative phase the Canada Basin. In addition, due to the strong Ekman of the AO caused persistent freshwater accumulation in pumping induced by anticyclonic circulation during the the basin, which even offset the impact of reduced sea negative phase of the AO, freshwater accumulated in the ice melt. However, the AO index returned to a posi- upper ocean affected the stable water below the wBSW tive phase in 2006 and, coupled with less sea ice melt, through strong mixing, causing an abnormal increase in this produced a slight F W C decline in most of the area. F W C in these layers.

Figure 8 AO index time series for 2003–2008.

3 Summary ing. In 2003–2008, the F W C of the upper ocean in the Canada Basin increased significantly except in 2006, re- We analyzed inter-annual variability of the F W C of the flecting a tendency toward rapid freshening. The F W C − upper ocean in the Canada Basin, based on data from increased by more than 1 m·a 1 in the southern basin. the 2nd and 3rd CHINARE-Arctic of 2003 and 2008 and In the exceptional year of 2006, F W C in the basin exhib- the Canadian LSSL cruises from 2004 to 2007. We also ited an overall decrease, but there was a slight increase examined major contributors to the F W C variability in in the southeast. the upper ocean, such as sea ice concentration and AO Given their small magnitudes and annual variations, changes. We also touched briefly on the impact of Bering Bering Strait inflow, net precipitation and river runoff Strait inflow, net precipitation and river runoff on the have little apparent impact on the F W C variability. F W C variability. In summary, we conclude the follow- Our analysis shows that rapid sea ice melt, caused Summer freshwater content variability of the upper ocean in the Canada Basin 163

by global warming, was one of the major contributors to of the Arctic and Subarctic freshwater cycle. Science, 2006, the F W C increase over the study period. In 2007 and 313, 1061–1066 2008, the sea ice extent reached record lows. In response, 8 Steele M, Ermold W. Steric sea level change in the Northern Seas. J Climate, 2007, 20:403–417 the upper ocean F W C in these two years was higher than 9 Polyakov I V, Alexeev V A, Belchansky G I, et al. Arctic that in previous years. Ocean Freshwater Changes over the Past 100 Yeas and Their AO variability was another major control on the up- Causes. J Climate, 2007, 21:364–384 per ocean F W C in the Canada Basin. During the posi- 10 McPhee M G, Proshutinsky A, Morison J H, et al. Rapid tive phase of the AO, the Canada Basin released fresh- change in freshwater content of the Arctic Ocean. Geophys water under cyclonic wind forcing, and vice versa. A Res Lett, 2009, 36, L10602, doi:10.1029/ 2009GL 037525 11 Swift J H, Jones E P, Aagaard K, et a1. Waters of the weakening of the dominant positive phase of the AO, Makarov and Canada basins. DeepSea Res, Part II,1997, beginning with the 21st century, induced the freshwa- 44, 1503–1529 ter increase in the basin. In 2003, the freshwater release 12 Woodgate R A, Weingartner T, Lindsay R. The 2007 caused by positive phase of the AO offset the impact of a Bering Strait oceanic heat flux and anomalous Arctic relatively large sea ice melt. In contrast, a freshwater in- sea-ice retreat, Geophys Res Lett, 2010, 37, L01602, doi:10.1029/2009GL 041621 crease triggered by a rapid sea ice retreat in 2007 was too 13 Guay C K, Falkner K K. Barium as a tracer of Arctic halo- strong to be offset by the effect of the dominant positive cline and river watersDeepSea Res, Part II, 1997, 44,1543– phase of the AO. 1569 14 Shi J X, Zhao J P, Jiao Y T, et al. Pacific inflow and its links with abnormal variations in the Arctic Ocean (In Chi- nese with English abstract). Chin J Polar Res, 2004, 16(3): Acknowledgments The authors wish to thank the 2nd and 3rd 253–260 CHINARE-Arctic and the Canadian LSSL JOIS cruises from 2004 15 Shi J X, Zhao J P. Advances in Studies on the Arctic Halo- to 2007 for providing CTD data for our analysis. We are also grate- cline (In Chinese with English abstract). Adv Earth Sci, ful to the National Snow and Ice Data Center (NSIDC) for sea ice 2003,18(3): 351–357 concentration data, and to NOAA/CPC for AO index data. This work was supported by the National Natural Science Foundation 16 Shi J X, Cao Y, Zhao J P, et al. Distribution of Pacific- of China (Grant nos. 40631006, 40976111) and the China’s Pro- origin water in the region of the Chukchi Plateau in the gram for New Century Excellent Talents in University (Grant no. Arctic Ocean in the summer of 2003. Acta Oceanol Sin, NCET-10-0720). 2005, 24(6): 12–14 17 Smith D M. Recent increase in the length of the melt sea- son of perennial arctic sea ice. Geophys Res Lett,1998,25: References 655–658

1 Dickson R R, Osborn T J, Hurrell J W, et al. The Arctic 18 Belchansky G I, Douglas D C, Platonov N G. Duration of Ocean response to the North Atlantic Oscillation. J Cli- the Arctic sea ice melt season: Regional and interannual mate, 2000, 13:2671–2696 variability, 1979–2001. J Climate, 2004, 17: 67–80 2 Mauritzen C, Ha¨kkinen S. Influence of the sea ice on the 19 Rigor I G, Wallace J M. Variation in the age of Arctic sea- thermohaline circulation in the North Atlantic Ocean. Geo- ice and summer sea-ice extent. Geophys Res Lett, 2004, 31, phys Res Lett, 1997, 24: 3257–3260 L09401, doi:10.1029/2004GL019492 3 Aagaard K, Carmack E C. The role of sea ice and other fresh 20 Belchansky G I, Douglas D C, Platonov N G. Spatial water in the Arctic circulation. Geophys Res, 1989, 94(14): and temporal variations in the age structure of Arctic sea 485–498 ice. Geophys Res Lett, 2005, 32, L18504, doi:10.1029/2005 4 Proshutinsky A, Bourke R H, McLaughlin F A. The role of GL023976 the Beaufort Gyre in Arctic climate variability: Seasonal to 21 Belchansky G I, Douglas D C, Platonov N G. Fluctuating decadal climate scales. Geophys Res Lett, 2002, 29, 2100, Arctic Sea Ice Thickness Changes Estimated by an In Situ doi:10.1029/2002GL015847 Learned and Empirically Forced Neural Network Model. J 5 H¨akkinen S, Proshutinsky A. Freshwater content variability Climate, 2008, 21: 716–729 in the Arctic Ocean. Geophys Res, 2004, 109, C03051, doi: 22 Serreze M C, Maslanik J A, Scambos T A, et al. A record 10.1029/2003JC001940 minimum arctic sea ice extent and area in 2002. Geophys 6 Swift J H, Aagaard K, Timokhov L, et al. Long term vari- Res Lett, 2003, 30, 1110, doi:10.1029/2002GL016406 ability of Arctic Ocean waters: Evidence from a reanalysis 23 Stroeve J C, Serreze M C, Fetterer F, et al. Tracking the of the EWG data set. Geophys Res, 2005, 110, C03012, Arctic’s shrinking ice cover: Another extreme September doi:10.1029/2004JC002312 minimum in 2004. Geophys Res Lett, 2005, 32, L04501, 7 Peterson B, McClelland J, Holmes M, et al. Acceleration doi:10.1029/2004GL021810 164 GUO GuiJun, et al. Adv Polar Sci September(2011) Vol.22 No.3

24 NSDIC. Weather and feedbacks lead to third-lowest extent. beaufortgyre/pdfs/FWC−BG−draft.pdf 2010. http://nsidc.org/arcticseaicenews/2010/100410.html 28 Proshutinsky A, Krishfield R, Timmermans M L, et al. 25 NSDIC. Global Sea Ice Extent and Concentration:What Beaufort Gyre freshwater reservoir: State and variabil- sensors on satellites are telling us about sea ice. 2010. ity from observations, J Geophys Res, 2009, 14, C00A10, http://nsidc.org/sotc/sea−ice.html doi:10.1029/2008JC005104 26 Proshutinsky A Y, Johnson M A. Two circulation regimes 29 Carmack E, McLaughlin F, Yamamoto-Kawai M, et al. of the wind-driven Arctic Ocean. J Geophys Res, 1997, 102 Freshwater storage in the Northern Ocean and the spe- (C6): 12493–12514 cial role of the Beaufort Gyre, in Arctic Subarctic Ocean 27 Krishfield R, Proshutinsky A. Freshwater Content in the Fluxes: Defining the Role of the Northern Seas in Climate, Beaufort Gyre: Data from Observations, Russian Cli- edited by R. R. Dickson et al., New York: Springer, 2008: matology and Models. 2004. http://www.whoi.edu/ 145–169 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00165 September 2011 Vol.22 No.3 165–174

Chemical composition of marine aerosols of the 26th Chinese National Antarctic Research Expedition

XU GuoJie1,2, CHEN LiQi1,2∗, ZHANG YuanHui1,2, WANG JianJun1,2, LI Wei1,2 & LIN Qi1,2

1Key Laboratory of Global Change and Marine-Atmospheric Chemistry, SOA, Xiamen 361005, China; 2Third Institute of Oceanography, SOA, Xiamen 361005, China

Received July 22, 2011; accepted August 16, 2011

Abstract The ionic compositions of aerosol samples collected during the 26th Chinese National Antarctic Re- − − − search Expedition were analyzed and the sources of ions were distinguished. Cl , Na+, SO2 , NO , and Mg2+ − 4 3 were the most abundant ionic components in the marine aerosols. Cl and Na+ contributed over 70% in the total ionic composition, indicating the sea salt is still the primary composition in marine aerosols, followed by the sulfate as the secondary ionic component existed as NH4NO3, NH4HSO4, (NH4)2SO4. The maximal sea ◦ salt concentrations were found at around 40 S and could be attributed to greater winds. The concentrations of methane sulfonic acid (MSA) appeared increasing trend from the low to high latitudes, possibly caused by lower temperature in air and higher marine biological processes in the marginal waters in Antarctica. The correlation − − and factor analyzes were used to investigate possible sources of these ions. Cl , Br , Na+, K+, Mg2+ and Ca2+ − − + had predominantly marine sources; while F , NO3 and NH4 had mostly anthropogenic sources; MSA had marine 2− biogenic sources. The concentrations of SO4 were influenced by both marine and anthropogenic sources. Keywords Soluble ions, MSA, Southern Ocean, 26th Chinese National Antarctic Research Expe- dition

Citation: Xu G J, Chen L Q, Zhang Y H, et al. Chemical composition of marine aerosols of the 26th Chinese National Antarctic Research Expedition. Adv Polar Sci, 2011, 22: 165–174, doi: 10.3724/SP.J.1085. 2011.00165

0 Introduction rine aerosols in polar areas[3−5]. During the first Chinese Marine aerosol is the major component of the global at- National Antarctic Research Expedition (CHINARE), mospheric aerosol, and the specific physical and chemical from November 1984 to March 1985, 31 marine aerosol properties of aerosols have a key role in atmospheric pro- samples from around the Pacific Ocean and Antarctic cesses and processing[1]. Aerosol is the main route of Peninsula area were analyzed by Ion Chromatography transferring marine species onto land, and a major chan- (IC)[3]. The results showed that the concentration of 2− nel through which terrestrial species are transported to non-sea-salt sulfate (nss-SO4 ) in the marine aerosol the oceans[2]. So an understanding of the composition rapidly decreased from the North Pacific Ocean to the and provenance of marine aerosol is important to evalu- South Pacific Ocean. The sea-air flux of sulfur was cal- ate the relative impact of anthropogenic and biogenic culated in the waters of the Antarctic Peninsula. To un- processes on the global biogeosphere and climate. derstand the latitudinal distribution of marine aerosols, There were many studies of the composition of ma- several aerosol samples were collected during the first

∗Corresponding author (email: [email protected]) journal.polar.gov.cn 166 XU GuoJie, et al. Adv Polar Sci September(2011) Vol.22 No.3

Chinese National Arctic Research Expedition cles determine the cloud albedo, which can influence the (CHINARE-Arctic, between July 1 and September 9, global climate greatly[12]. Precipitation samples from [4] + + + 2+ 2+ − − 1999) , Na , NH4 , K , Mg , Ca , Cl , NO3 the 12th CHINARE cruise showed that MSA exhibited 2− and SO4 were determined using IC. During the 2nd an increasing trend from Tropic Ocean to Sub-Antarctic CHINARE-Arctic (between July 15 and September 28, Ocean, with two abruptly increasing areas corresponding 2003), aerosol samples were also collected[5], and the to around 15◦S and 60◦S–65◦S, respectively[13]. These + + + 2+ 2+ − 2− concentrations of Na , NH4 , K , Mg , Ca , Cl , coincided with areas of upwelling MSA/nssSO4 ratio 2− − 2− − MSA, SO4 , NO3 , C2O4 and CH3COO were deter- was found to be sensitive to the temperature change, mined and the distribution and potential sources were which is shown by δD values of the samples and the discussed. The results showed that sodium and chloride simultaneous temperature records. Generally speaking, ions were the dominant species in marine aerosols, ac- the MSA research in the atmosphere of remote open counted for 60.2% of the total aerosol loading, followed Oceans and Antarctic area is relatively sparse in China. by sulfate. This work discusses the composition and prove- To understand the global sulfur cycle, and better nance of ionic components of high volume (hi-vol) ma- quantify the atmospheric sulfur fluxes, the effects of the rine aerosol samples along the cruise track of the 26th main marine and terrestrial sources of sulfur (dimethyl CHINARE, from October, 2009 to April, 2010. There is 2− sulfide (DMS) and hydrogen sulfide (H2S) respectively) a focus on MSA and nss-SO4 . on the aerosol have been a matter of keen interest. In particular, methane sulfonicacid (MSA) and DMS pre- 1 Sampling and methods cursors have been the subject of many measurements, because MSA is found in precipitation and in aerosol The 26th CHINARE started from Shanghai on October samples. DMS is formed in the oceans by the photo- 11, 2009, and returned to Shanghai on April 15, 2010. chemical and enzymic cleavage of Dimethyl sulfonium The cruise was approximately through Christchurch to propionate (DMSP), a phytoplanktonic metabolite, into Great Wall Station, to Ushuaia-Zhongshan Station to DMS and acrylic acid. DMS is very insoluble in seawa- Casey Station to Melbourne to Zhongshan Station to Fre- ter, and is released quickly from the sea surface to form mantle to Shanghai, the detailed cruise track and sam- atmospheric DMS. This DMS subsists in the atmosphere pling points are presented in Figure 1. for about 24 h before it is oxidized by either the OH 48 aerosol samples were collected by using a high- or IO radical to form MSA and SO2 (the ratio being volume sampler M400 (made in USA), and Whatman 41 dependent on temperature, and degree of anthropogenic filters (20.3 cm×25.4 cm, Whatman International Ltd, influence). Because MSA is one of the most water solu- England), see Table 1. The filters were placed on the up- ble compounds in the atmosphere, it is quickly scavenged per deck of M/V Xuelong about 25 m above the sea sur- onto aerosol particles. The final proportions in remote face. The sampling head was also covered by a cylindri- non-polar areas is about 85% MSA and 15% sulfate[6]. cal polyethylene hood to protect from sea surf or rainfall. MSA was first measured using IC by Saltzman et During the cruise, most of major samples were collected al.[7] in marine aerosol. According to many observations, in 3 d. The sampling date, latitude and longitude, air MSA was ubiquitous in Marine Boundary Layer (MBL) temperature, pressure and other meteorological parame- and even in coastal cities[8]. Some research showed that ters were recorded at the start and end of the sampling MSA concentrations were closely related to acidic pre- periods. After sampling, filters were removed from the cipitation, and global climate change[9−10]. The flux filter holder with disposable gloves and stored in cleaned of marine sulfur into the atmosphere has a great im- airtight plastic containers. After labeling, the samples ◦ pact on acid precipitation formation processes and the were stored in the refrigerator at 4 C aboard the M/V [11] 2− oxidizing ability of the atmosphere . SO4 particles Xuelong. To avoid contamination by the products of fuel produced by the oxidation of reduced sulfur compounds combustion from the vessel’s generator set etc, the are the main source of the cloud condensation nuclei sampling system was controlled with a wind speed and (CCN) in the remote marine atmosphere. These parti- direction system which only allowed sampling when the Chemical composition of marine aerosols of the 26th CHINARE cruise 167

Figure 1 Cruise track of the 26th CHINARE with sampling points. The solid line represents the cruise track. Symbols represent the start of sampling periods; the triangles on the outbound journey and the squares on the inbound journey. wind was from a sector between left and right 90◦ down ions of aerosol samples are presented in Figures 2 and the middle line of the ship’s head and at wind velocities 3. The concentration sequences for various ions were: · −1 3 − + 2− 2+ − + 2+ + >0.2 m s . Each filter sampled about 5 000 m of air Cl >Na > SO4 >Mg >NO3 >NH4 >Ca >K at a flow rate about 1 m3·min−1. >MSA>Br−>F−. The average concentrations for major A Dionex ICS-2500 ion chromatograph was used to water soluble ions were: Na+ 1 136.37 ng·m−3, Mg2+ · −3 + · −3 2+ · −3 analyze the soluble aerosol ions. The cations were an- 148.32 ng m , NH4 91.33 ng m , Ca 75.34 ng m , + · −3 − · −3 2− alyzed with a CS12A analytical column and a CG12A K 45.44 ng m , Cl 1 999.36 ng m , SO4 587.76 · −3 − · −3 · −3 − guard column, and the anions were analyzed with an ng m , NO3 94.31 ng m , MSA 23.10 ng m , Br AS18 analytical column and an AG18 guard column. 4.83 ng·m−3, F− 1.56 ng·m−3. The major water sol- − + 2− 2+ − Experimental methods were as follows: One eighth uble ion constituents (Cl , Na , SO4 , Mg , NO3 ) of each filter was rinsed with 25 mL of deionized water, accounted for the 94.3% of the total aerosol loading. ultrasonicated for about 40 min and leached overnight. Sea salt particles were the primary marine aerosol con- Then samples were injected into the IC system via 0.45 stituents, followed by sulfate. µm filters. The time sequence for the elution of the The charge balance was calculated using equivalent + + + 2+ 2+ cations was Na , NH4 , K , Mg , Ca , and that for concentrations. The usual method was used, i.e., equiv- − − − − 2− · −3 anions was F , MSA, Cl , Br , NO3 , SO4 . The detec- alent concentration (neq m ) = chemical valence mass + · −3 + · −3 tion limits for above ions were Na 0.015 mg m , NH4 concentration(ng m )/molecular weight. Strong linear 0.009 mg·L−1, K+ 0.017 mg·L−1, Mg2+ 0.020 mg·L−1, correlations existed between the two electrically charged Ca2+ 0.046 mg·L−1, F− 0.000 9 mg·L−1, MSA 0.000 4 groups of anions and cations with a slope of 1.04, tending · −1 − · −1 − · −1 − 2 mg L , Cl 0.009 5 mg L , Br 0.000 5 mg L , NO3 to 1(R =0.992), see Figure 4. This suggests that a good · −1 2− · −1 0.006 9 mg L , SO4 0.003 2 mg L . To consider the charge balance existed between the two groups. There- Whatman 41 blank, ion concentrations from 7 blank fil- fore the sampling and measurement methods in the study ters were determined, and the average blank was (unit: were reliable[14]. · −1 + + + 2+ 2+ mg L ): Na 330, NH4 330, K 40, Mg 50, Ca From the results, it can be seen that sulfate − − − − 2− 100, F 10, MSA 30, Cl 690, Br 30, NO3 350, SO4 was an important aerosol component of the marine 190. These blank values have been subtracted from all atmosphere[15]. Previous work suggested that the sul- aerosol results. fate aerosol had both direct and indirect impacts on so- lar radiation. It also acted as the CCN source, which 2 Results and discussions had greatly affected the radiation balance and hence cli- mate change. For these reasons, scientists of atmospheric 2.1 Majorions constituents and formation chemistry have focused on sulfate aerosol. Generally sul- Concentrations and percentages of major water soluble fate in tropospheric aerosol can be divided into two parts. 168 XU GuoJie, et al. Adv Polar Sci September(2011) Vol.22 No.3

Table 1 Sampling dates of aerosol samples of the 26th CHINARE cruise

Sample number Sampling time Sample number Sampling time A1 2009/10/12 02:23–10/15 01:37 A25 2009/12/25 22:08–12/28 14:10 A2 2009/10/15 01:42–10/18 02:05 A26 2009/12/28 14:18–12/31 05:12 A3 2009/10/18 02:09–10/23 01:51 A27 2009/12/31 05:18–2010/01/04 13:15 A4 2009/10/23 01:56–10/26 01:57 A28 2010/01/04 13:20–01/07 16:45 A5 2009/10/26 02:03–10/28 10:58 A29 2010/01/07 16:50–01/10 11:16 A6 2009/10/28 11:02–10/30 21:06 A30 2010/01/10 11:16–01/12 04:50 A7 2009/10/30 21:11–11/02 22:02 A31 2010/01/12 04:50–01/15 04:38 A8 2009/11/02 22:07–11/05 23:23 A32 2010/01/15 04:43–01/17 13:10 A9 2009/11/05 23:30–11/09 00:55 A33 2010/01/17 13:17–01/20 11:10 A10 2009/11/09 01:00–11/11 23:35 A34 2010/01/20 11:16–01/24 04:09 A11 2009/11/11 23:40–11/15 09:57 A35 2010/01/24 04:13–01/27 18:55 A12 2009/11/15 10:01–11/17 02:50 A36 2010/02/03 07:00–02/07 10:50 A13 2009/11/17 02:55–11/20 22:40 A37 2010/02/07 10:55–02/11 06:43 A14 2009/11/20 22:45–11/23 22:33 A38 2010/02/11 06:48–02/14 07:30 A15 2009/11/23 22:46–11/26 18:42 A39 2010/02/14 07:34–02/19 08:45 A16 2009/11/26 18:45–11/29 18:08 A40 2010/03/06 03:14–03/09 07:15 A17 2009/11/29 18:12–12/02 18:12 A41 2010/03/09 07:15–03/13 01:33 A18 2009/12/02 18:16–12/05 11:35 A42 2010/03/13 01:33–03/16 01:45 A19 2009/12/05 11:40–12/08 14:08 A43 2010/03/16 01:45–03/24 04:12 A20 2009/12/08 14:12–12/11 18:34 A44 2010/03/24 04:12–03/27 08:15 A21 2009/12/11 18:50–12/15 08:30 A45 2010/03/27 08:15–03/30 02:30 A22 2009/12/15 08:33–12/19 03:26 A46 2010/03/30 02:35–04/02 09:55 A23 2009/12/19 03:31–12/22 05:10 A47 2010/04/02 10:00–04/05 10:20 A24 2009/12/22 05:15–12/25 22:02 A48 2010/04/05 10:20–04/07 06:15

2− − One part sourced from sea salt, namely sea-salt SO4 (ss- NO3 is mostly produced by the chemical equation (1), 2− SO4 ), and the other part sourced from the emissions of and the reaction (4) occurs on the sea salt surface and can − + anthropogenic and biogenic activities, called non-sea-salt absorb additional NO3 . NH4 is produced by reactions 2− 2− SO4 (nss-SO4 ). Based on assumptions of the compo- (1), (2), (3). sition of seawater, − − → 2 2 − + × HNO3 + NaCl NaNO3 + HCl. (4) nss-SO4 =[SO4 ]Total [Na ] 0.2516 2− + where 0.251 6 is the value of SO4 /Na in seawater. According to the correlations between various wa- ter soluble ions (see Table 3), the secondary aerosol ions: − + 2− NO3 , NH4 and nss-SO4 had a significant positive cor- relation each other(Pearson Correlation Coefficient>0.5, + and p <0.01), which accounted for the NH4 existing as NH4NO3, NH4HSO4 and NH4(SO4)2. Several studies have shown that the likely chemical scheme of reactions + [16−17] to produce NH4 is as follows :

NH3(g) + HNO3(g) → NH4NO3 (1) → NH3 + H2SO4 NH4HSO4 (2) Figure 2 Major ions in marine aerosol along the Xuelong’s

NH3 + (NH4)HSO4 → (NH4)2SO4 (3) route during the 26th CHINARE. Chemical composition of marine aerosols of the 26th CHINARE cruise 169

that all of the Na+ and Cl− in the aerosol is sourced from the sea water, and all non-sea-salt components of K+, 2+ 2+ 2− − [18] Mg , Ca , SO4 and HCO3 have been excluded . Figure 5(a) shows the latitudinal distribution of sea salt on the route of the 26th CHINARE, and the high val- ues were found south of 40◦S. Some previous work[19] reports that Na+ and Cl− are mainly sourced from the sea, and their concentrations are controlled by the wind speed over the sea surface. In this study, the sea salt con- centration was also affected by the wind speed. In the south of 40◦S, which was close to the latitudes of pre- vailing westerly winds. Both wind-speed and waves were Figure 3 The pie map for major soluble ions in TSP during high and sea spray, which was enriched with sea salt, was the 26th CHINARE cruise. emitted into the atmosphere. So the general tendency of + sea salt concentrations was high. As for NH4 , shown in Figure 5(b), the general concentrations were about 50–100 ng·m−3, and high values(295.4 ng·m−3) occurred over the East China Sea. This sample was collected at the end of spring, 2010, when Western and Northern China were experiencing severe sandstorms, so the dust could be transported long distances to the Chinese marginal seas. Therefore, a series of atmospheric photochemical reactions occurred on the surface of the particles and produced high concentrations of secondary aerosols. The 2− − Figure 4 The charge balance between total anions and concentrations of SO4 and NO3 were also high, demon- cations during the 26th CHINARE cruise. strating that chemical reactions (1), (2) and (3) took place on the aerosols surface. Hence these samples were In addition, the concentrations of Na+, K+, Mg2+ cor- − − very altered by the influence of the transported terrestrial related obviously with Cl , Br (Pearson Correlation − substances. The latitudinal distribution of NO3 is shown Coefficient>0.6, p <0.01) from Table 3. This may in- − in Figure 5(c). The tendency of the NO is clearly simi- dicate that the ions were present as NaCl, KCl, MgCl , 3 2 + lar to that of NH4 , and high values also occurred in the NaBr, KBr, MgBr2 and so on. This would be similar to samples collected in Chinese marginal seas, Fremantle the components in sea water. port and the offshore of Philippines. These high concen- 2.2 The latitudinal distribution characteristics of trations were inevitably affected by the emissions from various components anthropogenic activities, and chemical reactions (1), (4) To examine the distribution of aerosol ions along the happened on the surface of particles with the participa- route of 26th CHINARE, latitudinal distributions of tion of HNO3. And chemical reaction (4) may the main − aerosol ions are shown in Figure 5. The sea salt concen- process of NO3 production in the remote ocean. tration was used to represent the concentration of marine The MSA in marine aerosols was produced by a se- aerosol sea salt particles, and the sea salt was calculated ries of oxidation reaction of DMS, and the latitudinal from: distribution is shown in Figure 5(d). According to the − − − distance of M/V Xuelong away from the continent and Sea Salt(ng · m 3) = Cl (ng · m 3) + − the specific characteristics of different ocean areas, the Na+(ng · m 3) × 1.47 voyage track could be divided into 3 parts, i.e. Shanghai Where 1.47 is the value of (Na++K++Mg2++ to Fremantle (32◦S–30◦S), Indian Ocean (30◦S–60◦S) 2+ 2− − + Ca +SO4 +HCO3 )/Na in sea water. This assumes and the Southern Ocean-Antarctic floating ice region 170 XU GuoJie, et al. Adv Polar Sci September(2011) Vol.22 No.3

+ − 2− Figure 5 Latitudinal distributions of aerosol ions concentration.(a) sea salt, (b) NH4 , (c) NO3 , (d) MSA and (e)nss-SO4 .

◦ [3] 2− (south of 60 S). The MSA concentration increased It has been reported that nss-SO4 concentrations de- 2− sharply from north to south in these three regions. That crease from north to south, and the nss-SO4 concentra- is, as the latitude increased, the MSA concentration in- tion in the West Pacific Ocean was twice as high as that in ◦ 2− creased, especially in the south of 60 S, where the con- South Pacific Ocean, while the nss-SO4 concentration in centration of MSA was three times higher than that South Pacific Ocean was twice as high as that in Antarc- in 30◦S–60◦S region. This could be explained by the tic Peninsula region. In the Northern Hemisphere, the air temperature and related algal emissions in Southern non-sea-salt sulfate accounted for 59% of the total sul- Ocean[12,20]. fate, while in Pacific Ocean that was 39%, and in the seas 2− The concentrations of nss-SO4 was low in the Open around Antarctica it was 28%. This study suggested that Oceans and the seas around Antarctica, while the con- pollution accounted for 90% of the source of sulfur to the centrations were high in the marginal seas (Figure 5(e)). atmosphere in the Northern Hemisphere, probably from Chemical composition of marine aerosols of the 26th CHINARE cruise 171

the combustion of fossil fuel which emitted SO2, which is not uniformly distributed, anthropogenic sources mostly 2− oxidized into SO4 in the atmosphere. In the Northern focused on marginal ocean, while the marine biogenic Hemisphere, sulfate aerosol is transported over the seas sources mostly focused on Southern Hemisphere (espe- by winds, and this gives a large contribution to the nss- cially Southern Ocean and the seas around Antarctica). 2− SO4 . By way of contrast, in the Southern Hemisphere Meanwhile the complicated air conditions and solar ra- the sea area spanned vastly greater distances than in the diation change together made the process of identifying North, not only across marginal oceans, but also across different air masses, and hence different sources of nss- 2−[28] the open ocean. Therefore, the average concentrations SO4 rather complex. 2− 2− of nss-SO4 during the whole 26th CHINARE was rela- The correlation between MSA and MSA/nss-SO4 tively higher than that in Open Oceans, but the value was was significant, and the Pearson correlation coefficient lower than that in Chinese marginal seas. Comparisons was 0.780, p <0.01. Our results indicate a good lin- 2− 2− of the marine aerosol nss-SO4 and MSA ions concentra- ear relationship between the MSA/nss-SO4 ratio and tion over the various concerned regions appears in Table MSA, R2 >0.6, see Figure 6. As the concentration of [21−27] 2− 2 . The results consistently show that the nss-SO4 MSA increased, the contribution of biogenic sulfur to 2− concentrations were high in the Chinese marginal seas, total nss-SO4 also increased. To some degree, as the 2− while the nss-SO4 concentrations were low in South- only precursor of MSA, the concentration and the oxida- 2− ern Ocean and in the seas around Antarctica. This tion processes of DMS determined the MSA/nss-SO4 . 2− [29] indicated that the nss-SO4 concentration was mostly Cosme et al. studied the aerosol sulfate over Dumont controlled by the distance away from the anthropogenic d’Urville station in East Antarctica and the inland Vos- sources; According to Table 2 and this study, the MSA tok Station. The contribution of DMS to the sulfate in concentration of marine aerosol in the Southern Ocean aerosols in that study was more than 90% in summer. and the seas around Antarctica were relatively high, ma- Contributions from anthropogenic and volcanic emissions 2− rine organisms contributed much to the nss-SO4 and contributed less than 3% to the sulfate, so the DMS con- 2− MSA concentrations in those regions. That is to say, in tributed mostly of the nss-SO4 in Antarctic area. 2− marginal seas, the concentration of nss-SO4 was high, In Figure 7, the correlation between the concentra- while the concentration of MSA was low. Hence, the tion of MSA and air temperature showed that the con- 2− contribution of biogenic sources to nss-SO4 was little, centration of MSA decreased when air temperature rose. 2− the main part of the nss-SO4 being sourced from an- There was a significant negative correlation between the thropogenic emissions. However, the concentrations of concentration of MSA and air temperature, and the Pear- 2− MSA were high, while the concentration of nss-SO4 was son correlation coefficient was –0.462, p <0.01. The lab 2− low in the Antarctic coastal area. Here, the nss-SO4 simulation showed that the rate of the abstraction re- to some extent may be formed by the oxidation of bio- action between DMS and OH was the function of air 2− genic sulfur. To sum up, the source of nss-SO4 was temperature, the rate of the addition reaction between

2− Table 2 Comparison of the nss-SO4 and MSA concentrations in marine aerosols for the various concerned regions 2− · −3 · −3 Area Location nss-SO4 /(µg m ) MSA/(µg m ) Reference East China Sea 26◦N–34◦N, 122◦E–130◦E 8.7§6.8 (0.96–24) 0.027§0.020 (N.D.-0.068) Nakamura T et al.[21] South China Sea 21◦N–23◦N, 113◦E–115◦E 14.6(3.9–29.8) 0.006–0.102 Zhang X et al.[22] New Zealand 41.42◦S, 174.88◦E 0.246 (0–0.96) 0.024 (0–0.162) Allen A G et al.[23] Southern Ocean 60◦S 0.02–0.032 0.018–0.037 Berresheim[24] (March-April) West Antarctica 64.77◦S, 64.05◦W 0.0985§0.121 0.0488§0.0892 Savoie D L et al.[25] (King George Island) East Antarctica 78◦S, 139◦E 0.021 (0.09–0.041) 0.088 (0.065–0.13) Mora S J de et al.[26] Mawson Station 67.6◦S, 62.6◦E 0.09 0.02 Savoie D L et al.[27] 172 XU GuoJie, et al. Adv Polar Sci September(2011) Vol.22 No.3

MSA[31−33]. But the especially high value of MSA in the 60◦S sea area deserves closer study. The other sources may derive from the oxidation of DMS in Southern Ocean and the coastal area of Antarctica should be studied further. 2.3 The source analysis of components As discussed above, marine aerosol ions generally can be divided into sea salt and non-sea-salt components. The correlations between aerosol ions in Table 3 showed that: 1. Sea salt components Na+, K+, Mg2+, Cl−, Br− Figure 6 The relationship between the concentration of had obvious correlations (Pearson value >0.6, p <0.01), 2− which represented the common characteristics of sea salt MSA and MSA/nss-SO4 . ions. 2− − + 2. Secondary aerosol ions nss-SO4 , NO3 , NH4 had obvious positive correlations each other (Pearson value >0.5, p <0.01), which demonstrated that these three ions had the similar sources. Factor analysis is a multiple statistical analysis method, it was used to explore the correlations among observed variables and potential major sources of chemi- cal species. The factor score indicated the relative impact of the factor on sample compositions. In this study, fac- tor analysis was carried out on 48 samples for 11 water soluble ions with measurable concentrations, and repre- Figure 7 Latitudinal distribution of air temperature and sented the various sources of aerosols. Three factors ex- MSA. (N represents air temperature, ¤ represents the con- plaining 91.3% of the variance in the data were obtained centration of MSA). and rotated according to the orthogonal varimax method. + 2+ 2+ + − 2− − the two increased as the air temperature fell. Na , Mg , Ca , K , Cl , SO4 , Br had a high Fac- + 2− − − At 300 K, about 25% of DMS reacted with OH, while tor 1 score; NH4 , SO4 , NO3 , F had a high Factor 2 at 250 K, 75% of DMS reacted with OH[30], so the concen- score; MSA had a high Factor 3 score. According to the tration of MSA in part depended on the air temperature discussion above, it could be concluded that Factor change, and the lower air temperature may produce more 1 may be concerned with marine sources, Factor 2

Table 3 Correlations between soluble ions concentration during the 26th CHINARE cruise

+ + + 2+ − − − − 2− 2+ Ions Na NH4 K Mg F MSA Cl Br NO3 nss-SO4 Ca Na+ 1 + NH4 –0.118 1 K+ 0.981∗ –0.015 1 Mg2+ 0.994∗ –0.079 0.980∗ 1 F− –0.253 0.691∗ –0.113 –0.218 1 MSA –0.271 0.215 –0.316 3 –0.277 –0.067 1 Cl− 0.997∗ –0.126 0.967∗ 0.988∗ –0.304 –0.274 1 Br− 0.975∗ –0.148 0.940∗ 0.965 1∗ –0.272 –0.264 0.977∗ 1 − ∗ ∗ NO3 0.076 0.511 0.246 0.116 0.669 –0.267 0.003 6 –0.032 1 2− ∗ ∗ ∗ nss-SO4 0.047 0.713 0.180 0.098 0.782 –0.050 –0.030 0.006 0.757 1 Ca2+ 0.707∗ 0.086 0.732∗ 0.734∗ 0.099 –0.311 0.669∗ 0.665∗ 0.467∗ 0.410∗ 1 ∗represents the obvious correlation under the 0.01 confidence interval (two-tailed test). Chemical composition of marine aerosols of the 26th CHINARE cruise 173 may be concerned with anthropogenic sources, and Fac- erally influenced by the distance from the continents and 2− tor 3 may be concerned with the emission from organic the anthropogenic activities. The nss-SO4 had low con- marine sources. centrations in open ocean and Southern Ocean area and To sum up, the water soluble ions of aerosols col- high concentrations in the marginal sea. lected on the route of the 26th CHINARE could be as- 3. Cl−, Br−, Na+, K+, Mg2+ and Ca2+ came mainly − − + cribed to marine sources, anthropogenic sources and ma- from marine sources; F , NO3 and NH4 were mostly 2− rine biogenic sources. The SO4 can be impacted both from anthropogenic sources; MSA was mostly from ma- 2− by Factor 1 and Factor 2, this demonstrated the impor- rine biogenic sources; The concentration of SO4 was 2− 2 tance of separating the ss-SO4 and nss-SO4. influenced both by marine and anthropogenic sources. − − Therefore separating ss-SO2 and nss-SO2 was impor- Table 4 Rotated component matrix of ions in aerosols 4 4 a tant for climate evaluation models. during the 26th CHINARE cruise

Ions Factor 1 Factor 2 Factor 3 Na+ 0.991 –0.053 –0.090 Acknowledgments This work was supported by the National Mg2+ 0.990 –0.005 –0.094 Natural Science Foundation of China (Grant no. 40671062), NH+ –0.016 0.842 0.406 4 the National High Technology Research Development Project 2+ Ca 0.738 0.347 –0.263 (Grant no. 2008AA121703), the Chinese International Coopera- K+ 0.971 0.102 –0.141 tion Project from the Ministry of Science and Technology(Grant Cl− 0.986 –0.112 –0.066 no. 2009DFA22920) and the Chinese International Cooperation 2− Project from the Chinese Arctic and Antarctic Administration, SO4 0.652 0.710 –0.047 − SOA (Grant no. IC201114) and the Special Fund for Marine Re- NO 0.080 0.856 –0.319 3 searches in the Public Interest(Grant no. 201205013). We also MSA –0.209 –0.051 0.933 thank all members of the 26th CHINARE for their supports and − Br 0.971 –0.115 –0.068 helps. F− –0.227 0.887 –0.058 Eigen values 6.111 2.843 1.089 References Variance /% 55.558 25.842 9.902 Accumulation /% 55.558 81.400 91.302 1 Fitzgerald J W. Marine aerosols: A review. Atmos Environ. Part A. General Topics, 1991, 25(3–4): 533–545 aValues of factor loading higher than 0.60 are in bold. 2 Duce R A, Liss P S, Merrill J T, et al. The atmospheric in- put of trace species to the world ocean. Global Biogeochem 3 Conclusions Cy, 1991, 5(3): 193–259 3 Chen L Q, Yang X L, Huang J H. The sources and the distri- Aerosol samples collected on the route of the 26th bution of sulfate in the Pacific atmosphere. Acta Scientiae CHINARE were analyzed by IC. The main results in- Circumstantiae(in Chinese), 1988, 8(1): 27–31 clude: 4 Sun J Y. Soluble species in aerosols collected on the route − + 2− − 2+ of the first Chinese National Article Research Expedition. 1. Cl , Na , SO4 , NO3 , and Mg were the most + − Journal of Glaciology and Geocryology(in Chinese), 2002, prevalent ion components, and Na and Cl contributed 24(6): 744–749 ∼ to 75% of the total ions. Sea salt is the primary com- 5 Xu J Z, Sun J Y, Qin D H. Characterization of solu- ponent of aerosols, and sulfate aerosol is the second ma- ble species and their sources in aerosols collected on the jor aerosol component. As a secondary aerosol ammo- route of the Second Chinese National Arctic Research Ex- pedition. Acta Scientiae Circumstantiae(in Chinese), 2007, nium existed in the forms of NH4NO3, NH4HSO4 and 27(9): 1417–1424 (NH ) SO . 4 2 4 6 Kim K H, Lee G, Kim Y P. Dimethylsulfide and its oxidation ◦ 2. The sea salt concentration peaked at 40 S, which products in coastal atmospheres of Cheju Island. Environ- may correlate with the wind speed. The concentration of mental Pollution, 2000, 110: 147–155 MSA increased from the low latitudes to high latitudes, 7 Saltzman E S, Savoie D L, Zika R G, et al. Methane sul- possibly caused by temperature change and algal emis- fonic acid in the marine atmosphere. J Geophys Res, 1983, 88(10): 897–910 sions. The concentrations and the oxidation processes 8 Pakkanen T A, Kerminen V M, Korhonen C H, et al. Ur- 2− of DMS determined the MSA/nss-SO4 ratio to some ban and rural ultrafine (PM0.1) particles in the Helsinki − + degree. The concentrations of NO3 and NH4 were gen- area. Atmos Environ, 2001, 35: 4593–4607 174 XU GuoJie, et al. Adv Polar Sci September(2011) Vol.22 No.3

9 Sasakawa M, Uematsu M. Relative contribution of chemi- 22 Zhang X, Zhuang G, Guo J, et al. Characterization of cal composition to acidification of sea fog (stratus) over the aerosol over the Northern South China Sea during two northern North Pacific and its marginal seas. Atmos Envi- cruises in 2003. Atmos Environ, 2007, 41(36): 7821–7836 ron, 2005, 39(7): 1357–1362 23 Allen A G, Dick A L, Davison B M. Sources of atmospheric 10 Andreae M O, Crutzen P J. Atmospheric Aerosols: Bio- methanesulphonate, non-sea-salt sulphate, nitrate and re- geochemical Sources and Role in Atmospheric Chemistry. lated species over the temperate South Pacific. Atmos Env- Science, 1997, 276: 1052–1058 iron, 1997, 31(2): 191–205 11 Doney S C, Mahowald N, Lima I, et al. Impact of anthro- 24 Berresheim H. Biogenic sulfur emissions from the sub- pogenic atmospheric nitrogen and sulfur deposition on ocean Antarctic and Antarctic oceans. J Geophy Res, 1987, acidification and the inorganic carbon system. PNAS, 2007, 92(13): 245 104: 14580–14585 25 Savoie D L, Prospero J M, Larsen R J, et al. Nitrogen 12 Charlson R J, Lovelock J E, Andreae M O, et al. Oceanic and sulfur species in Antarctic aerosols at Mawson, Palmer phytoplankton, atmospheric sulphur, cloud albedo and cli- Station, and Marsh (King George Island). J Atmos Chem, mate. Nature, 1987, 326(6114): 655–661 1993, 17(2): 95–122 13 Xiao C D, Sun J Y, Qin D H, et al. Latitudinal distribu- 26 Mora S Jde, Wylie D J, Dick A L. Methanesulphonate and tion of MSA in precipitation over the southern Pacific Ocean non-sea salt sulfate in aerosol, snow, and ice on the east and the sensitivityof its decomposition to temperature. Acta Antarctic plateau. Antarctic Science, 1997, 9 (1): 46–55 Oceanologica Sinica (Chinese Edition), 2001, 23(3): 112–116 27 Savoie D L, Prospero J M, Larsen R J, et al. Nitrogen and 14 Song F, Gao Y. Chemical characteristics of precipitation at sulfur species in aerosols at Mawson, Antarctica, and their metropolitan Newark in the US East Coast. Atmos Environ, relationship to natural radionuclides. J Atmos Chem, 1992, 2009, 43(32): 4903–4913 14: 181–204 15 Corinne L Q, Saltzman E S. Surface Ocean-Lower Atmo- 28 Savoie D L, Prospero J M. Comparison of oceanic and con- sphere Processes. Geophysical Monograph Series 187, Amer- tinental sources of non-sea-salt sulphate over the Pacific ican Geophysical Union, 2009: 17–35 Ocean. Nature, 1989, 339: 685–687 16 Seinfeld J H, Pandis S N. Atmospheric Chemistry and 29 Cosme E, Hourdin F, Genthon C, et al. Origin of dimethyl- Physics. New York: John Wiley, 1998: 313–323 sulfide, non-sea-salt sulfate, and methanesulfonic acid in 17 Dentener F J, Crutzen P J. A three-dimensional model of the eastern Antarctica. J Geophy Res, 2005, 110, D03302, doi: global ammonia cycle. J Atmos Chem, 1994, 19(4): 331–369 10.1029/2004JD004881 18 Quinn P K, Coffman D J, Bates T S, et al. Dominant aerosol chemical components and their contribution to extinction 30 Berresheim H, Eisele F L. Sulfur chemistry in the Antarctic during the Aerosols99 cruise across the Atlantic. J Geophys tropospheric environment: An overview of project SCATE. Res, 2001, 106(D18): 20783–20809 J Geophy Res, 1998, 103(D1): 1619–1627 19 Erickson D J, Merrill J T, Duce R A. Seasonal estimates of 31 Atkinson R, Pitts Jr J N, Aschmann S M. Tropospheric re- global atmospheric sea-salt distributions. J Geophys Res, actions of dimethyl sulfide with NO3 and OH radicals. J 1986, 91(D1): 1067–1072 Phys Chem, 1984, 88(8): 1584–1587 20 Grosjean D. Photooxidation of methyl sulfide, ethyl sulfide, 32 Grosjean D. Photooxidation of dimethyl sulfide, ethyl sul- and methanethiol. Environ Sci Technol, 1984, 18(6): 460– fide, and methanethiol. Environ Sci Technol, 1984, 18: 460– 468 468 21 Nakamura T, Matsumoto K, Uematsu M. Chemical charac- 33 Ayers G P, Cainey J M, Granek H, et al. Dimethylsulfide teristics of aerosols transported from Asia to the East China oxidation and the ratio of methanesulfonate to non-sea-salt Sea: An evaluation of anthropogenic combined nitrogen de- sulfate in the marine aerosol. J Atmos Chem, 1996, 25: position in autumn. Atmos Environ, 2005, 39(9): 1749–1758 307–325 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00175 September 2011 Vol.22 No.3 175–183

A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ magnetic latitude for solar minimum

HE Fang1∗, ZHANG BeiChen1, Joran Moen2 & HUANG DeHong1

1SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China; 2Department of Physics, University of Oslo, Norway

Received April 18, 2011; accepted August 25, 2011

Abstract Long-duration conjugate observations by the EISCAT Svalbard Radar (ESR) and the ionosonde at Zhongshan station from the International Polar Year (IPY) during solar minimum conditions are analyzed, with respect to variability in the F 2-layer peak parameters. A comparison between International Reference Ionosphere- 2007 (IRI-2007) and observation data clearly demonstrates good agreement in summer, but greater deviations in winter. The IRI model reproduces the F 2 peak parameters dominated by solar photoionization reasonably well, but it does not address the effect of electron precipitation. Hence, the discrepancies become large in the winter auroral ionosphere.

Keywords Solar minimum, polar ionosphere, IRI, electron precipitation

Citation: He F, Zhang B C, Joran M, et al. A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ magnetic latitude for solar minimum. Adv Polar Sci, 2011, 22: 175–183, doi: 10.3724/SP.J.1085.2011.00175

0 Introduction is an empirical model based on global observation data. The ground-based sounding is important for continu- Given geographic location, date and time, it provides ously monitoring ionosphere-magnetosphere coupling in altitude profiles of electron density, electron tempera- the polar region. During the 2007–2008 Interna- ture, ion density and ion temperature, from 50–1 500 km. tional Polar Year (IPY), observation instruments such Because of the complicated structure of the polar iono- as the European Incoherent Scatter Radar (EISCAT), sphere, non-uniform distribution of ionosonde stations, the Super Dual Auroral Radar Network (SuperDARN), and limited data coverage, comparative studies of the IRI and digital ionosondes were coordinated to investi- model and observation data have mainly focused on the gate the polar ionosphere. Efficient exchange of data mid-low latitude region[3−5]. There have been some vali- between the participants gave an excellent opportu- dation studies of the IRI model for polar regions, using nity to study the polar space environment. The EISCAT radar or satellite observations[6−9]. In general, joining of the Chinese to the EISCAT radar asso- however, there is better agreement with observations in ciation extends opportunities for continued conjugate mid and low latitudes than in high latitude polar cap re- observations. gions. As asserted by Moen et al.[9], the IRI model does The International Reference Ionosphere (IRI) not represent the transport of F-region plasma across model[1−2] is widely used in ionospheric research. IRI the polar cap from the EUV production region on the

∗Corresponding author (email: [email protected]) journal.polar.gov.cn 176 HE Fang, et al. Adv Polar Sci September(2011) Vol.22 No.3 dayside. time (MLT) at 10 universal time (UT) and 15 local time The motivation of the present paper is to validate (LT). Based on long-term observation, a series of studies the IRI-2007 model, using conjugate observations from focusing on the Antarctic ionosphere have been carried Longyearbyen, Svalbard and the Zhongshan stations dur- out by Chinese researchers[10−15]. For the conjugate ob- ing solar minimum conditions. We take advantage of the servation during the IPY, the ionosonde at Zhongshan IPY long run of the EISCAT Svalbard Radar (ESR) and station performed a long duration observation of 331 ef- the DPS-4 ionosonde at Zhongshan station, to compare fective days. The sampling rate was 5 min. diurnal and seasonal variations in the F 2 layer critical The geographic coordinates of the ESR are 78.15◦N frequency with the IRI-model. The paper is organized as and 16.05◦E, corresponding to 75.27◦ MLAT, which follows. Section 1 introduces the conjugate observation make the radar suitable for conjugate studies with the instruments and data in both hemispheres. Section 2 Zhongshan ionosonde. The ESR operated on 285 effec- configures the input parameters for the IRI-2007 model. tive days during the IPY. For ESR, we use 15 min av-

Section 3 presents the comparison of the conjugate ob- erages of the F 2 region maximum density (NmF 2) and servations and IRI model. In the discussion of Section 4, altitude hmF 2 diurnal profiles from the 42 m antenna. we try to explain the physical processes underlying the We selected simultaneous observations from both ESR model discrepancies. Finally, further work on IRI-2007 and the DPS-4 ionosonde on 122 days from 4 months is suggested in the conclusions. (one month from each season). The data periods are

shown in Table 1. NmF 2 data obtained from the ESR

1 Instruments were transformed to f0F 2 by

◦ ◦ e NmF 2 Zhongshan station, located at 69.4 S, 76.4 E in geo- f0F 2 = (1) ◦ 2π ε m graphic coordinates, corresponding to 77.2 magretic lat- r 0 e itude (MLAT), was equipped with a DPS-4 ionosonde in where e is the electron charge; ε0 is the vacuum permit-

1995. Zhongshan station crosses 12:00 magnetic local tivity; and me is the electron mass.

Table 1 Analysis dataset Spring Summer Autumn Winter

NmF 2 ESR 2007.3 2007.6 2007.9 2007.12  hmF 2  f0F 2 DPS-4 2007.9 2006.12 2007.3 2007.6  hmF 2 

2 IRI-2007 setup Figures 1 and 2; the variations of f0F 2 and hmF 2 from the DPS-4 ionosonde are shown in Figures 3 and 4. The The input parameters for IRI-2007 are given in Table 2. quartile difference is introduced as the error function, The monthly median values from the IRI model are gen- which indicates the median value of the data greater or erated by the same technique as the observation data, less than the median value of the entire data bin. which predicts 24 values at integral points for each day 3.1 Comparison of ESR observation and IRI in the month in which the monthly median value was model generated. Figure 1 compares the measured f0F 2 (from ESR) with 3 Observation results the IRI predicted value. Figure 1a shows f0F 2 diurnal variations measured by ESR in the spring equinox month Using the techniques introduced in the previous section, peaks around 4.2 MHz, occurring at 09 UT (10 LT/12 f0F 2 and hmF 2 diurnal variations of ESR are shown in MLT). This peak is higher than the IRI maximum of A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ 177

Table 2 IRI-2007 input parameters

Geographic coordinate Time period Height range/km 2007.3, 2007.6, ESR 78.15◦N, 16.03◦E 86–686 (step: 2 km) 2007.9, 2007.12 2007.3, 2007.6, DPS-4 69.40◦S, 76.40◦E 86–686 (step: 2 km) 2007.9, 2007.12

3.9 MHz at local noon. The measured f0F 2 hovers above very close to the IRI maximum, but delayed by 2 hours. the IRI model curve, with a larger discrepancy from the Notably, the discrepancy was smaller near the autumn daytime sector to the pre-midnight sector. For the au- equinox month than the spring equinox. Figure 1b shows tumn equinox month plotted in Figure 1c, a f0F 2 peak of the f0F 2 diurnal variation in summer month observed by 4 MHz occurred around 13 UT (14 LT/16 MLT), which is ESR, agrees well with the IRI prediction, except between

Figure 1 Diurnal variation of f0F 2 monthly median value of ESR observation and IRI prediction in different seasons (error bars indicate quartile difference). 178 HE Fang, et al. Adv Polar Sci September(2011) Vol.22 No.3

03–07 UT (04–08 LT/06–10 MLT) in the morning sector. spring and autumn. The minimum value of 230 km was

In winter, the f0F 2 diurnal variation observed by ESR reached near geographic noon, between 11–13 UT (12–14 is in anti-phase with the IRI prediction, which will be LT/14–16 MLT) in spring and 12–13 UT (13–14 LT/15– discussed in detail in Section 4. 16 MLT) in autumn. The average altitude in spring is

Figure 2 compares the measured hmF 2 (from ESR) higher than in autumn. There is another local minimum with predicted values. Figure 2b shows that the summer between 05–06 UT (06–07 LT/08–09 MLT), which is 15 hmF 2 observed by ESR remained at an almost constant km higher than the main valley. The IRI hmF 2 is system- altitude of 230 km between 11–16 UT (12–17 LT/14–19 atically higher than the observed value, and the largest MLT), and 250 km between 19–09 UT (20–10 LT/22– discrepancy occurs during the polar winter.

12 MLT). The diurnal variations of hmF 2 are similar in

Figure 2 Diurnal variation of hmF 2 monthly median values of ESR observation and IRI prediction in different seasons.

3.2 Comparison of DPS-4 observation at Zhong- imum of 5.3 MHz around 07 UT (12 LT/09 MLT), which is local noon, and a minimum of 4 MHz around 20 UT shan station and IRI model (01 LT/22 MLT). The measured f0F 2 hovers near the

Figure 3 compares the measured f0F 2 at Zhongshan with IRI model curve. The f0F 2 maximum value at Zhong- the IRI predicted values. Figure 3a shows that f0F 2 di- shan was 1.1 MHz higher than in Longyearbyen, and urnal variation in the summer at Zhongshan had a max- the diurnal variation scope at Zhongshan was greater, A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ 179

Figure 3 Diurnal variation of f0F 2 monthly median value from the DPS-4 at Zhongshan station and IRI prediction, in different seasons. because of the difference in geographic latitude of 8.8◦. There is another pronounced peak around 3 MHz, near

Figure 3d depicts f0F 2 diurnal variations at Zhong- 15 MLT. As will be discussed later, the relative impor- shan at the spring equinox month with the peak of 4.5 tance of particle impact ionization is greatest in winter.

MHz, which occurred at 09 UT (14 LT/11 MLT). This Figure 4 compares the measured hmF 2 at Zhong- peak is greater than the IRI maximum of 4 MHz that shan with predicted values. In general, the measured F 2 occurred at local noon. For the autumn equinox month layer peak altitude is lower than the predicted altitude.

(Figure 3b), a f0F 2 peak of 5.0 MHz occurred at 10 There is much closer agreement with the IRI predictions UT (15 LT/12 MLT). This peak is very close to the IRI at Zhongshan (largely within the error bars) than at Sval- maximum, but delayed 3 hours, i.e., it shifted from lo- bard. cal noon to magnetic local noon. Notably, the observed f0F 2 values at night are systematically greater than the 4 Discussion IRI model predictions, and the discrepancies were larger near the spring equinox than the autumn equinox. Fig- It is clear from the observations of Section 3 that the IRI ure 3c demonstrates that the IRI model underestimates model matches the observations of the polar ionosphere the winter f0F 2 throughout the day. The measured max- during solar minimum conditions much better in sum- imum value is 4 MHz around 9 UT (14 LT/11 MLT), in mer than in winter. This indicates that the IRI model contrast to the IRI model value of 3 MHz at local noon. performs well when solar EUV ionization dominates. In 180 HE Fang, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 4 Diurnal variation of hmF 2 monthly median value from the DPS-4 at Zhongshan station and IRI prediction, in different seasons. this section, we discuss the physical processes that can Longyearbyen as expected. This is because of the ∼10◦ explain the discrepancies between the model and obser- difference in geographic latitude and its resultant effect vations. To quantify seasonal variations of these discrep- on solar EUV ionization. In winter, solar EUV ioniza- ancies, we introduce the mean absolute deviation (σ1) tion is nearly absent above Longyearbyen, and hence the and the standard deviation (σ2), expressed as signature of particle impact ionization alone should be 1 most pronounced at the ESR in this season. Figure 1d σ1 = |(∆f0F 2)i| (2) f F 2 24 0 1 2 shows that the IRI model data are in anti-phase with the ( σ2 = 24P [(∆f0F 2)i] (3) winter ESR observations. The observed f0F 2 curve has σ q1 h F 1 = 24 P|(∆ m 2)i| (4) fmF 2 one maximum in the morning, between 04–06 UT (07–09 σ 1 h F 2 ( 2 = 24P [(∆ m 2)i] (5) MLT), and there is another broad region in the after- q where ∆f0F 2 and ∆hmPF 2 are the differences between noon with a peak between 14–18 UT (17–21 MLT). This the observation and IRI prediction. is most likely due to the radar beam crossing a contracted

Comparing Figures 1 and 3, we note that the day- auroral oval. A similar dayside f0F 2 anomaly in winter [20] time f0F 2 maximum at Zhongshan is larger than at also appears at high latitudes in austral summer . The Longyearbyen for all seasons. In addition, the differ- value around 09 UT (12 MLT) is very low, indicating ence between the daytime f0F 2 maximum and the night- either that the ESR was located south of the cusp pre- time f0F 2 maximum is greater at Zhongshan than at cipitation, or that cusp precipitation did not contribute A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ 181

Figure 5 Deviations between IRI f0F 2 and hmF 2 predictions, and observations of the ESR at Longyearbyen and of the DPS-4 at Zhongshan station. much to the electron density. There is a local max- Regarding the f0F 2 peak around noon, there is a imum around 12 UT (15 MLT). Previous statisti- prominent offset from geographic noon to magnetic lo- cal analyses[16−19] indicate that there are three high- cal noon in the spring ESR dataset (Figure 1a), and in incidence areas of auroral precipitation within the au- the autumn and spring dataset from Zhongshan (Fig- roral oval, one of which is around 15 MLT, 75◦ geomag- ures 3b and 3d). There are many factors that influence netic latitude, in proximity to the observatories at Zhong- hmF 2 in polar regions, especially the neutral wind and shan and Longyearbyen. Zhongshan encounters the “af- E×B drift[23]. However, since recombination decreases ternoon hotspot” region near 13 UT(15 MLT). In the with altitude, there is a simple rule of thumb to discrim- Zhongshan data of Figure 3a, there is a small local max- inate between local production within the radar beam imum around 13 UT (15 MLT). There is a pronounced and plasma transport through the beam: If the f0F 2 lo- local maximum in winter and spring, between 14–15 UT cal maximum corresponds to the hmF 2 local minimum, (16-17 MLT) (Figures 3c and 3d). Later crossing indi- it indicates that local plasma production dominates. For cates a more contracted polar cap. the cases above, however, the f0F 2 maxima near noon [9] Moen et al. reported that deviations between the did not correspond with the hmF 2 minima, indicating IRI model and ESR observations under solar maximum that the radar beam was situated outside the produc- conditions were caused by the transport of solar EUV tion peak while observing the f0F 2 maxima. This indi- ionized plasma. In fact, theoretical calculations and cates that plasma transport of solar EUV ionized plasma modeling[21−22] have revealed that the observed effects around magnetic noon is indeed an important factor[9]. may also be a consequence of the thermospheric wind In general, the higher the altitude of hmF 2, the longer system. This wind system induces a vertical ionospheric the transport time away from the production region. For downward drift from 09:00–18:00 LT, and is responsible winter at Zhongshan, and autumn and spring at both for the daytime decrease of f0F 2 in the aforementioned stations, there are elevated f0F 2 values around mag- regions. netic midnight (19–20 UT at Longyearbyen and 20–21 182 HE Fang, et al. Adv Polar Sci September(2011) Vol.22 No.3

UT at Zhongshan station). This indicates that plasma References transport across the pole is also an issue during solar 1 Bilitza D. International Reference Ionosphere 2000. Radio minimum conditions, but is less pronounced than during Sci, 2001, 36: 261–275 [9] solar maximum conditions . 2 Bilitza D, Reinisch B. International Reference Ionosphere 2007: Improvements and new parameters. Adv. Space Res, 5 Conclusion 2008, 42(4): 599–609 3 Lei J, Liu L, Wan W, et al. Comparison of the first long- duration IS experiment measurements over Millstone Hill We have taken advantage of the IPY operation of the and EISCAT Svalbard radar with IRI-2001. Advances in ESR and the DPS-4 ionosonde at Zhongshan station, Space Research, 2006, 37: 1102–1107 for conjugate measurements of the F 2 region ionosphere 4 Bertoni F, Sahai Y, Lima W L C, et al. IRI-2001 model pre- dictions compared with ionospheric data observed at Brazil- at 75 MLAT. This is a unique data set for solar min- ian low latitude stations. Annales Geophysicae, 2006, 24: imum conditions. We have used this dataset to study 2191–2200 seasonal and diurnal variation features of f0F 2 and 5 Zhang S, Holt J M, Bilitza D K, et al. Multiple-site compar- isons between models of incoherent scatter radar and IRI. hmF 2, and have compared those with IRI-2007 model Advances in Space Research, 2007, 39: 910–917 prediction. 6 Liu Y C, Ma S Y, Cai H T. The background ionospheric The IRI model agrees very well with summer obser- profiles at polar cusp latitudes-ESR observations. Chinese vations when solar EUV ionization dominates over par- Journal of Polar Research, 2005, 17: 193–202 ticle impact ionization through the day. The poorest 7 Cai H T, Ma S Y, Schlegel K. Climatologic characteristics agreement was for ESR at Longyearbyen during the polar of high-latitude ionosphere-EISCAT observations and com- parison with the IRI model. Chinese Journal of Geophysics, night, when solar EUV ionization is low and particle im- 2005, 48: 471–479 pact ionization is the dominant ionization source. Then, 8 Liu H, Stolle C, Watanabe S, et al. Evaluation of the IRI crossing of the auroral oval in the morning and evening model using CHAMP observations in polar and equatorial sectors becomes evident. No prominent signature of cusp regions. Advances in Space Research, 2007, 39: 904–909 precipitation is found. 9 Moen J, Qiu X C, Carlso H C, et al. On the diurnal variabil- F For the case when both solar EUV and particle ity in 2-region plasma density above the EISCAT Svalbard radar. Annales Geophysicae, 2008, 26: 2427–2433 impact ionization are important, the largest discrep- 10 Liu R Y, He L S, Qian S L. Ionosphere data collection ancies occurr in spring at both stations, and in win- at Zhongshan station in Antarctica: 1995–1999. Shanghai: ter at Zhongshan station. This is from the effect of Shanghai Scientific and Technological Literature Publishing plasma transport around magnetic noon and magnetic House, 2000, ISBN: 7900306587 midnight, and from crossing of the auroral oval in the 11 Liu R Y, He L S, Preliminary observation results of digital ionosonde at Zhongshan station in Antarctica. Progress in afternoon sector, both of which are key dynamic fea- Geophysics, 1997, 12: 109–118 tures of the polar ionosphere not represented by the 12 He L S, Liu R Y, Liu S L, et al. Overall properties of F re- model. gion around solar minimum at Zhongshan Station, Antarc- tica. Chinese Journal of Geophysics, 2000, 43: 289–295 For hmF 2, agreement between prediction and obser- vation at Zhongshan station in the southern hemisphere 13 Zhang B C, Liu R Y, Liu S L. Simulation study on the influ- ences of the precipitating electrons on the polar ionosphere. is superior to the agreement for ESR in the northern Chinese Journal of Geophysics, 2001, 44: 311–319 hemisphere. 14 Xu Z H, Liu R Y, Liu S L, et al. Variations of the iono- spheric F2 layer critical frequency at Zhongshan Station, Antarctica. Chinese Journal of Geophysics, 2006, 49: 1–8 15 Zhu A Q, Zhang B C, Huang J Y, et al. Comparative study F Acknowledgments This research was supported by the youth of winter polar ionospheric 2 layer in both hemispheres. fund of the State Oceanic Administration, People’s Republic of Chinese Journal of Polar Research, 2008, 20: 31–39 China (Grant no. 2010614), the Polar Strategic Research Founda- 16 Newell P T, Lyons K M, Meng C I. A large survey of elec- tion of China (Grant no. 20100201), the Public Science and Tech- tron acceleration events. Journal of Geophysical Research nology Research Funds Projects of Ocean (Grant no. 201005017), (Space Physics), 1996, 101(A2): 2599–2614 the National Natural Science Foundation of China (Grant no. 17 Newell P T, Lyons K M, Meng C I. Erratum: A large sur- 40874082, 40890164) and the National Basic Research Program vey of electron acceleration events. Journal of Geophysical of China (Grant no. 2010CB950503-06). Research (Space Physics), 1996, 101(A9): 19941–19942 A conjugate study of the polar ionospheric F 2-layer and IRI-2007 at 75◦ 183

18 Liou K, Newell P T, Meng C I, et al. Synoptic auroral dis- Terrestrial Physics, 2010, 72: 997–1007 tribution: A survey using polar ultraviolet imagery. Journal 21 Kohl H, King J W. Atmospheric winds between 100 and 700 of Geophysical Research (Space Physics), 1997, 102(A12): km and their effects on the ionosphere. Journal of Atmo- 27197–27206 spheric and Terrestrial Physics, 1967, 29: 1045–1062 19 Liou K, Newell P T, Meng C I, et al. Dayside auroral ac- 22 Pirog O M, Polekh N M, Chistyakova L V. Longitudinal tivity as a possible precursor of substorm onsets: A survey variation of critical frequencies in polar F-region. Advances using polar ultraviolet imagery. Journal of Geophysical Re- in Space Research, 2001, 27: 1395–1398 search (Space Physics), 1997, 102(A9): 19835–19844 20 Perevalova N P, Polyakova A S, Zalizovski A V. Diurnal 23 Rishbeth H. Thermospheric winds and the F-region: A re- variations of the total electron content under quiet helio- view. Journal of Atmospheric and Terrestrial Physics, 1972, geomagnetic conditions. Journal of Atomspheric and Solar- 34: 1–47 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00184 September 2011 Vol.22 No.3 184–191

Extraction of auroral oval boundaries from UVI images: A new FLICM clustering-based method and its evaluation

WANG Qian1, MENG QingHu1, HU ZeJun2∗, XING ZanYang2,3, LIANG JiMin1∗ & HU HongQiao2

1School of Life Sciences and Technology, Xidian University, Xi’an 710071, China; 2SOA Key Laboratory for Polar Science, Polar Research Institute of China, Shanghai 200136, China; 3School of Science, Xidian University, Xi’an 710071, China

Received June 28, 2011; accepted August 16, 2011

Abstract Based on the fuzzy local information c-means (FLICM) clustering algorithm, a new method is de- veloped for extracting the equatorward and poleward boundaries of the auroral oval from images acquired by the Ultraviolet Imager (UVI) aboard the POLAR satellite. First, the method iteratively segments the UVI image with the FLICM clustering algorithm, according to an integrity criterion for the segmented auroral oval. Then, possible gaps in the extracted auroral oval are filled, based on prior knowledge of its shape. To evaluate the method objec- tively, the extracted boundaries are compared with the precipitating electron boundaries determined from DMSP satellite precipitation particle data. The evaluation results demonstrate that the proposed method generates more accurate auroral boundaries than traditional methods.

Keywords Auroral boundary, UVI, image segmentation, evaluation

Citation: Wang Q, Meng Q H, Hu Z J, et al. Extraction of auroral oval boundaries from UVI images: A new FLICM Cl- ustering-based method and its evaluation. Adv Polar Sci, 2011, 22: 184–191, doi: 10.3724/SP.J.1085.2011.00184

0 Introduction intensity along the oval. The equatorward and poleward Aurorae are natural lights in the upper atmosphere that boundaries of the auroral oval in UVI images, referred to are produced by collisions between energetic particles as UVI boundaries, are closely related to the energy cou- precipitating from space with atoms or molecules in the pling between solar wind and the magnetosphere. The upper atmosphere. Observations from remote imager in- boundaries expand and contract in response to geomag- struments aboard satellites usually show the lights to netic and solar wind activity[3]. For auroral study, it is have the shape of an annular ring centered on the Earth’s important to accurately extract the boundaries of the au- magnetic poles, and they are called auroral ovals[1−2]. roral oval. Compared with local auroral observations from ground- To make full use of the large number of UVI im- based cameras, auroral images acquired by the Ultravi- ages, automatic image segmentation methods for extract- olet Imager (UVI) aboard the POLAR satellite can pro- ing auroral oval boundaries are of great appeal, because vide global auroral information, e.g., the overall config- manually determining the auroral oval of so many images uration of the oval and spatial distribution of auroral is tedious and impractical. Several methods have been

∗Corresponding authors: (Hu Zejun, email: [email protected]; Liang Jimin, email: [email protected]) journal.polar.gov.cn Extraction of auroral oval boundaries from UVI images 185 presented in recent decades. These include threshold- fuzzy local (both spatial and gray level) similarity mea- based methods[4−5], a histogram-based k-means sure to overcome the disadvantages of traditional fuzzy algorithm[6], an adaptive thresholding technique[7], c-means algorithms. a pulse-coupled neutral network[3] and shape-based To extract a complete and accurate auroral oval from methods[8−9]. The methods of references[4-7] do not a UVI image, our method contains three main steps: exploit auroral oval shape information and the spatial image preprocessing; auroral oval segmentation using relationship of pixels. Therefore, they are unable to FLICM; and gap filling, based on prior knowledge of the detect the complete auroral oval for some images, espe- auroral oval shape. cially those with large auroral intensity variations. Cao 1.1 UVI image preprocessing et al.[8−9] presented a shape-based method for extracting Effective preprocessing of the original UVI images greatly auroral ovals. They first segmented auroral images using aids the accurate extraction of auroral ovals, especially the algorithm of Li et al.[7], then selected some inner and for images with strong noise. The characteristics of UVI outer boundary points from the resultant foreground, images include blurry and elliptical outer boundaries[9], via radial-based processing. Finally, two ellipses fitted to blurry inner boundaries with complex shapes, and the the points using linear least-squares randomized Hough spatial distributions of the auroral oval centered on the transform[9] were used as the equatorward and poleward Earth’s magnetic poles, within a certain range (57.5◦ to boundaries, respectively. Although this method pro- 67◦[16]) of magnetic latitude. We considered these char- duces complete boundaries of the auroral oval, it may acteristics in the UVI image preprocessing, as follows. introduce errors if the boundaries are not strictly ellipti- (1) Background removal. Pixels corresponding to cal, or if only parts of the auroral oval are imaged. magnetic latitudes less than 50◦ were removed. In this paper, a new approach for extracting the (2) Negative pixel clearance. Pixels with negative UVI boundaries is proposed, based on fuzzy local infor- gray values, possibly caused by noise, were set to zero. mation c-means clustering (FLICM)[10]. In this method, (3) Small bright spot smoothing. A bright spot is the UVI image is iteratively segmented according to an defined as a connected region in which the pixel gray val- integrity measurement of the extracted auroral oval us- ues are greater than a given threshold. The threshold T ing FLICM, then auroral oval gaps are filled based on g is defined as prior knowledge of its shape. The method is not only applicable to images with whole auroral ovals, but also Tg = µA + 3σA (1) to images with partial ovals.

To objectively evaluate the performance of various where µA and σA are the mean gray value and standard algorithms, we defined particle precipitation boundaries deviation of all pixels in the image, respectively. If the (called DMSP boundaries) derived from DMSP satel- bright spot area is less than a predetermined threshold lite observations[11] as the real boundaries of the auroral (20 pixels), the pixels inside the spot are set to the aver- oval. The comparative experimental results demonstrate age value of pixels outside the spot and contained in the that the method outperforms previous methods. smallest rectangle covering the bright spot. Otherwise, the bright spot is not processed, to avoid destroying au- 1 Auroral oval extraction based on fuzzy roral substorm regions by mistake. clustering (4) Image smoothing. The pixels in each 3×3 neigh- borhood are partitioned into two classes, according to Auroral oval boundaries are usually blurry because of their gray values. The class with fewer pixels is consid- strong background noise. Fuzzy clustering techniques are ered the outlier, and its values are replaced by the average a good choice for UVI image segmentation. These tech- intensity of the other class. niques have been studied extensively and applied suc- Figure 1 demonstrates an example of the preprocess- cessfully in many fields[12−15]. In this paper, fuzzy local ing steps. Figure 1(f) shows the effective image region. information c-means clustering (FLICM)[10] is used to Only the data inside the effective region of the original segment the auroral oval in a UVI image. FLICM uses a image are meaningful. 186 WANG Qian, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 1 Preprocessing example. (a) Original UVI image. Images in (b) to (e) are preprocessed results of background removal, negative pixel clearance, small bright spot smoothing, and image smoothing, respectively. White region in last image (f) is the effective region of UVI image.

1.2 Auroral oval segmentation imaged (class C). The extracted auroral oval is incom- After preprocessing, the UVI image is segmented into plete, and will be postprocessed further. foregrounds (auroral regions) and backgrounds, using the (4) The number of segmentations by FLICM reaches FLICM method[10]. The resultant auroral regions may the predetermined upper limit number β (class D). be incomplete because of auroral intensity variation. To Both class B and class C images are further post- acquire a complete auroral oval from a UVI image, an processed to generate complete auroral oval boundaries. integrity measurement T of the extracted foreground is 1.3 Auroral oval gap filling defined as For a class B image, the whole auroral oval is imaged, m but not fully extracted, by iterative segmentation. An P θ(i) i T = =0 , 0 6 θ 6 2π, m > 0 (2) example is shown in Figure 2, where (a) is the original 2π image and (b) is the segmented result. There is a gap where θ(i) represents the field angle of the ith connected on the segmented auroral oval that requires filling. Mo- foreground region, and T ∈ [0, 1]. If T < ε (ε is a prede- tivated by the annular ring characteristic of the auroral termined threshold), the background will be segmented oval, Cao et al.[9] used two ellipses fitted to the inner again by FLICM. The resultant foregrounds are merged and outer boundaries to represent the equatorward and with former foreground regions, and the integrity T is poleward boundaries, respectively. Here, we only fit the recalculated. These procedures repeat until one of the segmented outer boundary to an ellipse (the red ellipse following criteria is satisfied. in Figure 2(b)), and used it to fill the gap. The width (1) T =1. This means the auroral oval has been com- of the gap is calculated as the linear interpolation of its pletely extracted. We call this type of UVI image as class starting and endpoint widths, which is defined as the av- A. erage width of a small auroral oval section (10 degrees) (2) ε 6 T < 1 and the whole auroral oval is imaged adjacent to the gap. Figure 2(c) shows the filled gap, (class B). The extracted auroral oval is incomplete and and Figure 2(d) shows the extracted auroral oval. After needs further postprocessing. Whether the entire auro- filling the gap, the inner and outer boundaries of the au- ral oval is imaged is assessed by the location of the fitted roral oval were easily obtained. ellipse of the outer auroral oval boundary. If the fitted The postprocessing of a class C image is similar to ellipse is completely inside the effective region of the UVI the procedures above, except that only the gaps of the image, we conclude that the entire auroral oval is imaged. auroral oval in the effective imaging region are filled. An (3) ε 6 T < 1 and only part of the auroral oval is example is shown in Figure 3.

Figure 2 Example of gap filling of a B class image. Extraction of auroral oval boundaries from UVI images 187

Figure 3 Example of gap filling of a C class image.

2 Evaluation local time (MLT). Each sector spans 2 h, and has a 1 h overlap with the neighboring sectors. The thresholds cal- Subjective and objective evaluations of the performance culated for each sector are smoothed, and then applied of various methods for auroral oval segmentation were to the original image. This method tends to generate [7] done. The five algorithms evaluated are AMET , a “block effect”, because each subsector has a different [17] [18] [9] OTSU , the classic FCM , the method of Cao et al. threshold. The OTSU and classic FCM methods easily (referred to as EF) and our method. detect bright regions, but have difficulty in extracting 2.1 Subjective evaluation aurorae with weak intensities. The EF method can de- Two type B images and two type C images were selected lineate the closed inner and outer boundaries, but its and segmented using the aforementioned five methods. precision needs improvement. For our method, the upper limit segmentation number Figure 4 gives a preliminary impression of the per- was β=4 and the integrity threshold was ε=0.65. As formance of each method. To objectively evaluate the shown in Figure 4, the auroral oval boundaries extracted algorithms, we used the DMSP boundaries as the real by our method are more complete and accurate than auroral oval boundaries, as follows. those of the other methods. The AMET method con- 2.2 Objective evaluation siders pixel intensities within image zones. Its first step divides the UVI image into 24 sectors, based on magnetic Cao et al.[9] used manually-segmented auroral oval

Figure 4 Auroral oval segmentation from the five methods. The first column (a) shows preprocessed UV images. Columns (b) to (f) are segmented images of AMET (b), OTSU (c), Classical FCM (d), EF (e) and our method (f), respectively. Blue and red curves in (e) and (f) indicate poleward and equatorward boundaries of the auroral oval. 188 WANG Qian, et al. Adv Polar Sci September(2011) Vol.22 No.3

boundaries as ground truth in an objective evaluation. between the two boundary types, denoted as Mt and Md, However, manual determination of the auroral oval for a respectively. The boundary matching procedures are as large number of images is tedious and impractical, and its follows. results are subject to disagreement. Fortunately, precip- (1) The DMSP and UVI boundaries are converted to itating electron boundaries can be distinguished by the the same altitude-adjusted, corrected geomagnetic coor- DMSP satellite[19−20], and some boundaries are closely dinates (AACGM), as described in Baker and Wing[25]. related to UVI boundaries[21−22]. To effectively evalu- (2) A selection is made of a point P at the DMSP ate methods of auroral boundary determination, we used boundary at time t. Its coordinate is (Mlat, Mlt), where the b1e and b6 boundaries defined by Newell et al.[23−24] Mlat and Mlt represent the magnetic latitude and mag- as the “real” equatorward and poleward boundaries, re- netic local time, respectively. spectively. It should be noted that the UVI and DMSP (3) All UVI images captured during the period of satellites operate at different heights and measure diffe- [t − Mt, t + Mt] are found. rent physical quantities. We call this bias as system bias. (4) Images satisfying two of the following conditions Therefore, auroral oval boundaries determined by these are selected, to construct the basic matched image set. satellites are theoretically different, but the system bias a) The minimum Euclidean distances Ld, between doesn’t affect our evaluation. P and every pixel in one image, must be less than Md. 2.2.1 Data b) The pixel corresponding to Ld must be within the effective imaging region of the UVI images. We used only nighttime UVI and DMSP data, since day- (5) Ld(i) is Euclidean distance between P and the time auroral images were usually greatly affected by day- ith image in the basic matched image set. The UVI im- glow, which degrades the performance of existing meth- age with the minimum distance in {Ld(i)} is assigned as ods in the extraction of aurora from UVI images[9]. For the matched image of P . nighttime, the b1e and b6 of Newell et al.[23−24] were The above steps are repeated until all DMSP bound- used to estimate the equatorward and poleward bound- aries are matched. Two matched databases are obtained, aries, respectively. corresponding to the equatorward (Edata) and poleward Two data sets were used in the evaluation. The first (Pdata) auroral boundaries. These databases contain consists of global aurora images acquired by the Lyman- nighttime data during the polar night period from De- Birge-Hopfield long (LBHL) filter of UVI. These images cember, 1996 to January, 2000, about 11 months in to- have an accumulation time of 37 s over a sample inter- tal. Information on Edata and Pdata is shown in Ta- val of about 3 min. The second is from precipitating ble 1. Figure 5 shows distributions of equatorward and electrons detected by the SSJ/4 sensor on DMSP satel- poleward boundaries with magnetic local time, magnetic lites. The detectors pointing toward the local zenith ob- longitude and magnetic latitude. serve precipitating particles that cause auroral emissions on the trace of satellites. It is apparent that the UVI 2.2.2 Evaluation experiments boundaries consist of two continuous curves (many pix- Because the AMET[7], OTSU[17], and classical FCM[18] els) in one image, while at the same time there may be methods cannot always generate complete auroral oval just one sample dot in the DMSP data. In addition, the boundaries, we only compared our algorithm to the EF sample of an SSJ/4 sensor is not always simultaneous method[9]. First, auroral boundaries were extracted us- with the UVI image. ing these two methods. The method precision is mea- To match DMSP and UV boundaries effectively, we sured by the smallest distance error between the used the upper limits of time error and spatial deviations DMSP boundary point and the segmented UVI auroral

Table 1 Information on the evaluation database Database Edata Pdata Number of matches 961 922 Number of aurorae imaged completely (percentage) 266 (27.68 %) 270 (29.28 %) Number of aurorae imaged incompletely(percentage) 695 (72.32 %) 652 (70.72 %) Extraction of auroral oval boundaries from UVI images 189

Figure 5 Distributions of equatorward and poleward boundaries with magnetic local time (MLT), magnetic longitude (MLON) and magnetic latitude (MLAT). Rows correspond to Edata and Pdata. boundary points, which is defined as where µ and σ indicate the mean and standard deviation of D(Mlat, Mlt), respectively. From Table 2, we con- 2 2 D(Mlat, Mlt) = pD−mlat + D−mlt , (3) clude that both equatorward and poleward boundaries D−mlat = Mlat−dmsp − Mlat−uv, (4) from our method are more accurate than those from EF.

D−mlt = Mlt−dmsp − Mlt−uv, (5) The average equatorward and poleward boundary errors of the former method are less than 0.5. where (Mlat−dmsp, Mlt−dmsp) are DMSP coordinates, Figure 6 depicts the error distribution of the equa- and (Mlat−uv, Mlt−uv) are UVI boundary point coor- torward and poleward boundaries determined by the two dinates, detected by EF or our method. methods. The origin in the figures represents the “real” The statistical results of EF and our method, using boundary determined by DMSP. This shows that the ac- the Edata and Pdata databases, are shown in Table 2, curacy of the equatorward boundary from the EF method

Table 2 Statistical boundary detection errors of EF and our method Database Method Edata Pdata EF method (µ, σ) (0.7019, 0.6654) (1.0646, 0.9275) Our method (µ, σ) (0.4542, 0.4244) (0.4951, 0.4239) is greater than that of the poleward boundary. The rea- proposed method is applicable to UVI images with either son for this may be that most poleward boundaries do not complete or partial imaging of auroral ovals. The method have a strictly elliptical shape. Our method has similar can be easily extended to other kinds of global aurora accuracies for both equatorward and poleward boundary images. In addition, an effective evaluation method is detection. described, which assumes that DMSP boundaries repre- sent the “real” auroral oval boundaries. The evaluation 3 Conclusion results show that our method is more accurate than pre- vious methods in the extraction of the auroral oval. We present an approach to extract auroral oval bound- The proposed method has been tested only on night- aries from UVI images. The approach features an iter- time data. If strong dayglow is present, performance may ative segmentation of the auroral oval using the FLICM deteriorate. To overcome this deficiency, blind signal sep- clustering algorithm, followed by gap filling based on aration techniques may be applied in the preprocessing prior information about the auroral oval shape. The stage, to remove interference from dayglow. 190 WANG Qian, et al. Adv Polar Sci September(2011) Vol.22 No.3

Figure 6 Distributions of equatorward and poleward boundary errors for EF-based method and our method in Edata (a, b) and Pdata (c, d).

Acknowledgments We are thankful to the Johns Hopkins 4 Frank L A, Craven J D. Imaging results from Dynamics Ex- University Applied Physics Laboratory and National Space Sci- plorer 1. Reviews of Geophysics, 1988, 26(2): 249–283 ence and Technology Center (NSSTC) in Huntsville for the pro- 5 Brittnacher M, Gillingim M, Parks G, et al. Polar cap area vision of DMSP and UVI data, respectively. This research was and boundary motion during substorms. Journal of Geo- supported by the National Natural Science Foundation of China physical Research, 1999, 104(A6): 12251–12262 (Grant nos. 60872154, 41031064, 40904041, 40974103), the Na- 6 Hung C, Germany G. K-means and iterative selection al- tional High Technology Research and Development Program of gorithms in image segmentation. IEEE Southeastcon 2003 China (Grant no. 2008AA121703) and the Ocean Public Welfare (Session 1: Software Development) Scientific Research Project, State Oceanic Administration of China 7 Li X, Ramachandran R, He M, et al. Comparing different (Grant no. 201005017). thresholding algorithms for segmenting auroras. Proceed- ings of the International Conference on Information Tech- References nology: Coding and Computing, 2004, 6: 594–601 8 Cao C, Newman T S, Germany G. A shape-based tecnique 1 Akasofu S I. The latitudinal shift of the auroral belt. Jour- for aurora oval segmentation from uvi images. American nal of Atmospheric and Terrestrial Physics, 1964, 26(12): Geophysical Union, Fall Meeting, 2005 1167–1174 9 Cao C, Newman T S, Germany G. A new shape-based auro- 2 Feldstein Y I. Some problems concerning the morphology of ral oval segmentation driven by LLS-RHT. Pattern Recog- auroras and magnetic disturbances at high latitudes. Geo- nition, 2009, 42: 607–618 magnetism Aeronomy, 1963, 3: 183–192 10 Krinidis S, Chatzis V. A robust fuzzy local information c- 3 Germany G A, Parks G K, Ranganath H, et al. Analysis of means clustering algorithm. IEEE Transaction on Image auroral morphology: substorm precursor and onset on Jan- Processing, 2010, 19(5): 1328–1337 uary 10, 1997. Geophysical Research Letters, 1998, 25(15): 11 Newell P T, Sotirelis T, Ruohoniemi J M, et al. OVATION: 3042–3046 Oval Variation, Assessment, Tracking, Intensity, and Online Extraction of auroral oval boundaries from UVI images 191

Nowcasting. Annales Geophsicae, 2002, 20(7): 1039–1047 tron observations of equatorward auroral boundaries and 12 Yang M S. A survey of fuzzy clustering. Mathematical and their relationship to magnetospheric electric fields. Journal Computer Modelling, 1993, 18(11): 1–16 of Geophysical Research, 1981, 86(A2): 768–778 13 Delbo S, Gamba P, Roccato D. A fuzzy shell clustering 20 Hardy D A, Burke W J, Gussenhoven M S, et al. DMSP/F2 approach to recognize hyperbolic signatures in subsurface electron observations of equatorward auroral boundaries and radar images. IEEE Transactions on Geoscience and Re- their relationship to the solar wind velocity and the north- mote Sensing, 2000, 38(3): 1447–1451 south component of the interplanetary magnetic field. Jour- 14 Dell’Acqua F, Gamba P. Detection of urban structures in nal of Geophysical Research, 1981, 86(A12): 9961–9974 SAR images by robust fuzzy clustering algorithms: the ex- 21 Carbary J F, Sotirelis T, Newell P T, et al. Auroral bound- ample of street tracking. IEEE Transactions on Geoscience ary correlations between UVI and DMSP. Journal of and Remote Sensing, 2001, 39(10): 2287–2297 Geophysical Research, 2003, 108(A1), doi: 10.1029/2002JA 15 Amini L, Soltanian-Zadeh H, Lucas C, et al. Automatic 009378 segmentation of thalamus from brain MRI integrating fuzzy 22 Baker J B, Clauer C R, Ridley A J, et al. The nightside pole- clustering and dynamic contours. IEEE Transactions on ward boundary of the auroral oval as seen by DMSP and the Biomedical Engineering, 2004, 51(5): 800–811 Ultraviolet Imager. Journal of Geophysical Research, 2000, 16 Galand M, Lummerzheim D, Stephan A W, et al. Electron 105(A9): 21267–21280 and proton aurora observed spectroscopically in the far ul- 23 Newell P T, Feldstein Y I, Galperin Y I, et al. The mor- traviolet. Journal of Geophysical Research, 2002, 107(A7), phology of nightside precipitation. Journal of Geophysical doi: 10.1029/2001JA000235 Research, 1996, 101(A5): 10737–10748 17 Otsu N. A threshold selection method from gray-Level his- 24 Newell P T, Feldstein Y I, Galperin Y I, et al. Correction tograms. IEEE Transactions on Systems, Man, and Cyber- to “Morphology of nightside precipitation”. Journal of Geo- netics, 1979, 9(1): 62–66 physical Research, 1996, 101(A8): 17419–17421 18 Bezdek J. Pattern recognition with fuzzy objective function 25 Baker K B, Wing S. A new magnetic coordinate system for algorithms. New York: Plenum Press, 1981 conjugate studies at high latitudes. Journal of Geophysical 19 Gussenhoven M S, Hardy D A, Burke W J. DMSP/F2 elec- Research, 1989, 94(A7): 9139–9143 Article · Advances in Polar Science · doi:10.3724/SP.J.1085.2011.00192 September 2011 Vol.22 No.3 192–198

Improved significance testing of wavelet power spectrum near data boundaries as applied to polar research

ZHANG ZhiHua1∗ & John C Moore2,3,4∗

1College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China; 2State Key Laboratory of Earth Surface Processes and Resource Ecology/College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China; 3Arctic Centre, University of Lapland, Rovaniemi, Finland; 4Department of Earth Sciences, Uppsala University, Sweden

Received March 7, 2011; accepted August 25, 2011

Abstract When one applies the wavelet transform to analyze finite-length time series, discontinuities at the data boundaries will distort its wavelet power spectrum in some regions which are defined as a wavelength-dependent cone of influence (COI). In the COI, significance tests are unreliable. At the same time, as many time series are short and noisy, the COI is a serious limitation in wavelet analysis of time series. In this paper, we will give a method to reduce boundary effects and discover significant frequencies in the COI. After that, we will apply our method to analyze Greenland winter temperature and Baltic sea ice. The new method makes use of line removal and odd extension of the time series. This causes the derivative of the series to be continuous (unlike the case for other padding methods). This will give the most reasonable padding methodology if the time series being analyzed has red noise characteristics.

Keywords Wavelet power spectrum, significance testing, Greenland winter temperature, Baltic sea ice

Citation: Zhang Z H, Moore J C. Improved significance testing of wavelet power spectrum near data boundaries as applied to polar research. Adv Polar Sci, 2011, 22: 192–198, doi: 10.3724/SP.J.1085.2011.00192

0 Introduction physical time series, an appropriate background noise or In the early 1980s, Morlet introduced the wavelet trans- null hypothesis is red noise, so in order to distinguish form and applied it in geophysics. After two decades their intrinsic feature from red noise, one needs to use a of fast development, the wavelet method has become significance test. routine[1]. The continuous wavelet transform possesses In practice, since one does not know the information the ability to construct a time-frequency representation of the time series in the past or in the future, the time of a time series that offers very good time and frequency series is usually padded with zeroes and then its wavelet localization, so wavelet transforms can analyze local- power spectrum is computed. After that, a significance ized intermittent periodicities of geophysical time series. test may find significant regions. However, discontinu- However, physical interpretation of the signals requires ities at data boundaries will distort the wavelet plausible statistical significance testing. For many geo- power spectrum in some regions which are defined as a

∗Corresponding authors(Zhang Zhihua, email: [email protected]; John C Moore, email: [email protected]) journal.polar.gov.cn Improved significance testing of wavelet power spectrum near data boundaries 193

− wavelength-dependent cone of influence (COI), so the sig- We call W (s) 2 wavelet power spectrum of x N 1. | n | { n}0 nificance test will be unreliable in the COI. Moreover, The wavelet transform is very useful for time series anal- since many geophysical time series are short and noisy, ysis where smooth, continuous variations in wavelet am- the COI will be a serious limitation in wavelet analy- plitude are expected. sis of time series. Many scientists have presented var- Liu et al.[7] believed that there existed some bias in ious methods to deal with boundary effects in wavelet wavelet power spectrum and adjusted it by dividing by analysis: Foster[2] and Frick et al.[3] constructed a new the corresponding scale. But the significant regions in wavelet near the gap to deal with the wavelet trans- the wavelet power spectrum will not be affected by using form of unevenly sampled time series, while Sweldens[4] Liu et al’s rectification algorithm if both the signal and presented the “lifting scheme” algorithm of constructing noise background are treated the same. new wavelet bases. Johnson[5] modified the traditional 1.1 Cone of influence wavelet transform by adapting wavelets through renor- malization and modifying the shape of the analyzing win- In the real world, since we do not have information on dow. Unfortunately, their methods cannot be applied to the time series in the past or in the future, we only can do significance tests and extract instinct features in the deal with the time series of finite length from T1 to T2. COI of wavelet power spectrum. In our paper, instead of In order to compute its wavelet transform, the simplest defining a new wavelet, we suggest a simple approach to algorithm is to pad the time series with zeroes and then the improvement to significance testing of wavelet power use the following approximation spectrum near data boundaries. In Section 1, we will re- ∞ view the cone of influence and significance test of wavelet 1 t b Wf (a, b) = f(t)ψ0 − dt power spectrum. In Section 2, we give a method to re- √a a −∞Z duce boundary effect. In Section 3, we apply our method ∞ ¡ ¢ 1 t b to research Greenland winter temperature and Baltic sea F (t)ψ − dt = W (a, b) (4) ≈ √a 0 a F ice. −∞Z ¡ ¢ 1 Cone of influence and significance test where of wavelet power spectrum f(t), t [T1, T2], In geophysics, due to the similar resolution in both time F (t) = ∈ (5) ( 0, otherwise and frequency, the most popular wavelet is the Morlet wavelet[1] whose representation is In this formula, we use the wavelet transform of F to 2 −1/4 iω0t −t /2 approximate to that of f. Because of discontinuities at ψ0(t) = π e e (1) the endpoints, errors will occur at the beginning and end where ω0 is the nondimensional frequency. In applica- of the wavelet transform. The COI is the region of the tion, one often takes ω0=6. Throughout this paper, the wavelet transform in which edge effects become impor- wavelet we use is the Morlet wavelet. Let f be a con- tant and is defined usually as the e-folding time for the tinuous time series. Then the wavelet transform of f is autocorrelation of wavelet transform at each scale[1]. So [6] defined as the values of the wavelet transform (or the wavelet power ∞ 1 t b spectrum) obtained in the COI are distorted. W (a, b) = f(t)ψ ( − )dt (a > 0, b R) (2) f √a 0 a ∈ −∞Z 1.2 Background noise and significance test where the overbar denotes a complex conjugate. We call For many geophysical phenomena, the plausible null hy- 2 Wf (a, b) the wavelet power spectrum of f. For the pothesis to test against is red noise[8]. A simple model | | − discrete time series x N 1 of time step δt, its wavelet { n}0 for red noise is the univariate lag-1 autoregressive [AR(1)] transform is defined as process as follows: N−1 0 δt 0 (n n)δt Wn(s) = xnψ0[ − ] (3) r s 0 s x0 = 0, xn+1 = λxn+ n+1, n = 1, 2, 3, . . . , N 1 (6) nX=0 ∈ − 194 ZHANG ZhiHua, et al. Adv Polar Sci September(2011) Vol.22 No.3

[11] where λ is a constant and εn is independent Gaussian “cosine damping” which preprocesses the time series white noise with mean 0 and variance σ2. Torrence f(t), t [T , T ] as follow ∈ 1 2 and Compo[1] showed that, for the Morlet wavelet, the 2 t T1 2 g(t) = f(t) [1 cos − π ] (9) wavelet power spectrum Wn(s) of an AR(1) process × − T T | | 2 − 1 satisfies that ¡ ¢ It is clear that at the endpoints, g(T1) = g(T2)=0. Al- 2 Wn(s) 1 2 though this method overcomes the boundary effect, at | | is distributed as Pkχ2 (7) σ2 2 the same time, we lose almost all the information from where σ2 is the variance of time series, χ2 is the dis- the wavelet power spectrum at the endpoints. This is e 2 tribution of the sum of the squares of two independent often an unacceptable artifact. The third algorithm is [11] standarde normal random variables, even symmetric extension . This algorithm guarantees that the signal is continuous at the endpoints. However, 1 λ2 [11] P = − (8) Meyers et al. indicated that it has the disadvantage of k 1 + λ2 2λcos(2πk/N) − artificially creating discontinuities in the first derivative and k is the Fourier frequency corresponding to the at the endpoints. wavelet scale s. For the Morlet wavelet (see formula Now, we will give a new extension algorithm to re- N(ω + 2 + ω2) move boundary effect in wavelet power spectra. The ad- (1)), k = 0 0 . If we take ω =6, then 4πs 0 vantage of our algorithm is that the first derivatives of N p k = . extended time series at the endpoints are still continu- 1.03s ous. Let a function f(t) be defined on [T , T ]. For a typical geophysical time series, If a region G 1 2 Step 1. We divide into two parts. The first is such that the values of the wavelet power spectrum part is a segment line joining two points (T , f(T )) and on G are all outside the α% confidence interval of the 1 1 (T , f(T )) distribution for wavelet power spectrum of an AR(1) red 2 2 noise, then we call G a significant region. In geophysical f(T2) f(T1) u(t) = − (t T ) + f(T ) (10) T T − 1 1 research, one always takes α =90 or 95. 2 − 1 Many statistical tests assume that the probability The second part is density function (pdf) is close to normal. Before one uses v(t) = f(t) u(t) (11) the wavelet transform to analyze typical geophysical time − series, one often transforms the original time series such From here, we know that that the pdf of the transformed data is normal[9−10]. A practical way of doing this is by taking the inverse normal v(T1) = v(T2) = 0 (12) cumulative distribution function (cdf). The main reason Step 2. We do the odd extension for v: for doing it is because otherwise the normally distributed red noise null hypothesis we use is wrong. Further the v (t) = v(t), t [T , T ] (13) odd ∈ 1 2 AR(1) estimators can be quite poor at dealing with non- Vodd(t) = v(2T1 t), t [2T1 T2, T1] (14) normal data. So, if the data are not normalized, then − − ∈ − the obtained significant regions will be misleading sim- Step 3. We do the periodic extension with period ply because the null-hypothesis can be trivially rejected. 2(T2 T1) for vodd(t), i. e., − ∗ v (t) = v (t), t [2T T , T ] (15) 2 Reducing the boundary effect odd ∈ 1 − 2 2 ∗ ∗ v (t + 2k(T2 T1)) = v (t), In order to reduce the data boundary effect and obtain − t [2T1 T2, T2], k = 1, 2, . . . (16) the better wavelet power spectrum in the COI, seve- ∈ − § § ral algorithms have been tried previously. The first Step 4. We do the wavelet transform for v∗ as [11] algorithm is to pad the time series on each endpoint ∞ 1 ∗ t b with zeroes. The algorithm artificially creates discon- W ∗ (a, b) = v (t)ψ − dt (17) v √a 0 a tinuities at the endpoints. The second algorithm is −∞Z ¡ ¢ Improved significance testing of wavelet power spectrum near data boundaries 195 where ψ is the Morlet wavelet. t0 T , t0 > T . So we obtain that 0 → 1 1 Below we give two propositions on our method. v (t) v (T ) lim odd − odd 1 = Proposition 1. In (17), for any polynomial h(t) = → − t T1 t T1 ct + d of degree one, we have W (a, b) 0. − 0 h ≈ v(T1) v(t ) 0 0 lim − 0 = v (T1) (22) Proof. The Morlet wavelet ψ0 has one vanishing t →T1+ T t 1 − moment approximately and the error is very small, then ∞ ∞ Combining (19) with (22), we know that the derivative ψ(t)dt 0 and tψ(t)dt 0. So −∞ ≈ −∞ ≈ of vodd exists at the endpoint T1 and R R 0 0 ∞ vodd(T1) = v (T1) (23) 1 t b Wh(a, b) = (ct + d)ψ0 − dt ∗ √a a By Step 3, v (t) = vodd(t), t [2T1 T2, T2] and −∞Z ∈ − ∞ ¡ ¢ 2T1 T2 < T1 < T2, we know that the derivative of ∗ − = √a [c(at + b) + d]ψ (t)dt 0 (18) v exists at T1. 0 ≈ ∗ −∞Z Next we prove that the derivative of v is continuous at T . From v (t) = v(2T t), t [2T T , T ], we 1 odd − 1 − ∈ 1 − 2 1 Remark 1. For a wavelet with vanishing moments have up to some power p, the corresponding wavelet transform 0 0 v (t) = v (2T t), t [2T T , T ] (24) of a polynomial of order p is zero. odd 1 − ∈ 1 − 2 1 Proposition 2. Let a function f(t) be defined on ∗ This implies that [T1, T2] and v (t) be defined in Step 3. If the derivative 0 ∗0 0 0 f (t) is continuous on [T1, T2], then the derivative v (t) lim vodd(t) = lim v (2T1 t) = t→T1− t→T1− − is continuous on ( , ). 0 0 0 −∞ 0∞ lim v (t) = v (T1) = vodd(T1) (25) Proof. Since f is continuous on the interval t→T1+ [T , T ], from the construction of v∗, we know that in 1 2 On the other hand, by vodd(t) = v(t), t [T1, T2], we ∗ ∈ order to prove the continuity of the derivative of v on have ( , ), we only need to prove that the derivative of v∗ −∞ ∞ 0 0 0 0 is continuous at the points T and T . By the similarity lim vodd(t) = lim v (t) = v (T1) = vodd(T1) (26) 1 2 t→T1+ t→T1+ of arguments, now we consider the point T1. 0 0 ∗ By (25) and (26), we get lim vodd(t) = vodd(T1), i.e., First we prove that the derivative of v exists at T1. t→T1− v0 (t) is continuous at t = T . By Step 3, we know that By Step 2, vodd(t) = v(t), t [T1, T2]. We consider the odd 1 ∈ the derivative of v∗(t) is continuous at T . Proposition 2 right derivative of vodd at the endpoint T1, 1 is proved. v (t) v (T ) lim odd − odd 1 = From Proposition 1, we know that although we re- t→T1+ t T − 1 move a linear function from the original signal, the cor- v(t) v(T1) 0 lim − = v (T1) (19) responding wavelet power spectrum will not be changed. t→T1+ t T − 1 By Proposition 2, our algorithm “linear removal+odd By Step 2 and Step 1, extension” guarantees that the derivatives of the time series are continuous at the endpoints. So our algo- v (t) = v(2T t), t [2T T , T ] (20) odd − 1 − ∈ 1 − 2 1 rithm can overcome boundary effect better than “zero- padding” and “even extension (even-padding)”. and vodd(T1) = v(T1) = 0. Considering the left derivative of vodd at the endpoint T1, we obtain that 3 Application v (t) v (T ) lim odd − odd 1 = We will examine Southern Greenland winter temperature t→T1− t T − 1 index and Baltic Sea ice index with the help of our al- v(T ) v(2T t) lim 1 − 1 − (21) gorithm, i.e., we will make a significance test against the → − t T1 T1 (2T1 t) − − null hypothesis of climate noise and determine significant Let t0 = 2T t. Then, for t T , t < T , we have regions near data boundaries. 1 − → 1 1 196 ZHANG ZhiHua, et al. Adv Polar Sci September(2011) Vol.22 No.3

3.1 Southern Greenland winter temperature the middle part to compute its wavelet power spectrum by “zero-padding”, “even extension”, and our algorithm index (Figures 2b–2d). In the COI, the significant region indi- The isotopic ratio δ18O measured in ice cores can be used cated by our method is more closer to the real significant as a temperature proxy because of the temperature de- region than that by “zero-padding” or “even extension”. pendent fractionation of oxygen isotopes, that takes place while moisture travels from its evaporation area to the Greenland ice sheet. Vinther et al.[12] analyzed ice core data from 7 different drill sites in the southern half of Greenland in order to get Southern Greenland winter temperature indices. We apply our method to analyze the wavelet power spectrum of this time series. So, first of all, we transform the original data such that the pdf of the transformed data is normal (Figure 1). In order to show the performance of our method, we consider the middle part of the full temperature indices. The wavelet power spectrum of the middle part can be computed by Figure 1 Thenormalized Southern Greenland winter tem- using full length data (Figure 2a). Now we only use perature indices.

Figure 2 Wavelet power spectrum of Southern Greenland winter temperature indices is obtained by using: (a) full-length data, (b) “zero-padding” method, (c) “even-padding” method, and (d) our method. The black contour designates the 90% significance level against red noise, and COI is just the region below the thick line. Improved significance testing of wavelet power spectrum near data boundaries 197

inal data such that the pdf of the transformed data is normal (Figure 3). We take the 1765–1870 part of the full-length BMI indices. It is obvious that we can obtain the true wavelet power spectrum and significant region of this 105-year data by using full length data. The true wavelet power spectrum of the 105-year data is shown in Figure 4a. Now we use the 105-year data to compute the wavelet power spectrum by using three approaches: “zero-padding”, “even extension” and our method (Fig- ures 4b–4d). If we compare the shapes of two significant regions on the top right corner of wavelet power spec- trum, it is obvious that the significant region obtained Figure 3 The normalized winter BMI percentile time se- by our extension algorithm is closer to true significant ries. region than that obtained by “zero-padding” or “even 3.2 Baltic Sea ice index extension” (Figure 4).

The maximum extent of the Baltic Sea ice (BMI) is char- 4 Summary and conclusion acterized by a bi-modal pdf with maximum likelihoods near 10 and 100% ice covered[13]. We transform the orig- Discontinuities at the data boundaries cause the

Figure 4 Wavelet power spectrum of BMI indices is obtained by using: (a) full-length data, (b) “zero-padding” method, (c) “even-padding” method, and (d) our method. The black contour designates the 95% significance level against red noise, and COI is just the region below the thick line. 198 ZHANG ZhiHua, et al. Adv Polar Sci September(2011) Vol.22 No.3

distortion of its wavelet power spectrum. The even- Environment Key Laboratory. extension method guarantees that the time series is con- tinuous at the endpoints. However, the first derivatives References on the endpoints are still discontinuous. In contrast with 1 Torrence C, Compo G P. A practical guide to wavelet anal- the even-extension method, our “linear removal+odd ex- ysis. Bull Am Meteorol Soc, 1998, 79: 61–78 tension” method has a continuous first derivative, so 2 Foster G. Wavelets for period analysis of unevenly sampled our algorithm can overcome boundary effect better than timeseries. Astron J, 1996, 112: 1709–1729 “even extension”. The desirable quality of continuous 3 Frick P, Baliunas S L, Galyagin D, et al. Waveletanalysis of time derivative means that our “linear removal + odd ex- stellar chromospheric activity variations. Astrophys J, 1997, 483: 426–434 tension” is a good choice of padding for time series that 4 Sweldens W. The lifting scheme, a construction of second have significant first order autoregressive components, or generationwavelets. SIAM J Math Anal, 1998, 29: 511–546 exhibit persistence (such as a Markov or fractional Gaus- 5 Johnson R W. Edge adapted wavelets, solar magnetic ac- sian process). tivity, andclimate Change. Astrophys Space Sci, 2010, 326: We tested the new method of “linear removal+odd 181–189 extension” with two different real-world climate proxies. 6 Daubechies I. Ten Lectures on Wavelets, CBMS-Conference Lecture Notes, Philadelphia, SIAM, 1992 One of which was unusually non-Normal in its pdf. We 7 Liu Y, Liang X S, Weisberg R H. Rectification of the bias extracted the middle parts of these times series and com- in thewavelet power spectrum. J Atmos Oceanic Technol, pared different methods of testing significance levels near 2007, 24: 2093–2102 the data boundaries with the known significant regions 8 Mann M E, Lees J M. Robust estimation of background from the complete time series. It is clear that the new noise andsignal detection in climatic time series. Climate method produces more reliable estimations of the signifi- Change, 1996, 33: 409–445 9 Grinsted A, Moore J C, Jevrejeva S. Application of the cross- cant regions of the wavelet power spectrum than existing wavelet transform and wavelet coherence to geophysical time alternatives. We therefore commend this new method as series. Nonlin Processes Geophys, 2004, 11: 561–566 worthwhile. 10 Jevrejeva S, Moore J C, Grinsted A. Influence of the Arc- No method can forecast future behavior completely tic Oscillation and El Nino-Southern Oscillation (ENSO) on accurately, especially if the considered time series is re- ice conditions in the Baltic Sea: The wavelet approach. J Geophys Res, 2003, 108(D21): 4677–4687 lated to climate which is non-stationary and potentially 11 Meyers S D, Kelly B G, O’Brien J J. An introduction non-linear. As a general rule, we recommend using ad- towavelet analysis in oceanography and meteorology: With ditional techniques to confirm possibly marginally sig- application to the dispersion of Yanai waves. Mon Wea Rev, nificant features affected by the COI, such as singular 1993, 121: 2858–2866. spectrum analysis[14] or Granger causality[15] that may 12 Vinther B M, Johnsen S J, Andersen K K, et al. NAO sig- nal recorded in the stable isotopes of Greenland ice cores. have different advantages and shortcomings. Geophys Res Letters, 2003, 30: 1387–1390 13 Seina A, Gronvall H, Kalliosaari S, et al. Ice seasons 1996– 2000 in Finnish sea areas /Jaatalvet 1996–2000 Suomenmeri- alueilla. Meri, Report Series of the Finnish Inst. of Marine Acknowledgments This research was partially supported by Res., 2001, 43 the National Key Science Program for Global Change Research (Grant no. 2010CB950504), the National High-Technology Re- 14 Ghil M, et al. Advanced spectral methods for climatic time search & Development Program of China (863 Program, Grant series. Rev Geophys, 2002, 40: 1003–1043 no. 2010AA012305), the National Natural Science Foundation of 15 Granger C W J. Investigating causal relations by economet- China(Grant no. 41076125), the Fundamental Research Funds for ric modelsand cross-spectral methods. Econometrica, 1969, the Central Universities (Key Program), and the Polar Climate and 37: 424–438 · Article · Advances in Polar Science

doi:10.3724/SP.J.1085.2011.00199 September 2011 Vol.22 No.3 199–204

Elevation determination of nunataks in the Grove Mountains

WANG ZeMin∗, AI SongTao, ZHANG ShengKai & DU YuJun

Chinese Antarctic Center of Surveying and Mapping, Wuhan University, Wuhan 430079, China

Received August 1, 2011; accepted August 16, 2011

Abstract A majority of the exposed nunataks located in the Grove Mountains of the Antarctic interior have yet to have had their elevations measured. The elevations of Mason Peak and Wilson Ridge were precisely deter- mined by the Grove Team of the 26th CHINARE in 2010, with Mason Peak turning out to be the highest of the Grove Mountains. Considering that both Mason Peak and Wilson Ridge are difficult to climb because of their cragginess, we first selected three control points on the ice surface near Mason Peak and positioned them with GPS. Thus, accurate elevations of Mason Peak and Wilson Ridge could be calculated from three directions using forward intersection and trigonometric leveling of a high-precision theodolite at the chosen control points. The results provide basic geodetic information that can be referred to as high-precision control points for surveying and mapping in this part of Antarctica. This paper elaborates on the process of measurement and computation of the mountains summit elevations, and also analyzes the details of the principal elements influencing the accuracy of trigonometric leveling, the determination of refraction coefficients k, and observations of structure and distance.

Keywords Grove Mountains, Mason Peak, Wilson Ridge, trigonometric leveling, GPS, forward intersection

Citation: Wang Z M, Ai S T, Zhang S K, et al. Elevation determination of nunataks in the Grove Mountains. Adv Polar Sci, 2011, 22: 199–204, doi: 10.3724/SP.J.1085.2011.00199

0 Introduction During four previous CHINARE surveys in the Mason Peak and Wilson Ridge are among the highest Grove Mountains, surveyors have carried out fieldwork on of the 64 exposed nunataks located in ice and snow ice surface topography and numerous exposed nunataks. fields about 2 000 m asl in the Grove Mountains, in Due to access difficulties caused by rugged topography the hinterland of East Antarctica. The relative heights and harsh weather conditions, the elevations of Mason of these nunataks, and in particular the determination Peak and Wilson Ridge had not been precisely deter- of the highest, had not been verified by previous sur- mined prior to this work. Despite preliminary estimates veys in the area. Through precise surveying and mea- provided by surveys decades ago, the elevation of Mason suring methods, the Grove Mountain Team of the 26th Peak based on satellite imagery had a large relative er- CHINARE (Chinese National Antarctic Research Expe- ror, e.g., a 200 m elevation gap exists between the two dition) in 2009/2010 successfully determined the eleva- maps published by the Australian Antarctic Division. tion of the summit of Mason Peak (2 362.9 m asl) show- Considering the difficulty of climbing Mason Peak ing that it is the highest nunatak in the Grove Moun- and Wilson Ridge, we first selected three control points tains, whereas the maximum elevation of Wilson Ridge on the ice surface near Mason Peak and positioned them is 2 325.1 m asl. with GPS. Thus, accurate elevations of Mason Peak and

∗Corresponding author (email:[email protected]) journal.polar.gov.cn 200 WANG ZeMin, et al. Adv Polar Sci September(2011) Vol.22 No.3

Wilson Ridge could be calculated from three directions using forward intersection and trigonometric leveling by setting a high-precision theodolite on the chosen control points.

1 Control point measurements and calcu- lations

To precisely determine the elevation and plane position Figure 1 Sketch map of elevation determination of of Mason Peak, we first set up a highly-accurate control nunataks in Grove Mountains. point MS1 at the foot of Mason Peak using comprehen- sive GPS measurements over a 12-hour continuous ob- tion is shown in Table 1. servation period. Then, based on MS1, two more GPS The coordinates of the two IGS stations were ac- control points MS2 and MS3 were established using GPS quired from the SOPAC website[1]. The coordinates RTK (Real Time Kinematic ) (Figure 1). of Zhongshan Station were calculated based on DAV1, The precise coordinates of MS1 were calculated with MAW1 and SYOG, and after adjustment with the global GAMIT/GLOBK. To do this, we chose two known IGS network of IGS stations, the coordinates for the first day sites near Zhongshan Station, e.g., DAV1 (Davis Station) of the year 2010 were obtained. and MAW1 (Mawson Station), and Zhongshan Station as Using DAV1, MAW1 and ZHON as control points, a tracking station ZHON. We then used these three sites we obtained a baseline for MS1 and then referenced its as origin points to calculate the coordinates of MS1. coordinates to WGS84. The precision of the GPS base- The coordinates of the three known stations above line, coordinates and precision of MS1 is shown in Tables are based on ITRF2000, 2010.001. The epoch informa- 2 and 3.

Table 1 Known stations and coordinates (ITRF2000, 2010.001 Epoch)

Site Station style X Y Z DAV1 IGS station 486 854.566 2 285 099.237 –5 914 955.733 MAW1 IGS station 1 111 287.188 2 168 911.229 –5 874 493.633 ZHON GPS tracking station 531 375.205 2 190 320.319 –5 946 671.112

Table 2 Precision of GPS baseline permitted the determination of the forward intersection Baseline Baseline length Root mean square positions of the theodolites and precisely measured the L/m error σ/m plane positions of the two mountain peaks. We used a MS1-ZHON 391 203.832 0 0.003 1 WILD T2 theodolite to measure the angles; two obser- vation sets were required, which resulted in angle mea- Table 3 Coordinate and precision of MS1 surements with an accuracy of <200. σ σ σ Site (X/ x)/m (Y/ y)/m (Z/ z)/m The formulas for forward intersection are as follows MS1 503 359.259 1 821 231.114 -6 073 275.652 (Figure 2)[2]. 0.007 0.007 0.016 xAcotβ + xBcotα − yA + yB xP = 2 Determining the plane position of the cotα + cotβ yAcotβ + yBcotα + xA − xB nunataks using forward intersection yP = cotα + cotβ To measure specific elevations, we first determined the For easy calculation using the formulas above, we plane position of the two nunataks. We set up theodo- purposely projected the geodetic coordinates of MS1, lites at stations MS1, MS2, MS3 to measure the hor- MS2, MS3 to a Gaussian coordinate system. To mini- izontal angle of Mason Peak and Wilson Ridge. This mize deformation in the survey area by projection, we Elevation determination of nunataks in the Grove Mountains 201

◦ chose the mean longitude as the central meridian L0=75 and the mean elevation as 2 100 m (set as the as elevation datum for the projection). The precise plane position and accuracy index are shown in Table 4. The accuracy of the plane position obtained by for- ward intersection for Mason Peak is better than Wilson Ridge due to the differences in the observation structure for the two mountains. For Mason Peak, the observ- ing stations of MS1, MS2, and MS3 were good enough for positioning during intersection, whereas the distance from the control points to Wilson Ridge is too long for Figure 2 Sketch map of angular forward intersection. an accurate positioning. In addition, the top of Wilson

Table 4 Coordinates and precision of Mason Peak and Wilson Ridge Target Latitude Accuracy/m Longitude Accuracy/m Mason Peak –72◦48059.7600 §0.219 74◦40040.4100 §0.189 Wilson Ridge –72◦48051.2200 §1.808 75◦02035.7300 §0.450

Ridge lacks undulatory features, which makes it diffi- cult to align and the aiming accuracy that is relatively low.

3 Measuring the height of a nunatak sum- mit using trigonometric leveling

3.1 Basic formulas for trigonometric leveling

The basic principles and formulas for calculating height differences by trigonometric leveling have been discussed in detail in surveying reports[3].

In Figure 3, suppose s0 is the observational horizon- tal distance between points A and B. The instrument is put on point A, and the instrument elevation is deter- mined to be i1. Point B is the point used for alignment 0 0 and the target elevation is v2; R is the radius of A B on the reference ellipsoid. PE and PF are the geoids that pass through points P and A, respectively. P C is the Figure 3 Principles of trigonometry leveling. tangent line to PE that passes point P, and PN is the points A and B is: curve of the optical path. Because of the effect of atmo- spheric refraction, when the telescope at point P aims in h1,2 = BF = MC + CE + EF − MN − NB (1) the direction of PM (i.e., tangent to PN), the rays pro- jected from point N arrive just at the horizontal line of where EF is the instrument elevation i1, NB is the target the telescope. This means that when the instrument is elevation v2, and CE and MN are the earth curvature put on point A, the vertical angle between points P and and the effect of atmospheric refraction.

M is α1,2. 1 2 1 2 Figure 3 shows that the height difference between CE = s0, MN = 0 s0 2R 2R 202 WANG ZeMin, et al. Adv Polar Sci September(2011) Vol.22 No.3 where R0 is the radius of optical path P N at point N. If that occur in the polar regions are extremely strong. To- R we let = K, then: d gether, these elements result in an atmosphere that is R0 even and steady, which facilitates the measurement of 1 R K MN = · S2 = S2 vertical angles all day long. 2R0 R 0 2R 0 According to the principles of atmospheric physics where k is defined as the refraction coefficient. and combining them with abundant statistics of mete- Due to the extremely low ratio of the horizontal orological observation, Brocks[4] produced a formula for distance between points A and B, s0 to radius R, PC calculating the refraction coefficient k: is supposed to be approximately vertical to OM, i.e., ∼ ◦ P CM =90 . Thus, ∆P CM is regarded as a right tri- P 1 ∆H k = 6.706 (3.42 + τ) 1 − (3.42 + 2τ) sinZ (3) 2 h i angle. Then MC in formula (1) is: T 3 T where P is atmospheric pressure (units: mmHg), T is MC = s0tanα1,2 absolute temperature (K) at the ground station, τ is the and the height difference between A and B is: vertical gradient of temperature [units: ◦C·(100 m)−1], 1 2 − K 2 − ∆H is the height difference between a low observation h1,2 = s0tanα1,2 + s0 + i1 s0 v2 2R 2R station and a high observation station (units: 100 m), − 1 K 2 = s tanα , + s + i − v (2) and Z is the zenith distance between the two stations. 0 1 2 2R 0 1 2 Brocks’ formula, derived for the situation where the rays From the formula above we conclude that the ele- are through a “free” atmosphere, works for the calcula- ments that influence the accuracy of trigonometric level- tion of k when the majority of the observation rays exceed ing are mainly the observational accuracy of the vertical the ground surface in a relatively large value. This for- angle and the verifying accuracy of the refraction coeffi- mula was applied to measure the elevation of Mount Qo- cient k. molangma many times and has been proven reliable[5−6]. 3.2 Determining the atmospheric refraction coe- It also works for the Grove Mountains with a mean ele- k fficient vation of over 2 000 m. Vertical refraction arises when rays pass through an at- Due to the limitations of the research conditions, we mosphere with variable density. The determining factor did not manage to release balloons to measure the tem- for the gradient of atmosphere density is the gradient of perature gradient during this expedition to the Grove atmospheric temperature, τ. Thus dealing with the verti- Mountains. So, to calculate refraction coefficients, we cal refraction problem, especially measuring the gradient applied the vertical gradient of mean temperature in the of atmospheric temperature, is of great importance in troposphere: obtaining acceptable trigonometric leveling results. The − ◦ atmospheric refraction coefficient k varies according to τ = 0.65 C/(100 m) (4) area, climate, season, ground cover and the height above By measuring atmospheric pressure and tempera- the ground. At present, it is impossible to precisely mea- ture, and by observing zenith distance, using formulas sure the value of k. Experiments that have been carried (3) and (4), we calculated a refraction coefficient for the out in China show that the value of k is the lowest and Grove Mountains: relatively steady about noon[5]; the value of k is rela- tively high and changes quickly during sunrise and sun- k = 0.15 (5) set. Therefore, the best time for observing vertical angles is 10:00 to 16:00 local time when the value of K recorded Consider that the distances from MS1, MS2 and is generally 0.08–0.14. However, the situation at Antarc- MS3 to Mason Peak are 5.1–5.6 km, and a 0.01 error tica, where the surface is dominated by ice and snow, is in k, leads to a 0.02 m height error in the Mason Peak el- different. The snow surface absorbs little heat, the po- evation. Therefore, the effect of the refraction coefficient lar day lasts for many months, and the katabatic winds at Mason Peak can be neglected. Elevation determination of nunataks in the Grove Mountains 203

The distances from MS1, MS2 and MS3 to Wil- as highly-precise control points for Antarctic surveying son Ridge are comparatively longer (i.e., about 15.4–17.1 and mapping. km), and due to a 0.01 error in k, this leads to a 0.2 m The geoid elevation of the summit of the Mason Peak height error in the Wilson Ridge elevation. The effect of is 2 363 m asl. It is the highest peak in the Grove Moun- the refraction coefficient on Wilson Ridge is too large to tains, 20 m higher than Mount Harding in the core area [7] be neglected. of the Grove Mountains . To improve the accuracy of elevation determina- 3.3 Determination of the Mason Peak and Wilson tions, we set instruments on three chosen control points Ridge summit elevations to observe the vertical angles in Mason Peak and Wilson As described above, WILD T2 theodolite stations at Ridge, enabling trigonometric leveling from three direc- MS1, MS2, and MS3 near Mason Peak were used to de- tions using redundant measurements. Thus the accuracy termine the precise elevation of Mason Peak and Wilson and reliability are both improved. Ridge using trigonometric leveling. We carried out two The property that affects the observational accuracy observation sets for the angles resulting in an accuracy of trigonometric leveling the most is the determination of angle measuring of §200. accuracy of the refraction coefficient k. In the Grove An error of §200 for an angle measurement can intro- Mountains, surrounded by the snow- and ice-fields of duce a §0.05 m height error to the Mason Peak elevation Antarctica, the snow surface primarily reflects sunlight and §0.16 m to that of Wilson Ridge. while absorbing little energy. In addition, the wind force After putting the observed values of vertical angles is extremely strong during the Antarctic day. Therefore, in Table 5 into formula (2), we calculated the elevations these elements all tend to make the atmosphere more of Mason Peak and Wilson Ridge. Comprehensively con- even and steady, which is not only helpful for the mea- surement of vertical angles all day long, but also enables sidering the observational error and atmosphere vertical the use of a reliable and fundamental theoretical formula refraction error, and after the application of appropriate that based on the freedom of the atmosphere to calcu- adjustments for redundant measurements, we derive an late refraction coefficients. Therefore, a refraction coeffi- evaluation of precision (Table 6). cient with a relatively high accuracy can also be obtained Table 5 Results of vertical angle observation without measuring the temperature gradient. This shows that shortening the distance from the observation site to Target Site MS1 MS2 MS3 the target is an efficient way to weaken the effect of at- Mason Peak 5◦4801800 6◦0905100 6◦0900600 mospheric refraction. Wilson Ridge 1◦4505900 1◦4501900 1◦5200000

Table 6 Elevation results of Mason Peak and Wilson Ridge Acknowledgements We thank all the members of Grove Moun- tains Team of the 26th CHINARE for their great help, and the Target Geoid elevation/m Precision/m Chinese Arctic and Antarctic Administration, SOA for field logis- Mason Peak 2 362.869 §0.179 tic support during the Grove Mountains expedition. This work Wilson Ridge 2 325.078 §0.418 was supported by the Polar Science Strategic Research Founda- tion of China (Grant no. 20080102), and the National High Tech- nology Research and Development Program of China (Grant no. 4 Conclusion 2009AA12Z133).

We have precisely determined the geoid elevation in the References area of Mason Peak and Wilson Ridge by combining GPS and trigonometric leveling using theodolite measuring 1 http://sopac.ucsd.edu/cgi-bin/sector.cgi angles. This has solved a local puzzle of geodetic heights. 2 Pan Z F, Yang Z R, et al. Principles and Methods of Digital The result provides information that can be referred to Mapping. Wuhan: Wuhan University Press, 2004 204 WANG ZeMin, et al. Adv Polar Sci September(2011) Vol.22 No.3

3 Chen J, Bo Z P. Geodesy for application. Beijing: Surveying 6 Chen J Y. On vertical refraction in Qumolongma Feng. Acta and Mapping Press, 1989 Geodaetica et Cartographica Sinica,1999, 22 (2) 4 Brocks K. Moteorolgische Hilfsmittel fur die geodatlische 7 Peng W J, Ding S J, et al. The application of post processing Hohenmessung. ZFV, 1950 differential GPS in plotting at Antarctic Grove Mountains. 5 Chen J Y. Height Determination of Qumolongma Feng. Bul- Chinese Journal of Polar Research, 2001, 13(4) letin of Surveying and Mapping, 1974 (4) Information for authors by the corresponding author, changes of authorship or in the order of the authors listed will not be accepted by Editorial Office of Advances in Polar Science. The copyright covers the exclusive right to reproduce Advances in Polar Science (APS) is an international, peer- and distribute the article, including reprints, microform, elec- reviewed journal jointly sponsored by the Polar Research In- tronic form (offline, online) or other reproductions of similar stitute of China and the Chinese Arctic and Antarctic Ad- nature. ministration. All articles published in this journal are protected by APS is a comprehensive academic journal dedicated to copyright, which covers the exclusive rights to reproduce and presentation of multi-disciplinary achievements in Arctic and distribute the article (e.g. as offprints), as well as all trans- Antarctic expeditions and research. Its primary purpose is lation rights. No material published in this journal may be to publish achievements in fundamental research, applied re- reproduced photographically or stored on microfilm, in elec- search and high-tech research focused or based on the polar tronic databases, video disks, etc., without first obtaining regions, and to report the latest discoveries, inventions, the- written permission from the publisher (respective to the copy- ories and methodologies in polar research. The scope of the right owner if other than the Editorial Office of Advances in journal covers a range of disciplines including polar glaciology, Polar Science). The use of general descriptive names, trade polar oceanography, polar atmospheric science, polar space names, trademarks, etc., in this publication, even if not specif- physics, polar geology, polar geophysics, polar geochemistry, ically identified, does not imply that these names are not pro- polar biology and ecology, polar medicine, Antarctic astron- tected by the relevant laws and regulations. omy, polar environment observation, polar engineering as well While the advice and information in this journal is be- as polar information service and management. lieved to be true and accurate at the date of publication, APS is a quarterly journal published in March, June, neither the authors, the editors, nor the publisher can accept September and December by the China Science Press and cir- any legal responsibility for any errors or omissions that may culated both nationally and internationally. Formerly known occur. The publisher makes no warranty, express or implied, as the Chinese Journal of Polar Science (English Edition), with respect to the material contained herein. founded in 1990, this journal will be known as Advances in Polar Science from Volume 22 (2011) on wards. The journal contains several sections: Reviews: Summarizes representative results and You can submit a manuscript by email. achieve-ments in a particular topic or an area, comments on the current situation, and advice on research directions. Au- thor opinions and related discussions are invited. Articles: Reports important original results in all areas ISSN: 1674-9928 of polar science. CN: 31-2050/P Letters: Briefly presents novel and innovative findings Price: RMB 30.00 (US $ 5.00) related to polar science. For more subscription information, please contact: Trends: Reports important scientific news, informa- Shanghai Branch of China Science Press tion, and academic affairs, as well as major international pro- 270 Fenglin Road, Shanghai 200032, China grams or conferences in all areas of polar science. Tel: +86 21 64042791, Fax: +86 21 64042791

Submission of a manuscript implies: that the work de- scribed has not been published before (except in the form of An electronic version is available at the websites: an abstract or as part of a published lecture, review or thesis); http://journal.polar.gov.cn/EN/volumn/current.shtml, that it is not under consideration for publication elsewhere; http://eng.cnki.net/grid2008/index.htm. that its publication has been approved by all co-authors, if any, as well as—tacitly or explicitly—by the responsible au- thorities at the institution where the work was carried out. Editorial Office of Advances in Polar Science The author warrants that his/her contribution is original 451 Jinqiao Road, Shanghai 200136, China and that he/she has full power to make this grant. The author Tel: +86 21 58713642 or +86 21 58713650 signs for and accepts responsibility for releasing this material Fax: +86 21 58713642 on behalf of any and all co-authors. Transfer of copyright E-mail: [email protected] to the Editorial Office of Advances in Polar Science becomes effective if and when the article is accepted for publication. After submission of the Copyright Transfer Statement signed Copyright°c 2011 EditorialOffice ofAdvances in Polar Science Information for authors by the corresponding author, changes of authorship or in the order of the authors listed will not be accepted by Editorial Office of Advances in Polar Science. The copyright covers the exclusive right to reproduce Advances in Polar Science (APS) is an international, peer- and distribute the article, including reprints, microform, elec- reviewed journal jointly sponsored by the Polar Research In- tronic form (offline, online) or other reproductions of similar stitute of China and the Chinese Arctic and Antarctic Ad- nature. ministration. All articles published in this journal are protected by APS is a comprehensive academic journal dedicated to copyright, which covers the exclusive rights to reproduce and presentation of multi-disciplinary achievements in Arctic and distribute the article (e.g. as offprints), as well as all trans- Antarctic expeditions and research. Its primary purpose is lation rights. No material published in this journal may be to publish achievements in fundamental research, applied re- reproduced photographically or stored on microfilm, in elec- search and high-tech research focused or based on the polar tronic databases, video disks, etc., without first obtaining regions, and to report the latest discoveries, inventions, the- written permission from the publisher (respective to the copy- ories and methodologies in polar research. The scope of the right owner if other than the Editorial Office of Advances in journal covers a range of disciplines including polar glaciology, Polar Science). The use of general descriptive names, trade polar oceanography, polar atmospheric science, polar space names, trademarks, etc., in this publication, even if not specif- physics, polar geology, polar geophysics, polar geochemistry, ically identified, does not imply that these names are not pro- polar biology and ecology, polar medicine, Antarctic astron- tected by the relevant laws and regulations. omy, polar environment observation, polar engineering as well While the advice and information in this journal is be- as polar information service and management. lieved to be true and accurate at the date of publication, APS is a quarterly journal published in March, June, neither the authors, the editors, nor the publisher can accept September and December by the China Science Press and cir- any legal responsibility for any errors or omissions that may culated both nationally and internationally. Formerly known occur. The publisher makes no warranty, express or implied, as the Chinese Journal of Polar Science (English Edition), with respect to the material contained herein. founded in 1990, this journal will be known as Advances in Polar Science from Volume 22 (2011) on wards. The journal contains several sections: Reviews: Summarizes representative results and You can submit a manuscript by email. achieve-ments in a particular topic or an area, comments on the current situation, and advice on research directions. Au- thor opinions and related discussions are invited. Articles: Reports important original results in all areas ISSN: 1674-9928 of polar science. CN: 31-2050/P Letters: Briefly presents novel and innovative findings Price: RMB 30.00 (US $ 5.00) related to polar science. For more subscription information, please contact: Trends: Reports important scientific news, informa- Shanghai Branch of China Science Press tion, and academic affairs, as well as major international pro- 270 Fenglin Road, Shanghai 200032, China grams or conferences in all areas of polar science. Tel: +86 21 64042791, Fax: +86 21 64042791

Submission of a manuscript implies: that the work de- scribed has not been published before (except in the form of An electronic version is available at the websites: an abstract or as part of a published lecture, review or thesis); http://journal.polar.gov.cn/EN/volumn/current.shtml, that it is not under consideration for publication elsewhere; http://eng.cnki.net/grid2008/index.htm. that its publication has been approved by all co-authors, if any, as well as—tacitly or explicitly—by the responsible au- thorities at the institution where the work was carried out. Editorial Office of Advances in Polar Science The author warrants that his/her contribution is original 451 Jinqiao Road, Shanghai 200136, China and that he/she has full power to make this grant. The author Tel: +86 21 58713642 or +86 21 58713650 signs for and accepts responsibility for releasing this material Fax: +86 21 58713642 on behalf of any and all co-authors. Transfer of copyright E-mail: [email protected] to the Editorial Office of Advances in Polar Science becomes effective if and when the article is accepted for publication. After submission of the Copyright Transfer Statement signed Copyright°c 2011 EditorialOffice ofAdvances in Polar Science