Journal of Asian Earth Sciences 114 (2015) 497–503

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Journal of Asian Earth Sciences

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Cooling trend over the past 4 centuries in northeastern waters as revealed by alkenone-derived SST records

Deming Kong a,b, Yuen-Yeung Tse a, Guodong Jia c, Gangjian Wei c, Min-Te Chen d, Yongqiang Zong a, ⇑ Zhonghui Liu a, a Department of Earth Sciences, The University of Hong Kong, Hong Kong, China b Guangdong Province Key Laboratory for Coastal Ocean Variation and Disaster Prediction, Guangdong Ocean University, Zhanjiang 524088, China c CAS Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China d Institute of Applied Geosciences, National Taiwan Ocean University, Keelung 20224, Taiwan article info abstract

Article history: The climate history over the past few centuries is important to be used to assess how regional climate Received 29 January 2015 responds to global forcing. Here we report three high-resolution alkenone-based sea surface temperature Received in revised form 11 May 2015 (SST) records over the past 4 centuries from three sediment cores collected in the Mirs Bay, northeastern Accepted 22 May 2015 Hong Kong. All three SST records consistently show a general cooling trend toward the present, with most Available online 14 June 2015 of cooling occurring over the last century. Alkenone-derived SST values stayed around 26.5–27 °C at the three sites prior to 1900s and decreased into the range of 25–26 °C. The magnitude of cooling approxi- Keywords: mately from the Little Ice Age (LIA) to present tends to be dampened from 2 °C nearshore to 1 °C off- Mirs Bay shore. The cooling trend, as identified in all three SST records, is thus opposite to the global temperature Upwelling Alkenone rise over the last century. Assisted with modern observations, we interpret that the alkenone-derived SST Summer monsoon reflects increasing upwelling in the Mirs Bay, which likely results from the strengthened East Asian sum- Global warming mer monsoon, in the context of global warming over the last century. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction greatly affected by regional climatic processes (Bao and Ren, 2014; Mann et al., 1998, 1995). For instance, several long-chain Climatic changes over the past few centuries have been used as alkenone-derived SSTs reveal notable cooling in the Yellow Sea key reference in the assessment and projection of climate change and East China Sea over the last 3 centuries (He et al., 2014). In under anthropogenic impact (IPCC, 2013; Mann et al., 1998). The some coastal areas, proxy-based SSTs exhibit significant cooling global climate is characterized with unprecedented temperature during the 20th century (Gutiérrez et al., 2011; McGregor et al., rise since the late 19th century as suggested by numerous instru- 2007). Particularly, observations and model simulations suggest mental records and the Intergovernmental Panel on Climate that surface temperatures over land rise more than over the ocean Change (IPCC) Assessment Report (IPCC, 2013). The start of global with a warming climate (Bakun, 1990; Dong et al., 2009; Sun et al., warming in the late 19th century is also the end of a relative cool 2010; Sutton et al., 2007). It is proposed that this different warm- period (1450 to 1850 AD), namely the ‘‘Little Ice Age’’ (LIA) ing over land and ocean could lead to stronger Asian summer mon- (Bradley and Jonest, 1993; Mann et al., 2009; Wanner et al., soon (Hu et al., 2000; Wang et al., 2012a), although other studies 2008). Rapid warming from the LIA to present has been extensively suggest weaker wind speed and monsoon precipitation over studied to test the response of regional climate and environment, China in recent several decades (Guo et al., 2011; Liu et al., 2011). such as monsoons, precipitation and regional surface temperature, The (SCS), a tropical marginal sea located to the global climate forcing (Bradley and Jonest, 1993; Jones and between the western Pacific and the Asian continent, is strongly Mann, 2004; Mann, 2007; Oppo et al., 2009). affected by the reversing East Asian monsoons. Paired Holocene Despite of the overall warming on Earth’s surface, temperature SST records from open ocean and the coast of the northern SCS variations are found in great divergence in various regions, and indicate a significant cooling trend at the coast toward the late Holocene, perhaps with maximum cooling during the LIA (Kong et al., 2014), suggesting that coastal regions might have responded ⇑ Corresponding author. Tel.: +852 2859 2831; fax: +852 2517 6912. to monsoonal changes more sensitively. However, due to limited E-mail address: [email protected] (Z. Liu). data resolution, coastal SST changes at centennial timescales could http://dx.doi.org/10.1016/j.jseaes.2015.05.026 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. 498 D. Kong et al. / Journal of Asian Earth Sciences 114 (2015) 497–503 not be assessed. To better understand coastal climatic changes over temperature and salinity is almost homogenous in winter, due to the past few centuries, we here reconstruct three high-resolution the strong winter monsoon-induced mixing effect (Yuan et al., alkenone-derived SST records from the Mirs Bay, northeastern 2011). Hong Kong, and assess the possible effects of East Asian monsoons on coastal SST changes. 3. Material and methods

2. Oceanographic settings Three short gravity cores, T3 (22°2605400N, 114°1504300E), T6 (22°2802400N, 114°2101000E) and M10 (22°2705500N, 114°2302400E), The study region is located in the Mirs Bay (22.50°N,114.70°E), were collected from the Mirs Bay of Hong Kong in September a semi-enclosed bay in the northeast of Hong Kong, northern South 2012 (Fig.1). The water depth at sampling sites T3, T6 and M10 China Sea (Fig. 1). The Mirs Bay is embedded into the terrene are 11, 15 and 20 m. The three core lengths are 66, 80 and 18 km between Hong Kong and in , 69 cm, respectively. All three cores were subsampled at 1 cm inter- covering an area 320 km2 (Li et al., 2014). The water depth is gen- val on board. erally less than 22 m, relatively deep in the southwest and shallow All the samples were freeze-dried and extracted following the in the northeast (Li et al., 2014). There is no big river discharging standard procedure described in Kong et al. (2014). The lipid frac- into the bay. The SST and salinity at the marine observation station tion containing alkenones was analyzed on an Agilent 7890 GC-FID

MM17 in the bay (Fig.1) range between 16–31 °C and 20–34 psu, at the University of Hong Kong. C36 n-alkane was used as with annual mean at 24 °C and 32 psu respectively (Hong internal standard for quantification of alkenones. The alkenone Kong EPD, www.wqrc.epd.gov.hk). K0 unsaturation index (U37) is calculated based on the definition: The temperature and salinity in the Mirs Bay and adjacent 0 UK =C /(C +C ), where C and C are contents of the waters around Hong Kong are strongly influenced by the season- 37 37:2 37:2 37:3 37:2 37:3 di- and tri-unsaturated C alkenones respectively (Prahl et al., ally reversing monsoons. The East Asian summer monsoon 37 K0 (EASM) lasts from mid-May to mid-September, with maximum 1988). SST was calculated using the U37-SST calibration equation: K0 intensity in June, July and August (Chu and Wang, 2003; Yin, U37 = 0.031 ⁄ SST + 0.092 for the SCS region (Pelejero and Grimalt, 2002). When summer monsoon prevails, southwest wind drives 1997). Replicate injections of standards in different batches show the surface water moving northeastward along the shore (Fang analytical errors less than 0.3 °C for the calculated SST (Kong et al., 2012; Su, 2004). It causes an offshore movement in the sur- et al., 2014). face water owing to the Ekman Effect. As a result, cooler and saltier The chronologies of core T6 and M10 were established based on bottom oceanic water intrudes shoreward and appears as a weak 210Pb and 137Cs dating. The radioactivity of 210Pb and 137Cs were upwelling (Jing et al., 2009; Yin, 2002; Zu and Gan, 2015). detected using gamma spectrometry equipped with a germanium Because of this upwelling effect, surface waters in the Mirs Bay, detector at the Institute of Polar Environment, the University of as well as the region to the northeast of Hainan Island, appear to Science and technology, China. The detailed procedure and param- be cooler by 1–2 °C, than surrounding waters (Jing et al., 2009). eters used are described by Xu et al. (2010) and He et al. (2014).As Despite the weak upwelling caused by summer monsoon, vertical 210Pb dating failed at a nearby core, possibly due to sediment dis- temperature gradient in the Mirs Bay reaches the maximum in turbance by human activities, 210Pb and 137Cs analyses were not summer time. Meanwhile, bottom and middle layer salinity reach conducted at core T3. Instead, the chronology of this core was ten- the maximum while surface salinity decreases apparently (Yin, tatively assigned through correlation with core T6, based on iden- 2002). In contrast to summer, the vertical distribution of tifiable common features in both SST records (Fig. 3).

Fig. 1. Location map of the study area, the Mirs Bay, Hong Kong. Core sites T3, T6 and M10 are indicated by stars and the HK observatory and marine observation station MM17 by filled circles. D. Kong et al. / Journal of Asian Earth Sciences 114 (2015) 497–503 499

4. Results 0 10 20 30 40 50 60 70 80

T3 4.1. Age models 27

The excess 210Pb (Fig.2) was processed using the CRS (Constant 26 Rate of Supply) model (Xu et al., 2010). Calculated sedimentation 210 25 rate based on the excess Pb is 0.26 cm/yr and 0.17 cm/yr at core 2000 AD 1900 AD 1800 AD 27.0 M10 and T6 respectively. The sedimentation rates of core T6 and 26.5 M10 are comparable to previous results based on 210Pb in this area (Owen and Lee, 2004). T6 26.0 Because the 137Cs radioactivity in analyzed samples was not 137 25.5 high enough to identify peak concentration, Cs results were 2000 AD 1900 AD 1800 AD 27.0 not used to construct the chronology. Despite lack of the age con- 25.0 trol by 137Cs, decreasing profile of excess 210Pb and high radioactiv- 26.5 ity in surface samples suggest that the surface sediments are quite young. Therefore, the top surface sediments were assumed to be 26.0 M10 present (2012 AD). The ages of T6 and M10 were then calculated based on the derived sedimentation rates. The bottom ages were 0 10 20 30 40 50 60 70 80 thus dated back to about 1550 AD and 1740 AD for T6 and M10, Depth (cm) respectively. Based on the correlation to core T6, core T3 at 6, 42 and 56 cm were tentatively assigned ages of 1937, 1707 and Fig. 3. Reconstructed SSTs from cores T3, T6 and M10 plotted against depth. The 1644 AD respectively (Fig. 3), yielding the bottom age of this core converted ages based on 210Pb dating (after 1800 AD) are also indicated for cores T6 at 1600 AD. We anticipate that potentially large uncertainties and M10. Tentative tie points between T3 and T6 are indicated by lines. exist in the derived chronologies. However, all the three sediment cores very likely span part of the LIA and the recent warming per- 210 2000 1950 1900 1850 1800 iod based on the excess Pb. The identified general trending in 24 our SST records, an unexpected finding in this study, should be less Air temperature influenced by chronological uncertainties. Due to chronological 23 uncertainties, secondary fluctuations in the SST records could not 22 27.3 be assessed here. NS02G 21 27.2 26 27.1 4.2. SST 27.0 25 0 Calculated UK -SST values at three sites were plotted against 26.9 37 Annual SST depth in Fig. 3 and against age in Fig. 4. The most prominent, com- mon feature in all three SST records is the general cooling trend 24 6.0 toward present, with much of the cooling occurring over the last Summer wind speed 6.5 century. SST at core T6 varies between 25.5 and 27.1 °C(Fig. 3). SST 7.0 values decreased over the past 4 centuries, with total cooling 27 amplitude of 1.5 °C. Following a relative cool period during 1580–1640 AD, SST started decreasing by 0.5 °C from 1640 to 1900 AD, and dropped significantly from 26.5 to 25.5 °C from 26 1900 AD to present. A minor pause occurred over the steady cool- T6 ing trend around the 1940s and 1950s. SST changes at core T3 M10 mimic changes at T6. SST varies between 24.5 and 27.5 °C. The 25 T3 total cooling amplitude is 2 °C, again with most of cooling occur- 2000 1950 1900 1850 1800 ring over the last century (Fig. 3). SST at M10 decreased from Year (AD) 27 °C to 25.8 °C since 1750 AD, showing good consistency with

Fig. 4. Comparison of reconstructed SSTs from the Mirs Bay with instrumental temperature and wind speed data. Annual mean air temperature is from Hong Kong 210 210 Pb (Bq/kg) Observatory; SST from core NS02G (Kong et al., 2014) is updated with higher Pb (Bq/kg) ex ex resolution; 5-year averaged Extended Reconstruction of SST and summer wind 0 50 100 150 200 250 0 50 100 150 0 0 speed (June, July and August) at 22°N, 114°E are reanalyzed data from apdrc.soest. hawaii.edu (http://apdrc.soest.hawaii.edu/las/v6/dataset). Core T3 chronology was tentatively assigned based on correlations with Core T6. 10 10

20 20 T6 and T3. Most of the cooling at M10 also occurred in the 20th 30 30

Depth (cm) Depth century. Thus the consistent cooling trend from the LIA to present

40 (cm) Depth 40 at all the three sites suggests a common phenomenon in the M10 T6 Mirs Bay over the past few centuries. Further, the cooling 50 50 amplitude also displays a spatial pattern, with larger amplitude at core T3 (2 °C), near shore at Tolo Harbor, than at offshore Fig. 2. Radioactivity of excess 210Pb vs. depth for cores T6 and M10. M10 (1 °C) (Fig. 3 and 4). 500 D. Kong et al. / Journal of Asian Earth Sciences 114 (2015) 497–503

5. Discussion records (Mann, 2007; Mann et al., 2009; Masson-Delmotte et al., 2013; Wanner et al., 2008), as well as the air temperature in K0 5.1. Interpretation on U37-SST in the Mirs Bay Hong Kong (Wong et al., 2011). Annual mean air temperature from the Hong Kong Observatory (HKO) increased by 2 °C from 1884 to The alkenone-producing coccolithophores (Gephyrocapsa ocean- 2013 (Wong et al., 2011)(Fig.4), much greater than the averaged ica and Emiliania huxleyi) are found to live in the upper 150 m of global warming (IPCC, 2013). The Extended Reconstruction of Sea sea water column in open ocean, and they have highest abundance Surface Temperature (ERSST) to the south of Hong Kong (22°N, in the water less than 20 m (Saavedra-Pellitero et al., 2014). 114°E) (data obtained from http://apdrc.soest.hawaii.edu/las/v6/ Unfortunately, the distribution and ecology of coccolithophores constrain?var=286) also indicates a warming trend over the last are still unclear in coastal areas of the northern SCS. As most part century (Fig. 4). of the Mirs Bay is less than 20 m deep, coccolithophores are sup- Therefore, before inferring any climatic implications from the posed to live in the whole water column, rather than uppermost K0 cooling trend in the Mirs Bay, the reliability of the U37-SST proxy K0 surface waters. Consequently, the alkenone proxy U37 could be has to be carefully assessed. The cooling magnitude in the bay, inferred to reflect temperature variability in both bottom and sur- 1to2°C, greatly exceeds the analytical error (0.3 °C). face waters. For the convenience of our discussion, the tempera- Preferential degradation of C37:3 over C37:2 alkenones over time K0 (Gong and Hollander, 1999) would give a warm bias for older ture derived from U37 is still taken as SST in this study. Modern observed seasonal SST near the sampling site T6 (station MM17) samples; however, this effect appears to be relatively small, (Fig. 1) varies from 16 °Cto31°C(Fig. 5), with annual mean at especially when a short time period of a few centuries is con- 24 °C. The SST of the topmost sample at core T6 is 25.5 °C, higher cerned. Further, within the northern SCS, alkenone records from than the observed annual mean surface temperature, as well as the the open ocean core NS02G (19.8°N, 113.9°E) (with updated res- average temperature (22.6 °C) of bottom, middle and surface olution from Kong et al., 2014) and from the continental shelf to water. Therefore, despite lack of studies on seasonal production the south of Hong Kong (Zhang et al., 2013), show either slightly 0 warming trend (Fig. 4) or no identifiable trending over the last in the Mirs Bay, we tend to interpret the UK -SST as 37 century, largely consistent with the global temperature rise. summer-biased mean annual temperature. This would essentially rule out the possible preferential degrada- K0 Substantial cooling in reconstructed U37-SST records from the tion effect as an explanation for the cooling trend identified in Mirs Bay is very uncommon compared to other temperature the Mirs Bay.

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18 18

16 16 Surface 14 14 Bottom 12 Middle 12 10 10 1991/1/1 1991/7/1 1992/1/1 1992/7/1 1993/1/1 1993/7/1 1994/1/1 1994/7/1 1995/1/1 1995/7/1 Date

34 34

32 32

30 30

28 28

26 26 Salinity (psu)

24 24 Salinity (psu) Surface 22 Bottom 22

20 Middle 20

1991/1/1 1991/7/1 1992/1/1 1992/7/1 1993/1/1 1993/7/1 1994/1/1 1994/7/1 1995/1/1 1995/7/1 Date

Fig. 5. Temperature and salinity of the surface, middle and bottom water at the marine station MM17 during 1991–1995. Data from Hong Kong Environmental Protection Department (http://wqrc.epd.gov.hk/tc/water-quality/marine.aspx). D. Kong et al. / Journal of Asian Earth Sciences 114 (2015) 497–503 501

Increasing human activities and nutrient loading to the coast similar to the monthly air temperature pattern in Hong Kong, but might have changed biological responses of coccolithophores to still 1 °C cooler than nearby waters. Bottom water cools down K0 up to 4 °C in July and August. Meanwhile, the bottom and middle environmental variables. However, low-resolution U37-SST records from the Pearl River Estuary (Kong et al., 2014) and the mouth of layer salinity reach the maximum in July and August. These dis- Yangzi River (He et al., 2014), the regions with more nutrient load- tinct features of water temperature and salinity were attributed ing, do not suggest a cooling trend over the last century. Shifts in to the intrusion of oceanic bottom water as a compensation of seasonal distribution or water depth of alkenone production over the offshore movement in surface (Ji et al., 2011; Wang et al., time, which is difficult to verify currently, could also plausibly 2012b; Watts, 1973; Yin, 2002). The surface water temperature explain the cooling trend from the LIA to present. For instance, does not decrease in summer months, partly because the upwel- increased cool season or deep water production toward present ling is not very strong, and the surface water could be warmed might have contributed to the apparent cooling signal. However, up fast by hot atmosphere during this time of the year. However, this phenomenon is not observed in nearby waters or open ocean the vertical changes of temperature and salinity support the occur- (Zhang et al., 2013; Kong et al., 2014). If this interpretation were rence of summer upwelling in the bay. proposed, one has to consider why such biological shift could have Although the upwelling-induced surface cooling appears to be occurred only in the Mirs Bay not nearby waters. Therefore, not very strong over seasonal cycles, inter-annual variability of without further evidence to suggest otherwise, we tend to not bottom and middle layer temperature in summer could be still 0 as large as 4 °C(Fig. 5). We thus speculate that, substantial question the reliability of the UK -SST proxy in this study. 37 EASM variability on centennial timescale, i.e. from the LIA to pre- Instead, we attempt to explain the cooling trend in the Mirs Bay sent, could have significantly affected the mean surface thermal with climatic factors/processes that appear to be specific for this conditions. Numerous proxy records indicate strengthened EASM particular region. during a warmer climate (Hu et al., 2000; Wang et al., 2008; Zhang et al., 2008). The instrumental wind data from the south 5.2. Influences of Asian monsoons of Hong Kong (22°N, 114°E) (data from http://apdrc.soest.hawaii. edu/las/v6/constrain?var=1508), still too short to be directly com- The strong contrast between the decreasing SST and warming pared with our SST records, indicate increasing wind speed toward air temperatures suggests that the coastal thermal condition in present over the last 50 years (Fig. 4). Although discrepancies the Mirs Bay is greatly influenced by factors other than air temper- indeed exist within the instrumental wind datasets (Guo et al., atures. A previous study suggests that the coastal SST in the Pearl 2011; Liu et al., 2011), possibly due to local non-climatic factors, River Estuary was strongly affected by the East Asian winter mon- the one shown here is in line with proxy records showing strength- soon (EAWM) as compared to open ocean SST, on a much longer ened EASM over the last century. Therefore, we speculate that the timescale, from the early to late Holocene (Kong et al., 2014). The strengthened EASM over the last century would have induced overall cool conditions in the Mirs Bay, with most of SST values intensified upwelling and thus surface cooling in the Mirs Bay, an lower than 27 °C over the last few centuries, relative to warm con- area with summer monsoon-induced surface cooling observed in ditions in open ocean (>27 °C at NS02G) (Fig. 4), probably also modern instrumental data (Jing et al., 2009). This proposal is con- reflect the coastal cooling induced by the EAWM. However, this sistent with the observation of stronger cooling at near-shore than concept is difficult to explain the cooling trend in the Mirs Bay over offshore sites (Fig. 4), and might also explain no apparent cooling a short timescale, from the LIA to present. trend over the last few centuries identified to the south of Hong As observational wind data in the Mirs Bay is too short Kong (Kong et al., 2014; Zhang et al., 2013), where summer K0 monsoon-induced surface cooling is minimal. (20 years) to be compared with the U -SST records, records from 37 Notable cooling over the last century has also been found in surrounding areas are used to examine the possible influence of K0 EAWM on the Mirs Bay SST. Coral-derived SST around the Xisha some coastal upwelling areas. U37-SST in the Northwest African Islands reveals declined EAWM velocity over the 20th century coast reveals a steady cooling by 1.2 °C through the 20th century (Liu et al., 2008). A longer coral record from the same area shows (McGregor et al., 2007). SST in the west coast of Peru exhibits sim- weakened EAWM from 1818 to 1954 (Song et al., 2012). ilar decreasing trend over the last 150 years, and particularly Modeling simulations also suggest that the EAWM is weakened prominent cooling in the recent 50 years (Gutiérrez et al., 2011). under the global warming background (Hori and Ueda, 2006). As Observed SSTs also exhibit distinct cooling in some coastal areas the EAWM could cause strong cooling in the coastal waters in the late 20th century (Belkin, 2009). The cooling trends in these (Kong et al., 2014), the weakened winter monsoon from the LIA areas are found to be mainly related to stronger monsoon or along- to present would be in an opposite sense to the cooling trend found shore wind induced upwelling (Bakun, 1990; Gutiérrez et al., 2011; in the Mirs Bay. Stronger EAWM during the LIA would have caused McGregor et al., 2007). further cooling, or insignificant SST changes as the bay waters are As observational wind data usually show large uncertainties already well mixed under present weak EAWM (Fig. 5). In either and inconsistencies (Hartmann et al., 2013; Zhou et al., 2009), case, EAWM variability over the past few centuries cannot account proxy-based climate reconstructions and climate modeling are for the cooling trend. Therefore the EAWM seems unlikely to be the sometimes helpful to better understand monsoonal changes in leading factor for the cooling trend over the past few centuries in the context of global warming. Oxygen isotopes of stalagmites in the bay. central China show higher precipitation during the Medieval Despite its weak strength relative to the EAWM over seasonal Warm Period compared to the LIA (Zhang et al., 2008), which is cycles, the EASM is found to account for the upwelling and thus likely due to stronger EASM under the warmer condition. From relatively cool summer SST in some areas of southeastern China the LIA to present, the global climate has been greatly warmed coast (Jing et al., 2009; Wang et al., 2014; Zu and Gan, 2015). The up as result of increased greenhouse gases (IPCC, 2013). Climate Mirs Bay is one of the areas, with another notable area to the simulations suggest that the increase of greenhouse gases would southeast of the Hainan Island (Jing et al., 2009). The effect of cause larger warming over land than sea (Dong et al., 2009; upwelling in the Mirs Bay could be identified from the vertical dis- Sutton et al., 2007). As a result, the land-sea thermal contrast tribution of temperature and salinity at the observation station would increase and lead to stronger EASM (Hu et al., 2000). MM17 (Fig.5). The surface temperature peaks in July and August, Based on the process between SST and EASM-induced upwelling, 502 D. Kong et al. / Journal of Asian Earth Sciences 114 (2015) 497–503 cooling in the Mirs Bay could also suggest intensified EASM in the He, Y., Zhou, X., Liu, Y., Yang, W., Kong, D., Sun, L., Liu, Z., 2014. Weakened Yellow context of global warming from the LIA to present. Understanding Sea Warm Current over the last 2–3 centuries. Quatern. Int. 349, 252–256. Hori, M.E., Ueda, H., 2006. Impact of global warming on the East Asian winter of this process would be helpful to predicting the monsoon trend in monsoon as revealed by nine coupled atmosphere-ocean GCMs. Geophys. Res. future climatic changes. 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