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Article Summer Changes in Water Mass Characteristics and Vertical Thermohaline Structure in the Eastern Chukchi , 1974–2017

Yayu Yang 1,2 and Xuezhi Bai 1,3,* 1 College of Oceanography, Hohai University, Nanjing 210098, China; [email protected] 2 Key Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources, Hohai University, Nanjing 210098, China 3 Key Laboratory of Coastal Disaster and Defence (Hohai University), Ministry of Education, Nanjing 210098, China * Correspondence: [email protected]; Tel.: +86-25-83787161

 Received: 5 April 2020; Accepted: 15 May 2020; Published: 18 May 2020 

Abstract: Hydrographic data from the World Database 2013 and the Chinese National Research Expedition were used to investigate the summertime changes in the eastern Chukchi Sea from 1974 to 2017. Owing to the Pacific inflow and timing of the sea ice retreat, water masses and vertical thermohaline structures in the eastern Chukchi Sea have changed but with regional differences. The entire eastern Chukchi Sea warmed up with significant temperature increase in the central shelf; however, the surface and bottom salinity in the southern, central, and northern shelves exhibited different trends. The northward extension of the Pacific Summer Water after 1997 influenced the summer hydrography significantly. Moreover, the data reveal changes in the characteristics of various water masses. Both Bering Summer Water (BSW) and Pacific Winter Water in the deeper layer became saltier, whereas the Alaskan Coastal Water in the upper layer became fresher after 1997. The previous definition of the BSW should be modified to include the warming water mass in the southern Chukchi Sea in the more recent years. Furthermore, the vertical thermohaline structure over the Chukchi shelves experienced considerable changes in its characteristics due to the combined effects of the Pacific inflow and surface forcing.

Keywords: Chukchi Sea; temperature; salinity; water mass; vertical thermohaline structure

1. Introduction As one of the cold sources of the system, the Arctic plays a significant role in ocean circulation and the global climate. The Pacific water that flows poleward through the is primarily driven by the pressure difference between the Pacific and [1]. The poleward flowing Pacific water is a predominant source of freshwater, heat, and nutrients, thereby exerting a strong influence on the melting of ice, thermohaline structures, and ecosystem functioning of the Arctic Ocean [2–5]. As a peripheral sea of the Arctic, the Chukchi Sea has several important features: it provides the pathway for the Pacific water to flow into the Arctic and therefore becomes one of the most important to mix the Pacific and Arctic water; since it has a seasonal to perennial sea ice cover, the minimum ice extent and edge in this area have been widely used as indicators of global climate change [6,7]. The Pacific inflow water is transformed seasonally by oceanographic processes over the wide continental shelves, with far-reaching implications for Arctic halocline formation and basin dynamics: the changed water in the Chukchi Sea will eventually ventilate the upper halocline of the western Arctic Ocean. Furthermore, the southern Chukchi Sea is one of the most biologically productive regions of the world’s because of nutrients supplied by the Pacific inflow water [8,9].

Water 2020, 12, 1434; doi:10.3390/w12051434 www.mdpi.com/journal/water Water 2020, 12, 1434 2 of 19

The different water masses that reside on the Chukchi shelf during summer have been defined by their potential temperature and salinity in numerous previous studies (e.g., [1,10–13]). The three main classes of Pacific summer water (PSW) are Alaskan Coastal Water (ACW; S < approximately 32, T > 3 ◦C), Bering Summer Water (BSW; T = 0–3 ◦C, S = 32–33) [10], and Chukchi Summer Water (CSW; T > 0 ◦C, S = approximately 31–32) [11,12]. The ACW is transported seasonally by the Alaskan Coastal Current (ACC) to the eastern-most part of the strait. The BSW is a mixed water mass, which occupies the bulk of the central Chukchi Sea [2]. The CSW, colder than the ACW and fresher than the BSW, is a combination of the warm and fresh PSW, and the cold and salty winter water. In addition, the Pacific Winter Water (PWW) is defined as water with a temperature below 1.6 C and salinity greater − ◦ than 31.5 [13]. The PWW is produced by cooling and brine rejection when ice forms during winter. The sea ice meltwater/river runoff (SMW, usually defined based on 24 kg m 3 isopycnal) [14] is typically · − observed in the upper part of the water column (usually shallower than 15 m) [13]. In recent decades, the Arctic Ocean has rapidly lost its summer sea-ice cover. The sea ice area has decreased by 3–4% every decade, while in summer, it has decreased by 7–9% [15–17]. Comiso [18] et al. (2008) compared and analyzed the decline rate of Arctic sea ice in two periods before and after 1997, and found that the decline rate of sea ice increased from 2.2% per decade before 1997 to 10.1% per decade after 1997. Stroeve [19] et al. (2012) further confirmed the trend of accelerated decline of Arctic sea ice extent. Different sea ice conditions will affect the heat flux exchange between the ocean and the atmosphere, causing changes in the local atmospheric circulation, and these changes will further affect the ocean process. There is evidence of increasing heat entering the Arctic Ocean through the Bering Strait during summer [20], and the upper ocean in the Arctic has warmed to unprecedented levels, including the shallow Chukchi Sea [21]. Model simulations indicate that in most areas, the main driver of seasonal ocean surface warming are downward atmospheric heat fluxes; some inflow areas like the Chukchi Sea warm up through ocean advection [22]. Since late 1990s, the Arctic has been warming more than twice as rapidly as the global average [23,24]. Steele et al. [25] reported that surface warming in the Arctic Ocean (since 1965) has been particularly pronounced since 1995. The retreating sea ice and warming environment may lead to significant oceanic changes, particularly in the marginal , where the edge of the sea ice is located. The decrease of sea ice will strengthen the upwelling near the Chukchi slope [26], thereby bringing the warm Atlantic water from deep basin to the shelf and affecting water properties there. Despite the studies to date, further work is required to better understand the specific changes in the Chukchi Sea where the retreat of sea ice has accelerated recently. The rapid retreat of sea ice results in more melt water and a longer exposure of the sea surface to surface heating and wind stress; thereby significantly modifying the water mass characteristics, stratification, and vertical mixing of the water column. In this study, we use available hydrographic data to investigate the oceanic changes in the eastern Chukchi Sea with a focus on the water mass characteristics and vertical thermohaline structures. We make a comparison between two periods: pre-1997 and post-1997, and attempt to reveal the unique responses in different regions of the Chukchi Sea. The structure of the paper is as follows: In Section2, datasets are described. In Section3, results are presented about the summer hydrography of the Chukchi Sea and oceanic changes surrounding the 1997 regime shift. Finally, Section4 provides a discussion and conclusions.

2. Data and Methods

The ocean temperature and salinity data of the 175◦ E–155◦ W, 60◦–75◦ N are obtained from the World Ocean Database (WOD13) at the National Oceanographic Data Centre (https://www. nodc.noaa.gov), which contains the high-resolution conductivity-temperature-depth (CTD) data from 1974 to 2017. The dataset provides the temperature and salinity data with a high level of quality control, including all preliminary and automatic quality control checks (i.e., minimum/maximum range assessment) [27]. An additional data source is the Chinese National Arctic Research Expedition (CNARE), which includes the observations from cruises conducted in 1999, 2003, 2008, 2010, and 2014 Water 2020, 12, x FOR PEER REVIEW 3 of 19

2003, 2008, 2010, and 2014 (http://www.chinare.org.cn). The hydrographic measurements from CNARE were collected using MarkIIIC and Sea-Bird SBE 9 CTD. After the removal of profiles that were positioned over land, wrong or empty stations, 10,518 profiles remain for the summer months (July, August, and September). The observation data do not cover all years. The time series of the number of stations counted by each year is shown in Figure 1a, and the distribution of all profiles in Waterthe study2020, 12 domain, 1434 is shown in Figure 1b; the purple dots represent the stations from 1974 to 1997,3 ofand 19 the yellow dots represent the stations from 1998 to 2017. In this paper, we used potential temperature and salinity to study water mass characteristics and hydrographic features. (http:The//www.chinare.org.cn data of monthly mean). The surface hydrographic net heat measurementsflux are provided from by CNARE Woods wereHole collectedOceanographic using MarkIIICInstitution and (WHOI) Sea-Bird OA SBEAir-Sea 9 CTD. Fluxes After (OAFlux, the removal http://oaflux.whoi.edu/descriptionheatflux.html). of profiles that were positioned over land, wrongThe net or heat empty flux stations, Qnet (positive 10,518 profiles downward, remain 1-degree for the summergrid) is computed months (July, as Qnet August, = SW and − September).LW − SH − TheLH, observationwhere SW denotes data do notthe covernet downward all years. shortw The timeave series radiation, of the LW number the net of stationsupward counted longwave by eachradiation, year isSH shown the upward in Figure sensible1a, and heat the flux, distribution and LH ofthe all upward profiles latent in the heat study flux. domain The sea is shownice extent in Figureand concentration1b; the purple data dots are represent based on the passive stations micr fromowave 1974 remote to 1997, sensing and the instruments, yellow dots representand the data the stationsare obtained from from 1998 the to 2017. National In this Snow paper, and we Ice used Data potentialCenter (NSIDC) temperature (https://nsidc.org/data/). and salinity to study water mass characteristics and hydrographic features.

Figure 1.1. (a) Temporal distributiondistribution ofof thethe numbernumber ofof observationobservation stations per year from 1974 to 2017 based on WOD13 and CNARE. (b) Distributions of all available T and S profiles profiles in the study region; purple dots represent observationsobservations duringduring 1974–1997, and yellow dots represent observations during 1998–2017; black dashed box represents the area for the T–S diagram; red boxes marked by A (66 –69 N, 1998–2017; black dashed box represents the area for the T–S diagram; red boxes marked by◦ A (66°–◦ 170 –160 W), B (69 –71 N, 170 –165 W) and C (71 –74 N, 170 –160 W) represent the three study 69°N,◦ 170°–160°W),◦ ◦ B (69°–71°N,◦ ◦ 170°–165°W)◦ and ◦C (71°–74°N,◦ ◦ 170°–160°W)◦ represent the three in Section 3.3.3. study subregions in Section 3.3.3. The data of monthly mean surface net heat flux are provided by Woods Hole Oceanographic The scattered profile data are gridded to a regular grid with mapping scales of 0.5° in both Institution (WHOI) OA Air-Sea Fluxes (OAFlux, http://oaflux.whoi.edu/descriptionheatflux.html). latitude and longitude. A linear interpolation function provided in MATLAB was used to interpolate The net heat flux Qnet (positive downward, 1-degree grid) is computed as Qnet = SW LW SH the scattered data (more detailed information is provided by the reference− −page: LH, where SW denotes the net downward shortwave radiation, LW the net upward longwave −https://ww2.mathworks.cn/help/matlab/ref/gridfit.html). Because many profiles of the surface layer radiation, SH the upward sensible heat flux, and LH the upward latent heat flux. The sea ice extent are not sufficiently accurate, the sea surface temperature (SST) and sea surface salinity (SSS) were and concentration data are based on passive microwave remote sensing instruments, and the data are defined as the mean temperature and salinity over the upper 10 m. In this study, the trends are obtained from the National Snow and Ice Data Center (NSIDC) (https://nsidc.org/data/). obtained using the linear regression analysis. Statistical significance is based on the p-value (i.e., the The scattered profile data are gridded to a regular grid with mapping scales of 0.5 in both latitude probability of observing the given result) of the F statistic. The F statistic is a test statistic◦ of the F-test and longitude. A linear interpolation function provided in MATLAB was used to interpolate the on the regression model, for a significantly linear regression relationship between the response scattered data (more detailed information is provided by the reference page: https://ww2.mathworks. variable and the predictor variables. To determine the statistical significance between means of two cn/help/matlab/ref/gridfit.html). Because many profiles of the surface layer are not sufficiently accurate, samples, the Student’s t-test was used in difference maps. Our null hypothesis is that the two samples the sea surface temperature (SST) and sea surface salinity (SSS) were defined as the mean temperature are statistically indistinguishable from each other. The null hypothesis is rejected if the value exceeds and salinity over the upper 10 m. In this study, the trends are obtained using the linear regression the 95% confidence interval. analysis. Statistical significance is based on the p-value (i.e., the probability of observing the given Because of the complicated physical processes and topography, the Chukchi Sea exhibits several result) of the F statistic. The F statistic is a test statistic of the F-test on the regression model, for a regions with different water characteristics. In our study, the eastern Chukchi shelf was divided into significantly linear regression relationship between the response variable and the predictor variables. three subregions (marked in Figure 1b): (A) southern (66°–69°N, 170°–160°W), (B) central (69°–71°N, To determine the statistical significance between means of two samples, the Student’s t-test was used 170°–165°W), and (C) northern (71°–74°N, 170°–160°W) Chukchi Sea. in difference maps. Our null hypothesis is that the two samples are statistically indistinguishable from each other. The null hypothesis is rejected if the value exceeds the 95% confidence interval. Because of the complicated physical processes and topography, the Chukchi Sea exhibits several regions with different water characteristics. In our study, the eastern Chukchi shelf was divided into three subregions (marked in Figure1b): (A) southern (66 ◦–69◦ N, 170◦–160◦ W), (B) central (69◦–71◦ N, 170◦–165◦ W), and (C) northern (71◦–74◦ N, 170◦–160◦ W) Chukchi Sea. Water 2020,, 12,, 1434x FOR PEER REVIEW 4 of 19

3. Results 3. Results

3.1. Hydrography during Summer: Sp Spatialatial Distribution Distribution of Water Masses The T-S diagram for all available hydrographic data in the study domain are are shown in in Figure Figure 22,, colored by the latitudes of the observations. The two two mixed mixed water water masses, masses, BSW BSW and and CSW, CSW, occupy the lower and higher latitudes, respectively. The SMW has a large range of temperatures depending on the season. Following Following previous previous study study [28], [28], we we distinguish distinguish between between newly newly melted melted water water (early_SMW; (early_SMW; with temperaturestemperatures approaching approaching the freezingthe freezing point) po andint) old and melted old water melted (late_SMW, water with(late_SMW, temperatures with temperaturesfar above the freezingfar above point). the freezing The PSW point). is comprised The PSW ofis ACW,comprised BSW of and ACW, CSW. BSW and CSW.

Figure 2. T–S plot of CTD data for the Chukchi Sea from July to September; dot colors represent the latitudes ofof thethe observations,observations, and and potential potential density density contours contours are are superimposed; superimposed; major major water water masses masses are aremarked marked by dashed by dashed and redand lines, red lines, and black and numbersblack numbers indicate indicate the water the mass water boundaries. mass boundaries. AW: Atlantic AW: Water;Atlantic ACW: Water; Alaskan ACW: Alaskan Coastal Water;Coastal BSW:Water; Bering BSW: SummerBering Summer Water; CSW:Water; Chukchi CSW: Chukchi Summer Summer Water; Water;SMW: SeasonalSMW: Seasonal Melt Water; Melt PWW:Water; PacificPWW: WinterPacific Water.Winter PSW: Water. Pacific PSW: Summer Pacific Summer Water, which Water, includes which ACW,includes BSW ACW, and BSW CSW. and CSW.

Figure 33a–da–d showsshows thethe mapsmaps ofof shelf-wideshelf-wide distributionsdistributions ofof thethe long-termlong-term (1974–2017)(1974–2017) summersummer mean surface temperature andand salinity.salinity. DuringDuring summer,summer, warmer warmer (T (T> >3 3◦ ℃C)) and saltiersaltier (S(S >> 30) surface PSW occupies approximately the entire Chukchi shelf, and colder (T << 3 ◦℃C)) and fresher (S < 30) melting water exists in the north of the shelf edge (100 m isobaths). There There are are two two relatively relatively cold areas over the Chukchi shelf; one extends along the east Siberian coast because of the cold East Siberian Current that flowsflows southward, and the other extendsextends over the Herald Shoal, which results from a Taylor columncolumn thatthat trapstraps iceice andand cold cold water water above above the the shoal shoal and and therefore therefore results results in in ice ice melting melting that that is islater later than than that that in in the the surrounding surrounding water water [29 [29]]. The. The warmest warmest ACW ACW flows flows through through the the eastern eastern channel channel of ofthe the Bering Bering Strait Strait and and continues continues to flowto flow along along the the Alaskan Alaskan coast. coast. The The ACW ACW has has two two branches branches owing owing to tothe the bifurcation bifurcation of theof the coastal coastal current current [10 [10,30,31],30,31]. One. One flows flows north-eastward north-eastward along along the the Alaskan Alaskan coast coast to tothe the Barrow ; Canyon; the the other other extends extends off shoreoffshore from from the the Alaskan Alaskan coast coast near near Point Point Hope Hope toward toward the thenorthwest, northwest, which which clearly clearly manifests manifests as a northwest-extendingas a northwest-extending warm warm tongue; tongue; subsequently, subsequently, the warm the warm water flows eastward along the northern part of the Herald Shoal. The distribution of the

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surface water properties clearly reflects the surface circulation pattern (see Figure 1 in Corlett and Pickart [32]). The bottom water is less influenced by the local air-sea fluxes; thus, it usually keeps its original water properties. The north edge of the PSW is located approximately 72°N, approximately along 50 m isobaths; north of the PSW is the PWW, which occupies the northeastern Chukchi shelf and slope (Figure 3b). Water 2020The, 12 vertical, 1434 distribution of water masses is reflected by a section in the central channel in 169°W5 of 19 direction (Figure 3e). Along the central channel, the north limit of the ACW is located at waterapproximately flows eastward 69°N owing along theto the northern bifurcation part of the the ACC. Herald The Shoal. bifurcation The distribution provides a ofbarrier, the surface which waterlimits properties the effects clearly of the reflects Pacific the inflows surface on circulation the northern pattern part (see of Figure the section;1 in Corlett thus, and the Pickart PWW [is32 not]). Theflushed bottom away water by isthe less PSW. influenced South of by 69°N, the local the wa air-searmest fluxes; ACWthus, occupies it usually the upper keeps layer, its original and the water BSW resides in the deep layer. North of 69°N, the water column consists of three water masses: the SMW properties. The north edge of the PSW is located approximately 72◦ N, approximately along 50 m isobaths;in the surface, north PWW of the in PSW the isbottom, the PWW, and CSW which in occupies the middle the layers. northeastern The approximate Chukchi shelf location and of slope each (Figurewater 3massb). is marked in Figure 3e.

Figure 3. Long-term mean (a) surface and (b) bottom temperature, (c) surface and (d) bottom salinity Figure 3. Long-term mean (a) surface and (b) bottom temperature, (c) surface and (d) bottom salinity according to available data of the summer seasons from 1974 to 2017; (e) vertical section of according to available data of the summer seasons from 1974 to 2017; (e) vertical section of temperature

(shaded) and salinity (white contour) along 169◦ W. The solid black contours in panels a–d represent the 100 m and 40 m isobaths.

The vertical distribution of water masses is reflected by a section in the central channel in 169◦ W direction (Figure3e). Along the central channel, the north limit of the ACW is located at approximately Water 2020, 12, 1434 6 of 19

69◦ N owing to the bifurcation of the ACC. The bifurcation provides a barrier, which limits the effects of the Pacific inflows on the northern part of the section; thus, the PWW is not flushed away by the Water 2020, 12, x FOR PEER REVIEW 6 of 19 PSW. South of 69◦ N, the warmest ACW occupies the upper layer, and the BSW resides in the deep layer.temperature North of 69 (shaded)◦ N, the and water salinity column (white consists contour) of along three 169°W. water masses:The solid the black SMW contours in the in surface, panels a– PWW in thed bottom,represent and the CSW100 m inand the 40 middle m isobaths. layers. The approximate location of each water mass is marked in Figure3e. 3.2. Temporal Changes in Temperature and Salinity 3.2. Temporal Changes in Temperature and Salinity The standard deviation of the summer mean surface temperature from 1974 to 2017 displays high Thevalues standard in the deviationcentral and of northern the summer Chukchi mean Sea surface (north temperature of 69°N) and from low 1974 values to 2017 in the displays south high(Figure values 4). The in thedistribution central and of the northern centers Chukchi of the local Sea maximums (north of 69 coincides◦ N) and with low valuesthe ACW in thepathway, south (Figureand the4 high). The values distribution across the of thenorthern centers Chukchi of the local shelf maximums are in accordance coincides with with the evident the ACW interannual pathway, andvariability the high of valuesthe ice acrosscover. theBy northerncontrast, Chukchithe southern shelf Chukchi are in accordance Sea is usually with ice-free the evident during interannual summer variabilityand is significantly of the ice influenced cover. By contrast, by the relatively the southern steady Chukchi Pacific Sea inflow. is usually Based ice-free on the during features summer of the andstandard is significantly deviation of influenced the surface by temperature, the relatively the steady long-term Pacific changes inflow. in Basedthe temperature on the features and salinity of the standardin the southern, deviation central, of the and surface northern temperature, Chukchi theSea long-termare studied. changes in the temperature and salinity in the southern, central, and northern Chukchi Sea are studied.

Figure 4. Standard deviation (shaded) of summer mean surface temperature from 1974 to 2017. TheFigure solid 4. Standard black contours deviation represent (shaded) 100 mof andsummer 40 m mean isobaths. surface temperature from 1974 to 2017. The solid black contours represent 100 m and 40 m isobaths. Figure5 shows the time series of the average temperature (T) and salinity (S) of the entire water columnFigure for 5 the shows three the regions. time series The trendsof the average of T and te Smperature are statistically (T) and not-significant salinity (S) of the in the entire southern water andcolumn northern for the Chukchi three regions. Sea. But, The in the trends central of shelf, T and the S warmingare statistically trend is not-significant significant and thein the temperature southern and northern Chukchi Sea. But, in the central shelf, the warming trend is significant and the increase is approximately 0.56 ◦C/decade; thus, the water temperature has increased by more than temperature increase is approximately 0.56℃/decade; thus, the water temperature has increased by 2.0 ◦C in 40 years. The salinity of the whole water column in the central Chukchi Sea, however, has not shownmore than any 2.0 significant ℃ in 40 changes years. The during salinity the pastof the 40 whole years. water Table 1column lists the in trends the central and their Chukchip-values, Sea, andhowever, the values has not in shown bold represent any significant the corresponding changes during trends the with past confidence 40 years. Table levels 1 abovelists the 95%. trends and their p-values, and the values in bold represent the corresponding trends with confidence levels above 95%.

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FigureFigure 5.5. Time series for thethe entireentire waterwater columncolumn inin thethe ((aa)) southern,southern, ((bb)) central,central, andand ((cc)) northernnorthern ChukchiChukchi Sea.Sea. 1 1 Table 1. The trends (◦C year− for temperature (T); psu year− for salinity (S)) and their p-values of the Table 1. The trends (℃·∙year−1 for temperature (T); psu year−1 for salinity (S)) and their p-values of the temperature and salinity of the whole water column, surface and bottom in the southern, central and temperature and salinity of the whole water column, surface and bottom in the southern, central and northern Chukchi Sea. The values in bold represent the corresponding trends with confidence levels northern Chukchi Sea. The values in bold represent the corresponding trends with confidence levels above 95%. above 95%. Southern Central Northern Southern Central Northern Whole Water Column Whole Water Column T 0.0107 (0.6609) 0.0558 (0.0476) 0.0106 (0.5354) S T 0.00470.0107 (0.6533) (0.6609) 0.0558 0.0030 (0.0476) (0.5751) 0.0106 0.0194 (0.5354) (0.1053) − S −0.0047 (0.6533) 0.0030Surface (0.5751) 0.0194 (0.1053) T 0.0135 (0.5253) 0.0668Surface (0.0254) 0.0340 (0.1319) S T 0.00490.0135 (0.6268) (0.5253) 0.06680.0229 (0.0254) (0.0140) 0.03400.0355 (0.1319) (0.1549) Bottom S 0.0049 (0.6268) 0.0229 (0.0140) 0.0355 (0.1549) T 0.0313 (0.1935) 0.0356 (0.2324) 0.0152 (0.1590) S 0.0204 (0.0483) 0.0156Bottom (0.0129) 0.0045 (0.5036) T − 0.0313 (0.1935) 0.0356− (0.2324) 0.0152− (0.1590) S −0.0204 (0.0483) −0.0156 (0.0129) −0.0045 (0.5036) To further our understanding of the long-term hydrography changes, the similar calculation has been done for the surface and bottom layers, the results are listed in Table1. The central shelf has the To further our understanding of the long-term hydrography changes, the similar calculation has most significant changes in both temperature and salinity. The linear trends of surface temperature been done for the surface and bottom layers, the results are listed in Table 1. The central shelf has the and surface salinity in the central shelf are +0.67 C/decade and +0.23 psu/decade, respectively. most significant changes in both temperature and sa◦linity. The linear trends of surface temperature While the trends in the bottom temperature are less evident than those of the surface layer, the bottom and surface salinity in the central shelf are +0.67 ℃/decade and +0.23 psu/decade, respectively. While salinity in the central shelf has a negative trend of 0.16 psu/decade. The opposite trends in surface the trends in the bottom temperature are less evident− than those of the surface layer, the bottom and bottom layer salinity leads to a statistically not-significant trend for the whole water column in salinity in the central shelf has a negative trend of −0.16 psu/decade. The opposite trends in surface the central shelf as mentioned above. The opposite trends also imply that the vertical structure of the and bottom layer salinity leads to a statistically not-significant trend for the whole water column in salinity has changed. In the southern and northern shelf, surface warming and salinification are not the central shelf as mentioned above. The opposite trends also imply that the vertical structure of the significant. Bottom salinity of the southern shelf decreases with a significant negative trend of 0.20 salinity has changed. In the southern and northern shelf, surface warming and salinification are− not psu/decade, probably because of the enhanced transport of the fresher Pacific inflow. significant. Bottom salinity of the southern shelf decreases with a significant negative trend of −0.20 psu/decade, probably because of the enhanced transport of the fresher Pacific inflow.

3.3. Oceanic Regime Shifts around 1997

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Water 2020, 12, x FOR PEER REVIEW 8 of 19 3.3. Oceanic Regime Shifts around 1997 InIn recent recent decades, decades, the the Arctic Arctic has has experienced experience dramaticd dramatic changes, changes, including including changes changes in in the the atmosphere,atmosphere, sea sea ice ice and and sea sea water. water. The The downward downward domain-averaged domain-averaged net net surface surface heat heat flux flux (NSHF) (NSHF) indicatesindicates the the Chukchi Chukchi Sea Sea absorbs absorbs heat heat in summer.in summer. Although Although the the trend trend over over the the whole whole period period is is increasing,increasing, it isit clearlyis clearly seen seen from from Figure Figure6a that6a that the the NSHF NSHF has has a di a ffdifferenterent trend trend in thein the two two periods periods beforebefore and and after after around around 1997. 1997. The The NSHF NSHF was was decreasing decreasing during during 1983–1997, 1983–1997, with with a negative a negative trend trend of of 1.1−1.1 W /W/mm2/year.2/year. But, But, a significanta significant increase increase started started after after 1997, 1997, with with a positivea positive trend trend of of 0.7 0.7 W /W/mm2/year,2/year, − leadingleading to to a significantlya significantly warmer warmer surface surface in in the the summer summer Chukchi Chukchi Sea Sea after after 1997 1997 that that impacts impacts the the atmosphere-ice-seaatmosphere-ice-sea interaction. interaction. The The net net shortwave shortwave radiation radiation contributes contributes the the most most to to the the trend trend shift shift (Figure(Figure6b). 6b).

FigureFigure 6. (6.a) ( Timea) Time series series of netof net surface surface heat heat flux flux (NSHF) (NSH inF) thein the Chukchi Chukchi Sea Sea during during summer summer from from 1983 1983 to 2009.to 2009. The The dash dash lines lines are are trends trends of of the the wholewhole (black(black line), pre- pre- and and post-1997 post-1997 (red (red lines) lines) periods. periods. The Thegray gray shading shading indicates indicates the the two-sided two-sided 95% 95% confidence confidence boundary. boundary. (b (b) )Time Time series series of of NHF NHF (red (red line), line), net netshort-wave short-wave radiation radiation (black (black line), line), net net long-wave long-wave radi radiationation (blue (blue line), line), latent latent heat heat flux flux (green (green line) line) and andsensible sensible heat heat flux flux (magenta (magenta line). line).

TheThe long-term long-term changes changes in sea-icein sea-ice extent extent further further confirm confirm the the regime regime shift shift around around 1997. 1997. Figure Figure7 7 showsshows the the sea-ice sea-ice extent extent anomaly anomaly since since 1978, 1978, with with significant significant trends trends at theat the two-sided two-sided 95% 95% confidence confidence level,level, indicating indicating the the decrease decrease in in thethe sea-icesea-ice extentextent accelerated accelerated after after late late 1990s 1990s (around (around 1997). 1997). The Thediminishing diminishing sea-ice sea-ice cover cover and and ice ice thickness aaffectffect advectionadvection andand fresh fresh water water flux flux in in the the Chukchi Chukchi Sea. Sea. ToTo characterize characterize the the oceanic oceanic changes changes of theof the 1997 1997 regime regime shift, shift, the the hydrographic hydrographic dataset dataset was was classified classified intointo pre- pre- and and post-1997 post-1997 periods periods in thein the subsequent subsequent research research (1997 (1997 is included is included in thein the former former period). period).

Water 2020, 12, 1434 9 of 19 Water 2020, 12, x FOR PEER REVIEW 9 of 19

Water 2020, 12, x FOR PEER REVIEW 9 of 19

FigureFigure 7.7. TimeTime series series of of monthly monthly sea-ice sea-ice extent extent anomaly anomaly in inthe the Arctic Arctic from from 1979 1979 to 2017. to 2017. The Thegray gray and andred shadings red shadings indicate indicate the two-sided the two-sided 95% 95% confidence confidence boundary. boundary. Figure 7. Time series of monthly sea-ice extent anomaly in the Arctic from 1979 to 2017. The gray and red shadings indicate the two-sided 95% confidence boundary. ConsideringConsidering regionalregional didifferencesfferences inin thethe ChukchiChukchi Sea,Sea, dailydaily evolutionsevolutions ofof thethe area-averagedarea-averaged iceice covercover inin thetheConsidering threethree subregionssubregions regional mentioned mentioneddifferences in aboveabove the Chukchi areare plottedplotted Sea, daily inin FigureevolutionsFigure8 .8. Theof The the southern southernarea-averaged Chukchi Chukchi ice Sea Sea experiencedexperiencedcover ain similar similarthe three ice-melting ice-melting subregions timementioned time in inpre- pre-above and and arepo st-1997.plotted post-1997. in In Figure the In central, the8. The central, southernthe daily the Chukchi evolution daily evolutionSea shows experienced a similar ice-melting time in pre- and post-1997. In the central, the daily evolution shows showsthat the that ice-free the ice-free date dateranged ranged fromfrom the the210th 210th to 280th to 280th day day of ofthe the year year after after 1997, 1997, which waswas that the ice-free date ranged from the 210th to 280th day of the year after 1997, which was approximately 50–60 days longer than before 1997. In the north, the ice-free season was prolongated approximatelyapproximately 50–60 50–60 days days longer longer than than before before 1997. 1997. InIn thethe north, north, the the ice-free ice-free season season was prolongated was prolongated afterafter 1997.1997.after TheThe 1997. change change The change of of the the ice-meltingof theice-melting ice-melting time time intime the inin Chukchi thethe ChukchiChukchi Sea Sea will Sea will a ffwill ectaffe thectaffe the surfacect surfacethe surface forcing; forcing; leadingforcing; toleading hydrographic leadingto hydrographic to changeshydrographic changes that changes will that be that presented will will be be presented presented and discussed andand discussed discussed in the in following. thein thefollowing. following.

Figure 8. Daily evolutions of the area-averaged ice cover in (a) southern, (b) central and (c) northern Chukchi Sea; black curves represent multi-year mean data from the pre-1997 period, while red contours represent the post-1997 period.

Water 2020, 12, x FOR PEER REVIEW 10 of 19

Figure 8. Daily evolutions of the area-averaged ice cover in (a) southern, (b) central and (c) northern Chukchi Sea; black curves represent multi-year mean data from the pre-1997 period, while red contours represent the post-1997 period.

3.3.1. Changes in Water Mass Characteristics The T–S diagrams of the southern, central, and northern shelves of the two periods are shown in Figure 9; the light blue and yellow dots represent the pre- and post-1997 periods, and the blue and red triangles represent the mean values of the water masses, respectively. The ACW and BSW dominated the southern Chukchi Sea during both periods (Figure 9a). A comparison between the post- and pre-1997 data reveals that the sea water warmed up after 1997. Moreover, Figure 9a exhibits a cluster of observations with water temperatures above 3 ℃ and salinity above 32 psu after 1997. The salinity and density of the water mass are similar to those of the BSW. Although the water temperature is higher than that in the previous definition of the BSW, this water mass can be still classified as “BSW.” Hence, the previous BSW definition should be modified to include the recently warmer water. The main water masses over the central Chukchi shelf includes ACW, BSW, CSW, and PWW (Figure 9b). The T–S diagram shows that the quantity of BSW increased after 1997, and the PWW had Watera higher2020, 12quantity, 1434 in pre-1997 than in post-1997; only little PWW was left in this area after 1997.10 of The 19 mean temperature of the BSW post-1997 was higher than before, while the salinity exhibited a slight increase. Moreover, the temperature of the ACW in this area changed insignificantly. 3.3.1. Changes in Water Mass Characteristics On the northern Chukchi shelf, PWW occupied a considerable portion of the observations in bothThe periods. T–S diagrams The salinity of the and southern, density central,of the PWW and northern increased shelves remarkably of the twoafter periods 1997 (Figure are shown 9c). The in Figurereason9; for the the light salinity blue and increase yellow is dotsstill unclear; represent a thepossible pre- and cause post-1997 could be periods, the increase and the in bluethe formation and red trianglesof represent over the the past mean few values decades of the [33], water thereby masses, leading respectively. to denser water.

Figure 9. T–S diagrams of (a) southern, (b) central and (c) northern shelf. The light blue and yellow dotsFigure belong 9. T–S to diagrams the pre- and of ( post-1997a) southern, periods, (b) central respectively; and (c) northern blue and shelf. redsolid The light triangles blue denoteand yellow the meandots valuesbelong ofto thethe waterpre- and masses post-1997 of the pre-periods, and post-1997respectively; periods, blue and respectively. red solid triangles denote the Themean ACW values and of the BSW water dominated masses of the the pre- southern and post-1997 Chukchi periods, Sea respectively. during both periods (Figure9a). A comparison between the post- and pre-1997 data reveals that the sea water warmed up after 1997. 3.3.2. Changes in Summer Hydrography Moreover, Figure9a exhibits a cluster of observations with water temperatures above 3 ◦C and salinity aboveThe 32 psuwarm after PSW 1997. in the The surface salinity layer and (T density > 3–4 of℃) the extended water mass further are north similar after to those1997; its of theedge BSW. was Althoughlocated at the approximately water temperature 72°N, isand higher approximately than that in along the previous 50 m isobaths definition (Figure of the 10a–d), BSW, thiswhereas water it masswas canlocated be still at approximately classified as “BSW.” 70°N Hence,before the1997. previous North of BSW the definition warm water should lies bethe modified melting towater. include The theextension recently of warmer the PSW water. has led to a significant warming and salting in the east of the Herald Shoal after 1997The (Figure main 10e,f) water because masses the over salt the and central warm Chukchi PSW replaces shelf includes the fresh ACW, and cold BSW, melting CSW, andwater. PWW The (Figure9b). The T–S diagram shows that the quantity of BSW increased after 1997, and the PWW had a higher quantity in pre-1997 than in post-1997; only little PWW was left in this area after 1997. The mean temperature of the BSW post-1997 was higher than before, while the salinity exhibited a slight increase. Moreover, the temperature of the ACW in this area changed insignificantly. On the northern Chukchi shelf, PWW occupied a considerable portion of the observations in both periods. The salinity and density of the PWW increased remarkably after 1997 (Figure9c). The reason for the salinity increase is still unclear; a possible cause could be the increase in the formation of polynyas over the past few decades [33], thereby leading to denser water.

3.3.2. Changes in Summer Hydrography

The warm PSW in the surface layer (T > 3–4 ◦C) extended further north after 1997; its edge was located at approximately 72◦ N, and approximately along 50 m isobaths (Figure 10a–d), whereas it was located at approximately 70◦ N before 1997. North of the warm water lies the melting water. The extension of the PSW has led to a significant warming and salting in the east of the Herald Shoal after 1997 (Figure 10e,f) because the salt and warm PSW replaces the fresh and cold melting water. The SST and SSS differences also show another remarkable warming and salinification phenomenon west of Point Hope, which may be due to the effect of the ACW. Only values passing the T-test (confidence levels: 95%) are shown in these difference maps (same in Figures 11 and 12). Water 2020, 12, x FOR PEER REVIEW 11 of 19

SST and SSS differences also show another remarkable warming and salinification phenomenon west of Point Hope, which may be due to the effect of the ACW. Only values passing the T-test (confidence levels: 95%) are shown in these difference maps (same in Figures 11 and 12). The striking feature in the bottom layer is the northward extension of the PSW and retreat of the PWW after 1997 (Figure 11a–d). The PWW was north of 69.5°N and covered the northeastern Chukchi shelf and slope before 1997 (Figure 11a,c). After 1997, the PWW shrank to north of 71.5°N, which has led to significant warming and freshening between the Herald and Hanna Shoal in the bottom layer Water 2020, 12, 1434 11 of 19 (Figure 11e,f).

Figure 10. Surface temperature of (a) pre-1997 and (b) post-1997 periods, and (c,d) for salinity; Figure 10. Surface temperature of (a) pre-1997 and (b) post-1997 periods, and (c) (d) for salinity; differences in (e) surface temperature and (f) surface salinity between the two periods. The solid black differences in (e) surface temperature and (f) surface salinity between the two periods. The solid black contours represent 100 m and 40 m isobaths. contours represent 100 m and 40 m isobaths.

Water 2020, 12, 1434 12 of 19 Water 2020, 12, x FOR PEER REVIEW 12 of 19

FigureFigure 11. 11. BottomBottom temperaturetemperature of (a) pre pre-1997-1997 and (b) post post-1997-1997 periods, and ( (cc), d()d) for for salinity; salinity; differences in (e) bottom temperature and (f) bottom salinity between the two periods. The solid black differences in (e) bottom temperature and (f) bottom salinity between the two periods. The solid black contours represent 100 m and 40 m isobaths. contours represent 100 m and 40 m isobaths.

The vertical sections along 169°W in July, August, and September of the two periods display the monthly evolutions and progress of the water masses (Figure 12). The PSW progressed northward, while the PWW retreated to the north. The mixed layer deepened, and the surface heating phenomenon shifted downward from July to September. Moreover, in September, the thermocline was not as sharp as those in July and August. The comparison of the vertical sections of the two periods shows that the seasonal progress of the water masses along 169°W after 1997 occurred approximately a month earlier than before, thereby leading to significant changes in the water properties. For example, in July, the north edge of the warm PSW was approximately at 69°N pre-1997 and approximately at 71°N post-1997. In September pre-1997, the subduction of the ACW led to a subsurface temperature maximum at a depth of

Water 2020, 12, x FOR PEER REVIEW 13 of 19 approximately 20–30 m north of 69°N; the feature was not evident post-1997 because the entire upper layer was heated. The difference in 95% confidence level shows that the temperature increased after 1997 each month (Figure 12g–i). The remarkable warming at the front near 69°–70°N in July was due to the advection of the PSW, and the warming in the upper layer (less than 20 m) in September was mainly due to the longer surface heating because of the earlier melting of the sea-ice cover after 1997. In July, salinity increased in the upper layer and decreased in the lower layer. In general, the changes in salinity were small in August and September along this meridional section (Figure 12j–l). The areas undergoing significant changes are small in August, especially in salinity difference maps. In August and September, PSW already occupied almost the whole shelf during the two periods, and the Watersalinity2020 ,of12 ,the 1434 PSW is more conservative than its temperature. The advection effect of the PSW become13 of 19 less important than surface forcing (wind mixing and surface heating) in August and September.

Figure 12. Multi-year mean vertical sections of the temperature (shaded) and salinity (contour) and Figure 12. Multi-year mean vertical sections of the temperature (shaded) and salinity (contour) and their their differences along 169°W from July to September in (a–c) pre- and (d–f) post-1997 periods, differences along 169◦ W from July to September in (a–c) pre- and (d–f) post-1997 periods, respectively; respectively; Differences in the (g–i) temperature and (j–l) salinity between the two periods. The color Differences in the (g–i) temperature and (j–l) salinity between the two periods. The color shaded areas andshaded theblank areas dots and in the diff erenceblank mapsdots in indicate difference 95% andmaps 99% indicate confidence 95% level, and respectively.99% confidence level, respectively. The striking feature in the bottom layer is the northward extension of the PSW and retreat of the PWW3.3.3. afterChanges 1997 in (Figure Vertical 11a–d). Thermohaline The PWW Structure was north of 69.5◦ N and covered the northeastern Chukchi shelf and slope before 1997 (Figure 11a,c). After 1997, the PWW shrank to north of 71.5◦ N, which has led to significant warming and freshening between the Herald and Hanna Shoal in the bottom layer (Figure 11e,f). The vertical sections along 169◦ W in July, August, and September of the two periods display the monthly evolutions and progress of the water masses (Figure 12). The PSW progressed northward, while the PWW retreated to the north. The mixed layer deepened, and the surface heating phenomenon shifted downward from July to September. Moreover, in September, the thermocline was not as sharp as those in July and August. The comparison of the vertical sections of the two periods shows that the seasonal progress of the water masses along 169◦ W after 1997 occurred approximately a month earlier than before, thereby leading to significant changes in the water properties. For example, in July, the north edge Water 2020, 12, 1434 14 of 19

of the warm PSW was approximately at 69◦ N pre-1997 and approximately at 71◦ N post-1997. In September pre-1997, the subduction of the ACW led to a subsurface temperature maximum at a depth of approximately 20–30 m north of 69◦ N; the feature was not evident post-1997 because the entire upper layer was heated. Water 2020The, 12 di, ffx erenceFOR PEER in REVIEW 95% confidence level shows that the temperature increased after 199714 eachof 19 month (Figure 12g–i). The remarkable warming at the front near 69◦–70◦ N in July was due to the advectionBecause of theof earlier PSW, andsea ice the retreat, warming longer in the exposu upperre layer of the (less shelf than water, 20 m) and in Septemberthe advancing was Pacific mainly inflow,due to thethe longerChukchi surface Sea exhibits heating several because regions of the earlier with different melting ofwater the sea-ice characteristics. cover after The 1997. southern, In July, centralsalinity and increased northern in subregions the upper layerof the and eastern decreased Chukchi in theshelf lower (CS) layer.were studied In general, to understand the changes the in differentsalinity were responses small in in vertical August thermohaline and September stru alongctures. this The meridional green and section red lines (Figure in Figures 12j–l). 13–15 The areas are regionalundergoing mean significant vertical changesprofiles areof smalltemperature in August, and especially salinity infor salinity the pre- di ffanderence post-1997 maps. In periods, August respectively.and September, PSW already occupied almost the whole shelf during the two periods, and the salinity of the PSW is more conservative than its temperature. The advection effect of the PSW become less (1)important Southern than CS surface forcing (wind mixing and surface heating) in August and September. This region is located near the entrance of the warm Pacific inflow (Figure 13). Owing to a similar ice-melting3.3.3. Changes time in before Vertical and Thermohaline after 1997, the Structure T and S profiles of the two periods are similar. Two main waterBecause masses of(ACW earlier and sea iceBSW) retreat, dominate longer exposurethe region of. theThe shelf two water, water and masses the advancing occasionally Pacific intersect inflow, withthe Chukchi each other Sea when exhibits flowing several poleward regions owing with ditoff theerent topography. water characteristics. The southern, central andThe northern T and subregions S profiles exhibit of the easternACW in Chukchi the surface shelf layer (CS) and were BSW studied in the to deeper understand layer during the diff erentboth periods.responses In inaddition, vertical the thermohaline water columns structures. were roug Thehly green mixed and with red weak lines instratification, Figures 13– 15and are the regional mixed layermean is vertical shallow profiles (5–10 m). of temperature The whole water and salinity columns for during the pre- post-1997 and post-1997 was warmer periods, than respectively. pre-1997.

FigureFigure 13. 13. TheThe regional regional average average (a)–( (ac–) cT) and T and (d)–( (df–)f S) profiles S profiles of the of thesouthern southern Chukchi Chukchi shelf shelf from fromJuly toJuly September. to September. The green The and green red and lines red represent lines represent the average the average values of values the pre- of and the pre-post-1997 and post-1997 periods, respectively.periods, respectively.

(2) Central CS This region is the frontier of the Pacific summer inflow. Owing to an ice melting time that was 50–60 days earlier during post-1997 than during pre-1997, the T and S profiles were strongly influenced by the Pacific inflow and surface heating. In the pre-1997 period (green lines in Figure 14), because of the early_SMW in the top, the mixed water (BSW/CSW) in the subsurface and the PWW in the bottom layer, the T-profiles in July and August feature a multi-layer structure (shallow mixed layer, entrainment zone, upper and lower thermoclines, and a sublayer [34]) with a nose-shaped curve in the upper thermocline that has a

Water 2020, 12, x FOR PEER REVIEW 15 of 19

maximal temperature at 10 m. The multi-layer structure in August is, however, not as obvious as that in July. Meanwhile, the S profiles show a steep halocline at 5–15 m depth, which separated the fresh water in the mixed layer and salty water in the deeper layer. In post-1997 period (red lines in Figure 14), the T profiles are of a two-layer structure. The maximal subsurface temperature and steep halocline in July and August pre-1997 disappeared after 1997. Salinity mixed from July, which was two months earlier than before. The strong mixing

Waterredistributed2020, 12, 1434 the salt in the water column, resulting in an increased salinity in the upper layer15 ofand 19 decreased salinity in the bottom layer.

Figure 14. The regional average (a–c) T and (d–f) S profiles of the central Chukchi shelf from Figure 14. As Figure 13, but for central Chukchi shelf. July to September. The green and red lines represent the average values of the pre- and post-1997 Water 2020, 12, x FOR PEER REVIEW 16 of 19 periods, respectively. (3) Northern CS No data from this region for July are available because the region is near the ice edge. After the acceleration of sea ice reduction around 1997, the ice-free season in this region was prolongated (Figure 8c), thereby resulting in significant changes in the vertical structure (Figure 15). The T and S profiles of pre-1997 Augusts are a two-layer system that consists of a surface mixed layer above a bottom boundary layer, separated by a sharp interface at 10 m depth. The ACW occupied the upper layer and the PWW the deep layer. The T profiles of Septembers are nose-shaped curves with the maximal subsurface temperature at 15–25 m depth, which is associated with the eastward-flowing BSW. After 1997, the T and S profiles become a two-layer system. Compared with that of the pre-1997 period, the two-layer system shifted downward, and the sharp interface deepened (from 10 m to 20 m) in August.

Figure 15. The regionalFigure average 15. As (a Figure,b) T and 13, (butc,d) for S profiles northern of Chukchi the northern shelf. Chukchi shelf in August and September. The green and red lines represent the average values of the pre- and post-1997 4. Discussionperiods, and respectively. Conclusions In this study, we have addressed the general hydrographic characteristics of the Chukchi Sea during summer and the associated regional changes from 1972 through 2017 by using a large amount of hydrographic data from the World Ocean Database 2013 and the Chinese National Arctic Research Expedition. The Arctic ocean has been losing its summer ice cover over the past decades. Daily evolution of sea ice concentration shows that ice-free seasonal phases in the central Chukchi shelf after 1997 is approximately 50–60 days longer than before 1997 (Figure 8). The results correspond to the facts that the decline rate of Arctic sea ice accelerated after 1997 [18]. One of the potential reasons may be that the trend of net surface heat fluxes over the Chukchi Sea has shifted from a negative trend of −1.1 W/m2/year in 1983–1997 to a positive trend of 0.7 W/m2/year in 1997–2017 (Figure 6). Steele et al. (2008) [25] found that the Arctic surface warming is particularly pronounced since 1995, and especially since 2000. All these phenomena suggest that perhaps there was a climate regime shift around 1997, though the cause of this shift is still unclear. The possible interpretation is that sea ice retreat creates a longer ice-free season on the shelf, which certainly affects the surface forcing by allowing enhanced wind mixing [35], turbulence heat fluxes and extra solar radiation absorbing, etc. In addition to the changes in surface forcing, heat transport from the through Bering Strait increased by 60% during 2001–2014, which was attributed to increases in both volume flux and temperature [4]. These two effects all contribute to the changes in the Chukchi shelf. As an inflow shelf, the Chukchi Sea is heavily influence by the Pacific inflow, and the oceanic advection effect is expected to be weaker approaching the northern shelf. Based on the comparison of summer hydrography between the pre- and post-1997 periods, our results suggest that the north extension of the PSW is the primary driver for the hydrographic changes in the Chukchi shelf. The data shows that the edge of PSW at the surface (bottom) was located at approximately 70°N (69.5°N) before 1997 and 72°N (71.5°N) after 1997 (Figures 10 and 11). The surface edge position of PWS over post-1997 period was close to the averaged dominate current using satellite-tracked drifters for 2010– 2015 [36], which indicates the Pacific inflow may be the factor causing the hydrographic change post-1997 period. The PSW replaced the fresh and cold melting water at the surface, and pushed the

Water 2020, 12, 1434 16 of 19

(1) Southern CS This region is located near the entrance of the warm Pacific inflow (Figure 13). Owing to a similar ice-melting time before and after 1997, the T and S profiles of the two periods are similar. Two main water masses (ACW and BSW) dominate the region. The two water masses occasionally intersect with each other when flowing poleward owing to the topography. The T and S profiles exhibit ACW in the surface layer and BSW in the deeper layer during both periods. In addition, the water columns were roughly mixed with weak stratification, and the mixed layer is shallow (5–10 m). The whole water columns during post-1997 was warmer than pre-1997.

(2) Central CS This region is the frontier of the Pacific summer inflow. Owing to an ice melting time that was 50–60 days earlier during post-1997 than during pre-1997, the T and S profiles were strongly influenced by the Pacific inflow and surface heating. In the pre-1997 period (green lines in Figure 14), because of the early_SMW in the top, the mixed water (BSW/CSW) in the subsurface and the PWW in the bottom layer, the T-profiles in July and August feature a multi-layer structure (shallow mixed layer, entrainment zone, upper and lower thermoclines, and a sublayer [34]) with a nose-shaped curve in the upper thermocline that has a maximal temperature at 10 m. The multi-layer structure in August is, however, not as obvious as that in July. Meanwhile, the S profiles show a steep halocline at 5–15 m depth, which separated the fresh water in the mixed layer and salty water in the deeper layer. In post-1997 period (red lines in Figure 14), the T profiles are of a two-layer structure. The maximal subsurface temperature and steep halocline in July and August pre-1997 disappeared after 1997. Salinity mixed from July, which was two months earlier than before. The strong mixing redistributed the salt in the water column, resulting in an increased salinity in the upper layer and decreased salinity in the bottom layer.

(3) Northern CS No data from this region for July are available because the region is near the ice edge. After the acceleration of sea ice reduction around 1997, the ice-free season in this region was prolongated (Figure8c), thereby resulting in significant changes in the vertical structure (Figure 15). The T and S profiles of pre-1997 Augusts are a two-layer system that consists of a surface mixed layer above a bottom boundary layer, separated by a sharp interface at 10 m depth. The ACW occupied the upper layer and the PWW the deep layer. The T profiles of Septembers are nose-shaped curves with the maximal subsurface temperature at 15–25 m depth, which is associated with the eastward-flowing BSW. After 1997, the T and S profiles become a two-layer system. Compared with that of the pre-1997 period, the two-layer system shifted downward, and the sharp interface deepened (from 10 m to 20 m) in August.

4. Discussion and Conclusions In this study, we have addressed the general hydrographic characteristics of the Chukchi Sea during summer and the associated regional changes from 1972 through 2017 by using a large amount of hydrographic data from the World Ocean Database 2013 and the Chinese National Arctic Research Expedition. The Arctic ocean has been losing its summer ice cover over the past decades. Daily evolution of sea ice concentration shows that ice-free seasonal phases in the central Chukchi shelf after 1997 is approximately 50–60 days longer than before 1997 (Figure8). The results correspond to the facts that the decline rate of Arctic sea ice accelerated after 1997 [18]. One of the potential reasons may be that the trend of net surface heat fluxes over the Chukchi Sea has shifted from a negative trend of 1.1 W/m2/year in 1983–1997 to a positive trend of 0.7 W/m2/year in 1997–2017 (Figure6). Steele et al. − Water 2020, 12, 1434 17 of 19

(2008) [25] found that the Arctic surface warming is particularly pronounced since 1995, and especially since 2000. All these phenomena suggest that perhaps there was a climate regime shift around 1997, though the cause of this shift is still unclear. The possible interpretation is that sea ice retreat creates a longer ice-free season on the shelf, which certainly affects the surface forcing by allowing enhanced wind mixing [35], turbulence heat fluxes and extra solar radiation absorbing, etc. In addition to the changes in surface forcing, heat transport from the Pacific Ocean through Bering Strait increased by 60% during 2001–2014, which was attributed to increases in both volume flux and temperature [4]. These two effects all contribute to the changes in the Chukchi shelf. As an inflow shelf, the Chukchi Sea is heavily influence by the Pacific inflow, and the oceanic advection effect is expected to be weaker approaching the northern shelf. Based on the comparison of summer hydrography between the pre- and post-1997 periods, our results suggest that the north extension of the PSW is the primary driver for the hydrographic changes in the Chukchi shelf. The data shows that the edge of PSW at the surface (bottom) was located at approximately 70◦ N (69.5◦ N) before 1997 and 72◦ N (71.5◦ N) after 1997 (Figures 10 and 11). The surface edge position of PWS over post-1997 period was close to the averaged dominate current using satellite-tracked drifters for 2010–2015 [36], which indicates the Pacific inflow may be the factor causing the hydrographic change post-1997 period. The PSW replaced the fresh and cold melting water at the surface, and pushed the PWW northward at the depth. The differences in the thermohaline along 169◦ W also demonstrate the influences of the extending PSW (Figure 12). The results are in accordance with the observed increase of the Pacific inflow [4,37]. Given differences in timing of sea ice melting, effects of the Pacific inflow [37] and local circulations that are steered by the complex local topography [32], the responses in the Chukchi Sea to the sea ice retreat are not uniform. On the southern shelf, due to the relatively stable effect of the Pacific inflow and similar ice conditions during the two periods, the area-averaged temperature and salinity as well as the vertical thermohaline structures did not show evident changes (Figures5 and 13). However, as the frontier area of the PSW, the central shelf has experienced significant changes due to the increased Pacific inflow. It has been warming since 1972 with a linear trend of +0.67 ◦C/decade. The surface salinity has a positive trend and the bottom salinity has a negative trend (Figure5), implying that the vertical structure of the salinity has changed. The possible explanation is the northward extension of the PSW. Before 1997, the central shelf water column consisted of fresh melting water in the surface and the PWW near the bottom. After 1997, the whole water column was replaced by the PSW. Previous study of the increase in the Pacific inflow to the Arctic over 1990–2015 [37] well supported the northward extension of the PSW after 1997. The T and S profiles also confirm such changes (Figure 14). The increasing Pacific inflow also acted as increasing the heat flux and study found that the ~67% of the total heat input to the sea water in Chukchi Sea was from Pacific inflow and then of 58% heat input was used to further heat sea water [38], which may interpret the warming in the central shelf. The oceanic advection effects on the northern shelf become weaker, while surface forcing may play a bigger role in water property and vertical structure changes. It is clearly that the Chukchi Sea are influenced by both oceanic advection and surface forcing simultaneously, and it remains a challenge to distinguish between the effects of oceanic advection and sea ice retreat. Using the coupled ocean and sea ice model, Wang et al., (2019) [38] found that the Pacific inflow heat flux acted more significant impacts on the interannual variations of Chukchi Sea ice cover than the net heat flux at ice surface during 1994–2015, but their contributions will change for different stages of ice melting. Our results also suggest that the relative roles of the two effects may vary with region and months during the summer season. For example, the changes in meridional section along 169◦ W (Figure 12) are most significant in July due to the north extension of the PSW, however, the effect of oceanic advection become less important in August and September, because the northern edges of the PSW are close in both periods, and the changes in hydrography are mainly induced by enhanced wind mixing and surface heating that are caused by sea ice retreat. The sparsity and discontinuity of the observations make it difficult to evaluate the relative roles of the oceanic Water 2020, 12, 1434 18 of 19 advection and sea ice retreat. Further analysis and idealized numerical experiments should be useful to discuss the mechanisms and relationships of these factors.

Author Contributions: Conceptualization, X.B.; data curation, Y.Y.; formal analysis, Y.Y. and X.B.; funding acquisition, X.B.; investigation, Y.Y.; project administration, X.B.; writing—original draft, Y.Y.; writing—review and editing, X.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key Research and Development Program of China (Grant 2017YFA0604600), Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant SJKY19_0417), the Fundamental Research Funds for the Central Universities (Grant 2019B63014) and National Natural Science Foundation of China (Grant 41676019). Acknowledgments: The authors wish to acknowledge the National Oceanographic Data Centre (NODC, https: //www.nodc.noaa.gov) and the National Arctic and Antarctic Data Center (http://www.chinare.org.cn) for providing the temperature and salinity data of the Chukchi Sea, the National Snow and Ice Data Center (NSIDC, https://nsidc.org/data/) for providing sea-ice data. We sincerely thank the anonymous reviewers for their critical and constructive comments, which helped significantly improve the analysis and presentation of the paper. Conflicts of Interest: The authors declare no conflict of interest.

References

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