Summer water temperature structures and their interannual variation in the upper Canada Basin

●ArticleAdvances in Polar Sciencedoi:10.3724/SP.J.1085.2011.0223December 2011 Vol.22 No.4 223 -234Summer water temperature structures and theirinterannual variation in the upper Canada BasinZHAO Jinping1,2 & CAO Yongl*'College of Physical and Environmental Oceanography, Ocean University of China, Qingdao 266100, China;2Key Laboratory of Physical Oceanography, Ministry of Education, Qingdao 266003, ChinaReceived September 16, 2011; accepted November 15, 2011Abstract Conductivity, temperature and depth (CTD) data from 1993- -2010 are used to study water tempera-ture in the upper Canada Basin. There are four kinds of water temperature structures: The remains of the winterconvective mixed layer, the near- surface temperature maximum (NSTM), the wind-driven mixed layer, and theadvected water under sea ice. The NSTM mainly appears within the conductive mixed layer that forms in winter.Solar heating and surface cooling are two basic factors in the formation of the NSTM. The NSTM can also appearin undisturbed open water, as long as there is surface cooling. Water in open water areas may advect beneath thesea ice. The overlying sea ice cools the surface of the advected water, and a temperature maximum could appearsimilar to the NSTM. The NSTM mostly occurred at depths 10 30 m because of its deepening and strengthening. duringsunmer,withighestrequeneyatonmTwoterstaontranatvaratenametentntentetere2003, most NSTMs were observed in marginal ice zones and open waters, so temperature maxima were usuallywarmer than 0C. After 2004, most NSTMs occurred in ice covered areas, with much colder temperature maxima.Average depths of the temperature maxima in most years were about 20 m, except for about 16 m in 2007,which was related to the extreme minimum of ice cover. Average temperatures were around -0.8C to -1.1°C, bincreased to around 0.5°C in 2004, 2007 and 2009, corresponding to reduced sea ice. As a no-ice summer in theArctic is expected, the NSTM will be warmer with sea ice decline. Most energy absorbed by seawater has beentransported to sea ice and the atmosphere. The heat near the NSTM is only the remains of total absorption, andthe energy stored in the NSTM is not considerable. However, the NSTM is an important sign of the increasingabsorption of solar energy in seawater.Keywords Canada Basin, upper ocean, near-surface temperature maximum, halocline, warmingCitation: Zhao J P, Cao Y. Summer water temperature structures and their interannual variation in the upper CanadaBasin. Adv Polar Sci, 2011, 22: 223-234, doi: 10.3724/SP.J.1085.2011.002230 Introductionthere are three main water masses- Arctic Surface WaterSea ice is one of the important factors infuencing the(ASW, 0 200 m) with low temperature and low salin-Arctic climate; it infuences strongly the heat flux at theity, Arctic Intermediate Water (AIW) or Atlantic Waterair-sea interface. Another key factor is the thermal struc-(AW, 200- 900 m) with higher temperature and higherture of seawater under the ice cover, which absorbs solarsalinity, and Deep Water (DW) or Bottom Water (BW,radiation penetrating through the sea ice and provides 900 m-bottom) with lower temperature and higher salin-heat to infuence the melting and freezing of sea ice above.ity. Usually, there are two temperature maxima in theBased on the criterion of Swift and Aagaard叫 andCanada Basin. One of these is at depth 60-80 m,Aagaard et al.[2] for the water mass in the Canada Basin,which originate中国煤化工), modifed in' Corresponding author (email: caoyong@ouc.edu.cn)YHCNMH Gjournal.polar. .gov.cn225(1993- 2002) isued by the Ocean Climate Laboratory of1.1 Winter water structure under sea icethe National Oceanographic Data Center, the LSSL dataIn the freczing season, convection occurs by brine reje-base (2003- 2010) issued by the Joint West Arctic Climatetion during ice freezing. This produces a uniform con-Study (JWACS) and Beaufort Gyre Exploration Projectvective mixed layer with maximum thickness about 40-(BGEP), the Mirrai data (1999, 2000, 2002, 2004) issued50 ml191. The heat of the upper ocean is released to theby the Japan Agency for Marine Earth Science and Tech-atmosphere, and the water temperature in the convectivenology (JAMSTEC), and Chinese National Aretic Remixed layer eventually approaches the freezing point(4.search Expedition (CHINARE-Arctic) data from 2003,There are two typical profiles of the winter mixed2008 and 2010.layer, as shown in Figure 2. The type appearing in mostof the Canada Basin exhibits a temperature maximum at1 Warm waters in the upper Canadadepths between 60 and 80 m (Figure 2a). The warm wa-Basinter called STM was advected into the Arctic Ocean fromthe Pacific during the previous summer. During winter,Here, the upper ocean means water from the surface water in the top 40 -50 m is cooled by convection, butdown to the cold core near 150 m. Solar heating, windthe temperature maximum from 60- 80 m remains. Themixing, vertical convection and ocean-ice air interactionsother type appears in the shelf and slope areas, without aall occur in this layer. The water structure in this level isSTM. The cold convective layer links with the cold corenot only a result of changing sea ice and climate, but also at 150 m, with a uniform temperature near (feeding back ice melting and climate change. The ψ ing point (Figure 2b). This type indicates that PW hasper Arctic Ocean has experienced rapid change in recent not been transported into the area, or that the wateryears. Interannual variation of the temperature profle there was replaced by Pacific Winter Water. A mixedhas become an indicator of this change. In this section,layer caused by convection appears in the top 40- -50 mwe discuss the types of water temperature structures in for both profle types. These two fundamental structuresthe upper Canada Basin, as a foundation of the changing provide the background for the summer water structure.summer water profiles.n summer, the remnants of the two winter structuresSalinity29303235a)b)0F12060 F16-1.6.-1.2.-0.8200 o1.6-1.2 -0.8 -0.4 0.0 0.Temperature/CTemperature/Figure 2 Two typical structure of the rennants of winter convection. (2) Convectively mixed layer with the STM, at156°27 .0w, 75° 15.40'N (on 17 April 199); (b) convectively mixed layer withou中国煤化工，8536.50N (on20 April 199 Thick and thin lines are for temperature and salinit, respectivelYHCNMH G226ZHAO Jinping, et al. Adu Polar SciDecember(2011) Vol.22 No.4still exist under the thick pack ice, as important water emerge with or without the STM. The NSTM and STMstructures.are distinguishable because the NSTM is within the con-vective mixed layer, and its peak is always shallower than1.2 NSTM structure in summer40 m. The maximum NSTM under sea ice is usually low,In spring, solar radiation penetrates the sea ice and enters but it can become much greater with decreased thicknessthe seawater, producing the NSTM. More solar energy and concentration of sea ice. Since ice melt water remainsenters from leads of marginal ice zones, causing higher in the top of the ocean, there usually is a shallow, lowtemperature peaks.Corresponding to the two typicalsalinity layer from 10 -20 m,mixed by the turbulencewinter structures of the upper ocean, two kinds of typi- of drifting ice. There is a strong, thin halocline undercal NSTM appear, as shown in Figure 3. One embodiesthe fresher layer. The NSTM usually occurs under thea NSTM+STM structure (Figure 3a), and the other has halocline, because the weak turbulence of the haloclieonly the NSTM (Figure 3b). Therefore, the NSTM mayprevents the upward loss of heat content.SalinitySalinit303232_(a)二(b)40 -40台80一。8(上120-F 120162005-1.4Temper:re/e-0.8200-1.8-1.6 -1.4 -1.2 - 1.0Temperature/CFigure3 TwO tpical srucures of the NSTM. (@) NSTM with the STM, at 100.60W, 75°59 64'N (on 23 August 20);(b) NSTM without the STM, at 140*11.29W, 7026.56N (on 17 September 196). Thick and thin lines are for temperatureand salinity, respectively.In undisturbed open water, more solar energy entersNSTM from the STM. These criteria apply to developedthe ocean. As long a8 the air temperature is less than the NSTMs. Emerging NSTMs have very weak temperaturewater temperature, the NSTM can appear quickly, and peakss (Figures 5a and 5b), but they represeant an impor-the temperature extreme is much greater than that undertant stage in the NSTM annualcycle.sea ice (Figure 4). The observed maximum temperatureThe NSTM under sea ice should exhibits a sin-extreme exeeds 5C. It is vrifed that both solar heating gle temnperature peak, since the coling above is nearlyand surface cooling are principal mechanisms for produc-constant. The NSTM in ice-free water, however, maying the NSTM.have a multi-peak structure, such as the double peaks inThe lifetimne of the NSTM is dynamic. It emerges,Figure 6a and triple peaks in Figure 6b. We speculatestrengthens, and vanishes. Jackson et al.!1] proposed that this multi-peak中国煤化工Y variablevariablethree criteria for the NSTM in termns of salinity and cooling at the surfacor of ice-minimum value of temperature peak, to ditinguish the covered water with vfYHCNMHGSummer water temperature structures and their interannual variation in the upper Canada Basin227Salinityof insufficient solar heating. There is also no NSTM when.邵232_34nair temperature is greater than water temperature, because of insufficient surface cooling.Solar radiation penetrating sea ice and leads in sum-Smer is the main heat source for the upper ocean, as widelyaddressed by previous studies[11-13,20]. Under the infu-，80-ence of global warming,the Arctic is experiencing rapidchanges, such as increasing air temperature, and decreas-ing ice extent!21- 22| and thickness[23- -24]. Sea ice extent120 tparticularly declined in summer 2007, by 37%[25]. Thesefactors will greatly infuence the structure of the NSTM.Furthermore, the NSTM is not only related to so-160lar radiation, but also to surface cooling[14]. If there isno surface cooling, the temperature maximum should ap-pear at the surface. Therefore, heating by solar radiation200and surface cooling are two basic factors in NSTM for-Temperature/Cmation. Solar heating occurs when solar energy entersthe ocean, whether it is ice-covered or ice-free. SurfaceFigure 4 NSTM in open water. Observed at 158° 59.28'W,cooling occurs when the surface temperature is less than73° 1.56'N (on 19 September 1999). Thick and thin lines arethat of seawater; whether this condition is caused by seafor temperature and salinity, respectively.ice or cold air.Therefore, the NSTM could appear in either ice-In previous studies, only the NSTM in the Pacificcovered or ice free water, when solar radiation heating was reported. Solar radiation heating and surface coolingand surface cooling exist simultaneously. The NSTM un-also occur in the Atlantic. There, however, in summer,der sea ice is much more stable and longer lasting than the warm current influences the marginal ice zone, andin open water, where there is no wind stirring and weakwater with higher temperature submerges the NSTM.turbulence. There is no NSTM under thick ice, because Therefore, the NSTM should only appear in the upper26(a(b40 F0-卤120E 120200 L00 L1.2-0.8-0.40.0Developing NSTM. (a) Observed at 156° 17.05'W, 75°35.26'N (on 19 A中国煤化工t 139*59.34'W,74°59.82N (on 2 September 2003). Thick and thin lines are for temperature and:YHCNMH G .228ZHAO Jinping, et al. Adv Polar Sci Decermber(2011) Vol.2 No.4Salinity。2S (b)40 F0号E 12016016之-1.3-1.2-1.1-1.0-0.4 .0.0”Temperature/ cTemperature/ cFigure 6 NSTM with multiple temperature peaks. (a) Observed at 99 19.73' W, 69*49.31'N (on 31 July 2006); (b) observedat 129°58.39'W, 73*46.61'N (29 July 2007). Thick and thin lines are for temperature and salinity, respectively.water with low temperature.exceeds 31.2. However, the salinity becores much less inIn other regions of the world oceans, there are sev-the Arctic because of mixing with ice melt water(6].eral kinds of water with a temperature maximum in sub-In the Canada Basin, runoff from the Mackenziesurface level. They are caused by seasonal cooling orRiver controls the upper ocean along the coastl27]. Thesubduction of warm and salty water. The mechanism of river water is a source of warm water but with very lowNSTM with solar heating and suface cooling only ap-salinity. By mixing with ice melt water, the river waterpears in polar regions, including the Arctic and Antarc-generates a very low salinity area. When the river watertitravels far from the river delta and mixes with ice melt1.3 Wind-mixed structure in summerwater, it is dificult to distinguish it from PW by salinityalone.In ice-free areas, wind stirring usually produces a mixedThe third source is local ice melt water in open wa-layer of 15- -20 m or more during storms. The mean tem-ter. After ice melt, water with low salinity and low tem-perature of the wind-driven mixed layer depends on airperature is heated by local solar radiation and mixed bytemperature, as shown in Figure 7. In cold conditions, awind. A mixed layer forms quickly in open water, whenrelatively cold wind-driven mixed layer (Figure 7a) willacted upon by strong wind.be produced as a typical upper ocean temperature proAs PW entering the Arctic follows the retreatingfile in summer. Sometimes, a relatively warm mixed layermarginal ice zone, the PW distributes widely in openappears, as shown in Figure 7b. The mixed layer waterwater in the Canada Basin, and mixes with ice meltcomes from three possible sources.PW as a main source in the upper ocean usuallywater. Thus, the three types of water are dificult tooriginates from the Bering Strait, with a relatively warmdistinguish in open water. Isotope analysis is somesurface temperature. It is mixed by wind in the northerntimes more effective to reveal the mixing ratio in a waterBering Sea, before entering the Arctic with a uniformsamplel28].mixed layerl26]. PW always maintains a warm mixed1.4 Advected water under sea icelayer during its northward journey, until its heat is ex-中国煤化工hausted. The original salinity of PW in the Bering StraitNot all temperatureMYHCNMHGtheNSTM.229Salinity。232_34242628(a4(上12016200 !-1.2-0.8-0.40.0Temperature/CFigure 7 Colder (a), and warmer (b) wind-driven mixed layers. (a) Observed at 143° 16.86'W, 75°02.70'N (on 30 August2003); (b) observed at 150900.01'W, 73°00.00'N (on 26 July 2008). Thick and thin lines are for temperature and salinity,respectively.We have stressed that the NSTM is formed by solar heat- originates elsewhere and has a much greater heat con-ing and surface cooling. If the heat of a temperature tent and upward heat fux. The heat release from thismaximum is produced by other mechanisms, it shouldwater can accelerate ice melt and retard ice freezing overbe distinguished from a NSTM. A typical case is water a large area. Therefore, we should carefully distinguishunder sea ice that is advected from open water, which wethe NSTM from the warm advected water, to accuratelycall advected water.calculate the heat budget of the upper ocean. NevertheIn a marginal ice zone, mixed water can advect be-less, it is difficult to distinguish the two by CTD dataneath sea ice, or sea ice could drift above the mixed wa-alone.ter, so it may be observed in ice-covered regions. TheSometimes, the temperature structure is more com-open water may infiltrate under sea ice, because of theplex. When there is no strong wind, a NSTM can againbarotropic pressure gradient along the marginal ice zonedevelop, superposing on the wind-driven mixed layer un-established by wind. The advected water, with mixedder solar heating and surface cooling conditions. A tem-layer structure, replaces winter water when it is trans-perature peak can arise from the advected water underported under sea ice. The original mixed layer structuresea ice (not shown).is easily distinguished from the NSTM; the NSTM tem-1.5 Categories of upper temperature structuresperature peak is normally sharp, whereas the tempera-ture profile of the advected water has no peaks (FigureIn the Canada Basin during summer, there are four ba-8a). However, since the overlying sea ice cools the ad-sic kinds of water structures: remnant winter convectionvected water, the uniform temperature profile of the ad-under thick ice, the NSTM in marginal ice zones, thevected water will be sharpened, and a temperature max-wind-mixed layer in open water, and advected water un-imum will appear similar to the NSTM (Figure 8b).der sea ice (Table 1).The thermal contributions of the NSTM and ad-NSTMs are found under sea ice, in ice-free water,vected water to sea ice are very diferent. The heat trans-and superposed on mixed or advected water, all of whichport of the NSTM to sea ice is from local heating, result-are related to sol中国煤化工ling. Anothering in a relatively small heat fAux. The advected watertemperature maYHC N M H Gurface cooling230ZHAO Jinping, et al. Adu Polar SciDecember(2011) Vol.22 No.4Salinity。26_2832_34_3034_36(a)(b三。8(:120上工12060 F6000 t-Temperature/CTemperature/ CFigure 8 Advected water under ice cover. (a) Advected water observed at 158°29.91'W, 73° 30.05'N (on 25 September2000); (b) Advected water similar to the NSTM observed at 159° 35.88'W, 73° 12.06'N (on 19 September 1999). Thick andthin lines are for temperature and salinity, respectively.Table 1 Structures of upper ocean water in summerWinterSunmerLocationMechanismConvective mixed layerPack iceInsufficient solar radiationConvectiveNear surface temperatureIce covered water andSolar radiation heatingmixed layerundisturbed open waterand surface coolingWind-driven mixed layerOpen waterWind-driven mixingAdvected waterIce covered waterSurface coolingfrom advected water. It is not formed by solar heating, climatel291.but it is very similar to the NSTM and there is no reliableway to distinguish the two. Consequently, this tempera-2.1 Occurrence of temperature maximumture maximum is sometimes taken to be the NSTM.2 Multiyear variation of the NSTMThe frequency of NSTM depth during 1993 2010 isshown in Figure 9. Most NSTMs were from 10 -29 m,Among the four summer water structures, the remnant with greatest frequency at 20 m. Those NSTMs shallowerwinter water, wind-driven mixed layer, and advected wa-than 10 m are developing; they deepen and strengthenter are less dependent on the sea ice condition. The during summer. At depths below 30 m the NSTMs aremultiyear variation of upper ocean structure is mainlydeveloped, or formed from advected waters. The dates ofexpressed by the NSTM.the obtained data are denoted in Table 2 by blue marks,The NSTM furnishes an important indication of in- and data with the NSTM are marked by red diamonds.creased absorption of solar energy by seawater. DeMost NSTMs before 2001 appeared in September, becreased ice coverage, concentration and thickness permit cause of heavy summer sea ice that nersisted until then.more solar energy transmission into the water, increasingFrom 2002 onward,中国煤化工ier, and oc-ice melt, and producing positive feedback to the Arctic currences were mosYHCNMHGSummer water temperature estructures and their interannual variation in the upper Canada Basin23155mits more solar energy to enter the ocean. Although thes0NSTM is correlated with ice concentration and thickness,the correlation coefficient of NSTM and ice concentration45is very low, R2=0.18l171. The reason is that the NSTM40is infuenced by ice conditions over a period of time, notinstantaneous ice conditions. As a result, the NSTM con-tains information from previous ice conditions, providing25a method for understanding the heating history.2.3 Distribution of depth and temperature20extremes of NSTM15Figure 11 displays the maximum value of the NSTM vs.10depth in all years. This again shows that most NSTMsoccurred at depths between 10 -29 m. Prior to 2003, tem-perature maxima (marked by black symbols) were much101520。2530.354045warmer than the freezing point at most stations, becauseice was heavy and most NSTMs occurred in ice free wa-Figure 9 Frequency of depths of NSTM temperature max-ter. However, during 2004 -2010, the NSTM in most ofima during 1993- 2010.the Canada Basin had very low temperatures (most were2.2 Spatial distribution of observed NSTM from less than 0°C). This was because most NSTMs appeared1993 to 2010under sea ice, where the penetrating solar energy wasBecause of limited CTD data, it is impossible tomuch weaker.provide a gridded spatial distribution of the NSTM.Figure 12 shows temperature peaks colder than 0.5'CFigure 10 shows locations of CTD measurements (blue and their depths, from the period 2004 2010. This showssquares) and the NSTM (red dots) during 1993- 2010.a quasi-linear scattering, lower temperatures correspond-The NSTM showed claer regional and year-to-year dif ing to greater depth, and vice versa. The average tem-ferences.perature and depths of NSTMs in each year are markedFrom 1993 to 2003,0 sea ice was sill heavy, and most by black dots. The average depths in most years wereobservations were around the margin of ice cover. There-about 20 m. Only in 2007 Was it about 16 m, which wasfore, most NSTMs during that period occurred aroundrelated to an extreme minimum of ice cover that year.the margin of the basin, such as the slope of the Chukchi Average temperatures were around -0.8C to -1.1rC, butSea, Barrow Canyon, and Beaufort Sea shelf. There are increased to about -0.5C in 2004, 2007 and 2009. Thisfew data from the central Canada Basin over the tenindicates that the temperature extreme of the NSTM hasyears. In 1993 and 1997, there were some stations in theincreased in recent years.southwest basin, which showed the NSTM in the deep3 Conclusion and discussionpart of the basin. From 2003 to the present, Canada hascarried out Arctic cruises each year, for long-term obser- CTD data from 1993- 2010 have been analyzed to studyvation in the central basin. In 2003, the NSTM did not the spatial distribution and interannual variation of up-appear in central basin. During 2004 -2010, the NSTMper water in the Canada Basin during summer. Thereappeared in most of the basin, even at the northernmost are four kinds of water temperature structures: (1) thestation around 85.5°N. The NSTM has been a normal remains of a winter convective mixed layer under packphenomenon in the central basin since then.ice; (2) water with 8 NSTM under sea ice or in undis-The large area over which the NSTM appears is turbed open water; (3) a wind-driven mixed layer formnedrelated to the rapid reduction of ice concentration and by PW, river discharge or ice melt water in open waterthickness in summer. Since the albedo of ice is about 5-6 areas; (4) water中国煤化工m open water.times that of the water, the lower ice concentration perThe NSTMI YHC N M H Gthe convective232ZHAO Jinping, et al. Adu Polar Sci December(2011) Vol.22 No.4001993199419951996 .2018151997199199920001618001.200200312A16060___2160_050620072008140in2010 .190 .Figure 10 NSTM spatial distribution during 1993- 2010. CTD locations are indicated by blue squares, and NSTM positionsby red dots.mixed layer formed in winter. Solar radiation penetrat- appear in undisturbed open water, as long as surfaceing sea ice heats seawater and increases its temperature; cooling persists. Under sea ice, the NSTM is simple, withat the same time, sea ice on the surface cools the near-only a single temperature peak. In marginal ice zones andsurface water. Solar heating and surface cooling are two open waters, howeve中国煤化工mplicated,basic factors in NSTM formation. The NSTM can alsowith multiple tempe:YHCNMHG234ZHAO Jinping, et al. Adu Polar Sci December(2011) Vol.22 No.4References16 Chen Z H, Zhao J P. Simulation for the thermodynam-ics of subsurface warm water in the Arctic Ocean. Acta1 Swift J H, Aagaard K. 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