upper-ocean hydrography of the nordic seas during the international polar year (2007–2008) as...

Upper-ocean hydrography of the Nordic Seas during the InternationalPolar Year (20072008) as observed by instrumented sealsand Argo floats

Pl E. Isachsen a,n, Signe R. Srlie b, Cecilie Mauritzen c, Christian Lydersen d,Paul Dodd d, Kit M. Kovacs d

a Norwegian Meteorological Institute, P.O. Box 43 Blindern, N-0313 Oslo, Norwayb Norwegian Meteorological Institute, P.O. Box 6314, N-9293 Troms, Norwayc DNV GL, P.O. Box 300, N-1332 Hvik, Norwayd Norwegian Polar Institute, Fram Centre, N-9296 Troms, Norway

a r t i c l e i n f o

Article history:Received 24 October 2013Received in revised form21 June 2014Accepted 25 June 2014Available online 17 July 2014

Keywords:Nordic SeasTemperature changeSalinity changeAnimal-borne instrumentationSealsArgoInternational Polar YearMEOP

a b s t r a c t

Following indications of recent warming trends in the Nordic Seas, we have studied the hydrography ofthese marginal seas from the summer of 2007 until the fall of 2008, using observations gathered byinstrumented seals and Argo floats. The combined dataset shows that the upper ocean was indeed bothwarmer and saltier over much of the Nordic Seas in 20072008 compared to the average ocean state forthe period 19562006 (based on the World Ocean Atlas 2009). There are also indications that the surfacePolar Waters of the East Greenland Current were colder and fresher than the climatology, though thequality of the climatology is questionable for this region given the low number of historical observations.Dynamic height calculations suggest that the observed hydrographic changes were associated withenhanced northward upper-ocean thermal wind transport in the east and possibly also enhancedsouthward transport in the west.

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1. Introduction

The Nordic Seas (Fig. 1) act as the main gateway between theArctic Ocean and the rest of the world oceans. Warm and salinewaters from the North Atlantic are transported northward alongthe Norwegian coast in the east, into the Arctic, and cold and freshwaters are transported southward along the Greenland coastin the west, out of the Arctic. A mid-ocean ridge system, theJan-MayenMohnKnipovich Ridge (JMK Ridge), sets up a stronghydrographic front between the two domains. The temperaturedifferences between the oppositely directed currents reflect astrong surface heat loss in the north, and the salinity differencesreflect a net input of fresh water from rivers and from an excess ofprecipitation over evaporation. So the ocean circulation throughthese seas is responsible, in part, for considerable sensible andlatent heat transports to high northern latitudes (Blindheim andsterhus, 2005).

Analyses of historic observations and data from long-termmonitoring programs point to systematic changes in the hydro-graphy of the Nordic Seas over the last few decades (Mauritzenet al., 2013). The seas appear to be getting both warmer and saltier,at least in the east where Atlantic Water dominates. Time seriesfrom repeated transects off the Norwegian coast, from the BarentsSea opening between North-Norway and the top of Svalbard, aswell as from the west coast of Svalbard, show clear positive trendsin temperature and salinity from the beginning of measurementsin the late 1970s. The positive trends flattened out or reversed inthe early 1990s but strengthened again in the late 1990s and havecontinued up until the present (Dye et al., 2012). According toDickson and sterhus (2007), these last decades have beendominated by a warm and moist southerly airflow directed alongthe eastern boundary of the North Atlantic during the prevailingNAO-positive conditions (enhanced pressure difference betweenthe Icelandic Low and the Azores High; Hurrell et al., 2001). Thesuggestion is that this airflow has been responsible for driving awarmer (Dickson et al., 2000) and stronger (Mork and Blindheim,2000; Orvik et al., 2001) flow of Atlantic Water northwardsthrough the Nordic Seas.

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Deep-Sea Research I

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n Corresponding author.E-mail address: [email protected] (P.E. Isachsen).

Deep-Sea Research I 93 (2014) 4159

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Observational coverage is much less dense in the north-western parts of these seas; regular monitoring has been limitedto a mooring array and yearly transects across the Fram Strait at791N (de Steur et al., 2009) and to semi-regular transects thatcross the Greenland Sea at 751N (Ronski and Budus, 2005). Datafrom these programs suggest that upper-ocean temperature andsalinity trends here may be much weaker than in the east or evennon-existent. Finally, monitoring transects off the north coast ofIceland again show positive upper-ocean temperature and salinitytrends over these same periods, especially in the Atlantic Waterthat enters the Nordic Seas through the Denmark Strait west ofIceland (Dye et al., 2012).

So the existing data suggest both warming and salinification ofwaters flowing from the North Atlantic into the Arctic Ocean.But the information regarding changes in waters flowing out of theArctic is less conclusive, which is not surprising given the scarcityof observations. The presence of sea ice in the west hampers ship-based observations and introduces seasonal biases into those thatexist. In particular, the southward flow of waters in the EastGreenland Current (EGC) has been seriously under-sampled in allseasons. Satellite observations of sea surface temperature (SST), ascompiled in e.g. the OSTIA (Donlon et al., 2011), OSI SAF (2013) and

OISST (Reynolds et al., 2007) reanalyses or operational analyses,can give information with unrivaled spatial extent and detail. Butthese observations are also hampered by the presence of sea iceand, importantly, give information pertaining only to the actualsea surface. More year-round hydrographic observations fromdepth are still very much needed, especially from the westernparts of the Nordic Seas. In the study presented herein, we explorehydrographic data from the entire Nordic Seas region from twocost-effective all-weather sources, Argo floats and instrumentedseals.

The gradual development of a global array of Argo floats(http://www.argo.ucsd.edu) has been a significant step towardscontinuous monitoring of the oceans at depth. In the Nordic Seasthere are now anywhere from five to twenty floats operationalat any one time. The ocean flow at high latitudes is stronglytopographically steered, and Argo floats therefore tend to followtopographically guided boundary currents or tend to circle aroundwithin deep basins (Voet et al., 2010). But Argo floats consistentlyunder-sample shelf regions (see e.g. Roquet et al., 2013) and,importantly, are not designed to collect data where there isheavy sea ice. The Ice-Tethered Profiler (ITP; Toole et al., 2011) isspecifically designed to obtain hydrographic profiles under pack

Fig. 1. Map of the Nordic Seas with schematic surface currents: Warm and salty Atlantic Water (red) flows northward along the eastern boundary while cold and fresh PolarWater (blue) flows southward in the west. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

P.E. Isachsen et al. / Deep-Sea Research I 93 (2014) 415942

http://www.argo.ucsd.edu

ice, but this instrument, which is attached beneath a specific icefloe, tends to drift quickly away from the marginal ice zone (MIZ)once the ice breaks up or melts.

Animal-borne instruments provide an additional possibility toobserve ocean hydrography in the MIZ as well as in the open ocean(Nicholls et al., 2008; Ohshima et al., 2013; rthun et al., 2012,2013). Depending on the particular species selected, such instru-ments can obtain vertical oceanographic profiles down to morethan 2000 m depth (McIntyre et al., 2010). The animals often moverapidly in a directed fashion, delivering long-distance transects ofnear-synoptic data. As they travel between breeding, foraging andresting locations, their tracks often cut across important frontalregions that are of interest to both biologists and physicaloceanographers. Individual animals often retrace previous tracks,thus providing repeated sections over a variety of time scales ina cost-effective manner. Importantly, diving animals obtain mea-surements from all seasons and often from data-sparse regions,e.g. the MIZ (Boehme et al., 2009). Data from instrumentedanimals have already been used extensively in the SouthernOceans (Boehme et al., 2008a,b; Charrassin et al., 2008; Biuwet al., 2010; Meredith et al., 2011; Nst et al., 2011; Roquet et al.,2013) and also, to a somewhat lesser extent, in the north (Lydersenet al., 2002, 2004; Laidre et al., 2010; Grist et al., 2011).

During the summer of 2007 and spring of 2008, 20 hoodedseals (Cystophora cristata) were tagged with CTDs and released inthe central Nordic Seas, as a part of the IPY-MEOP (InternationalPolar YearMarine mammals Exploring the Ocean Pole to pole)project. These seals produced 6030 hydrographic profiles withinthe region. When combined with concurrent Argo float data(another 1535 profiles) an extensive coverage of the upper-oceanhydrography in 20072008 was achieved.

We use this dataset to compare the hydrography in 20072008with climatological mean conditions. Specifically, we ask whetherwarming and salinification trends observed in the repeated sec-tions along the eastern boundary of the Nordic Seas are repre-sentative of larger areas or merely limited to the boundary currentitself. Below, we first present the various datasets used, includingthe gridded climatology that we use as a comparative reference.Calibration of salinities from the seal-borne instruments is dis-cussed briefly, with details spelled out in the appendix. Then, wecompare the new data from seals and Argo floats with theclimatology by testing for significant differences east and west ofthe central ridge system and by plotting temperature and salinity

anomaly maps. Finally, a qualitative look at changes in upper-ocean circulation, diagnosed from the difference in dynamicheight between 20072008 and the long-term mean, is presented.The various findings are then summarized and discussed.

2. The hydrographic data

2.1. The WOA09 climatology

The World Ocean Atlas 2009 (WOA09; Locarnini et al., 2010;Antonov et al., 2010) is a gridded climatology of ocean hydro-graphic data from 1956 to 2006. The product is based on in situobservations from a range of platforms which have been objec-tively interpolated onto a 1111 horizontal grid at a set ofstandard depths. Quality control includes checks for duplications,checks for absolute ranges and the strength of gradients, checksfor statistical outliers and, finally, checks for static stability ofprofiles. For each horizontal grid point and depth the climatologyoffers an objectively analyzed (smoothed) mean temperature andsalinity of quality-controlled data, as well as raw sample statistics(number of observations, sample mean, standard deviation andstandard error). We use monthly WOA09 temperature and salinityvalues in the analysis below, but start off with a look at howannual mean conditions (obtained by averaging over 12 months)are represented in this product.

Fig. 2 shows the WOA09 annual-mean temperatures and salini-ties at 10 m depth. The front between Atlantic and Arctic waters,following the JMK Ridge, is described by temperature contrasts of68 1C and a salinity contrast of about 0.5. The EGC, and indeed theentire western parts of the Nordic Seas, is heavily influenced byPolar Water, ice melt waters (with some contribution from riveroutflows) characterized by near-freezing temperatures and verylow salinities. The topographic constraints posed by the centralridge system ensure that this fresher water is largely confined tothe west.

The low-salinity waters in the west are extremely buoyant andlargely confined to depths shallower than 200 m depth. This canbe seen in Fig. 3 which shows the climatological hydrography overa section crossing the Nordic Seas at 72.51N. Waters below thisdepth are modified return Atlantic Water and intermediate anddeep waters exported from the Arctic Ocean (Mauritzen, 1996).The bottom waters (below 2000 m depth) west of the JMK Ridge

2 0 2 4 6 8 10Temp. [C]

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Fig. 2. Annual-mean temperature (left) and salinity (right) at 10 m depth, as compiled in the WOA09 gridded climatological atlas. White regions in the north and northwestof the right panel have salinities lower than 33.5. The black line at 72.51N indicates the location of the vertical transect shown in Fig. 3.

P.E. Isachsen et al. / Deep-Sea Research I 93 (2014) 4159 43

are the coldest waters in the entire region; they are formed bydeep convection in the Greenland Sea during winter (Aagaardet al., 1985). East of the ridge, warm and salty Atlantic Waterdominates down to 600800 m, approximately the depth of theGreenlandScotland Ridge in the south. Below this depth the sameintermediate waters that are found in the west reside.

WOA09 is based on 50 years of observations and gives athorough description of large-scale climatological mean condi-tions. But, the data coverage is uneven. Fig. 4 shows the number ofindividual temperature profiles that have gone into creating thestatistics for each 1111 grid cell. The historical coverage east ofthe central ridge system is impressive, but it drops off drastically inthe west, particularly during the winter months when the icecover makes ship-borne observations hard to obtain. Large regionson the East Greenland Shelf and Slope have less than 10 historicalobservations. The data density for salinity is generally lower thanthat for temperature. As will be seen below, a few Argo floats andinstrumented seals have been able to sample these same regionsrather well, over the course of only 15 months.

2.2. The new data from 2007 to 2008

2.2.1. ArgoThe global Argo array system currently consists of about 3500

profiling floats. These floats drift at a predefined parking depth,typically 1000 m. Every 10 days they descend to about 2000 mbefore performing an ascent to the surface, during which theymeasure conductivity, temperature and pressure (depth) with aCTD sensor. The floats then stay at the surface long enough totransmit the measured data to Argos satellites (http://www.argos-system.org) before returning to the drift depth. The Argostracking system also estimates the position of the floats by a leastsquares method from the Doppler shift of signals received (CLS,2014). The lifetime of an Argo float is about 4 years, with a batterycapacity that allows for the collection of more than 150 profiles(Boehme and Send, 2005).

The accuracy of the position fixes of the Argo floats is typicallybetter than 1.5 km (CLS, 2014), and the accuracies of pressure,temperature and salinity data are about 5 dbar, 0.005 1C and 0.01,

1 0 1 2 3 4 5 6Temp. [C]

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th [m

]

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Fig. 3. WOA09 annual-mean potential temperature (left) and salinity (right) for a section crossing the Nordic Seas at 72.51N (as shown in Fig. 2).

Fig. 4. The total number of historical temperature observations at 10 m depth that have gone into making the 1111 WOA09 climatology: (left) MayOctober and (right)NovemberApril. A logarithmic color scale has been used (white regions have no profiles at all). The black dashed lines show the 25% contour for the seasonal-mean iceconcentrations in 2008 (ice data from http://osisaf.met.no). (For interpretation of the references to color in this figure caption, the reader is referred to the web version of thispaper.)


http://www.argos-system.orghttp://www.argos-system.orghttp://www.argo.ucsd.edu

respectively (http://www.argo.ucsd.edu/FAQ.html). The salinitydata, however, can suffer from drift in the conductivity sensors.These values are often improved at the central Argo data centersthrough post-calibration that follow procedures developed byWong et al. (2003), Boehme and Send (2005) and Owens andWong (2009). When considered appropriate, time-varying correc-tions to the conductivity sensors are applied via optimal inter-polation and piece-wise least squares regression with respect tohistorical data from within well-mixed mode waters or deephomogeneous water masses. Such procedures generally allowdelayed-mode salinities to be corrected to better than 0.01(http://www.argo.ucsd.edu/Data_FAQ.html).

For our study Argo float data fromMay 2007 until January 2009were used, starting two months before the seal sampling period(see below) and ending two months after. Fig. 5 shows the tracksof the 41 floats that happened to be in the Nordic Seas at somepoint during this period. For the analysis presented below we useddelayed-mode data when available and real-time data (whichhas only undergone automatic quality checks) when these werethe only data available. We only used data with position and timequality flags equal to one (good data) and also only used pressure,temperature and salinity data with quality flags equal to one.Furthermore, we discarded data with salinities lower than 32 (thelower range of the seal data) and data from unstable densityprofiles. Using these criteria, 1535 profiles were retained in thedataset.

2.2.2. Seal-borne CTD-SRDLsCTD-SRDLs (http://www.smru.st-andrews.ac.uk/Instrumenta

tion/CTD) are small, relatively inexpensive data recording andrelaying devices that can be deployed on marine mammals. Whenthe animals ascend from a dive, the instrument records verticalprofiles of conductivity, temperature and pressure (depth). Dataare then relayed to the Argos satellite systemwhile the animal is atthe surface breathing. Due to limited data transfer rates of theArgos system and energy constraints of the CTD-SRDL itself, only asub-set of pressure, temperature and conductivity triplets fromeach profile are transmitted to the satellite. The sampling protocol

and the energy budget are typically designed to obtain a max-imum of one year of data (Boehme et al., 2009).

CTD-SRDLs were deployed on 20 hooded seals in this study, 17of which provided useful observations from our region of interest.We deployed instruments on three of the hooded seals immedi-ately following their annual moult in 2007 and set the datasampling protocol such that the battery life of the tag wouldprovide approximately one year of data collection, taking therecord up to the timing of the next period when the seals wouldshed their hair again (see Boehme et al., 2009 for details regardingthe energy budget of the tags). The other 17 seals were taggedfollowing breeding and the tags were set to sample and send datamuch more intensively for the months between the breeding(March) and the annual moult (JulyAugust).

All animal handling was approved by the Norwegian AnimalResearch Authority. Animals were captured on an ice floe using asling net, before being weighed using a tripod and a Salter springscale. After weighing, animals were immobilized with an intra-muscular injection of Telazol (0.8 mg per kg body mass) before theCTD-SRDL was glued to the head or the neck region of the animalusing quick-setting epoxy. Blood and blubber samples werecollected for another study, then each animal was tagged with aRototag in each hind flipper (for possible later identification), andthen free to go when the immobilizing agent stopped working.All animal-handling protocols were approved by the NorwegianAnimal Research Authority.

A total of 6112 CTD profiles were obtained from the seal tracksshown in Fig. 6. The seals crisscrossed most of the deep parts ofthe Nordic Seas. The interior basins were often crossed in a verydirected fashion, and the seals did most of their diving along shelfedges. The shelf slopes are important upwelling and downwellingfrontal areas that undoubtedly concentrate prey of interest to theseals, as discussed by Bost et al. (2009). In the Nordic Seas the iceedge near the shelf break in the west also plays a particularlyimportant role since the sea ice in this area is where seals in thispopulation breed. After the breeding season they do foraging tripsacross the Nordic Seas for a month or two before returning to the

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Fig. 5. Argo float tracks in the Nordic Seas from May 2007 to January 2009.

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Fig. 6. Tracks of hooded seals in the Nordic Seas from mid-July 2008 to end ofNovember 2008.


http://www.argo.ucsd.edu/FAQ.htmlhttp://www.argo.ucsd.edu/Data_FAQ.htmlhttp://www.smru.st-andrews.ac.uk/Instrumentation/CTDhttp://www.smru.st-andrews.ac.uk/Instrumentation/CTD

ice in the west once again to moult (Lavigne and Kovacs, 1988;Salberg et al., 2008).

The seal data loggers were programmed to transmit 18 observa-tion triplets (pressure, temperature and salinity) per profile to thesatellite. The first point in a dive occurs at a pre-selected depth (7 min our study). Another dive point marks the deepest depth reachedin a dive. Eight additional points are spread at pre-set intervalsbased on the maximum depth of the individual dive. The remainingpoints are randomly chosen between these other values. Themeasurements ranges were set to be from 3 1C to 37.5 1C andfrom 32 to 37 for temperature and salinity, respectively.

The instrument accuracies for temperature and salinity, whentested in controlled environments, are better than 0.005 1C and0.02, respectively (Boehme et al., 2009). However, errors in datareturned from the sea can be considerably higher, up to 0.03 1Cand 0.1 (Roquet et al., 2009, 2011), due to the pressure effects onsensors and due to an interference from the animals head on theinductive field of the conductivity sensor. For our study we werenot able to correct for pressure effects by comparison with ship-borne CTD casts, so we adopted an instrumental temperature errorof 0.03 1C. The CTD-SRDL temperature data were not scrutinizedfurther, except for a manual inspection of TS diagrams. From suchinspection, obvious outliers as well as data that fell below thefreezing line were removed.

The salinity data were calibrated against nearby ARGO observa-tions to estimate constant offsets for each instrument caused bythe head effect on conductivities. For this study calibration had tobe done on the derived salinities rather than actual conductivities,as salinity was the value stored and disseminated. Our calibrationwas based on a simplified version of procedures developed byWong et al. (2003), Boehme and Send (2005) and Owens andWong (2009) for calibrating the Argo data. Details are given inAppendix A, but essentially involved comparing seal salinities tonearby Argo observations objectively mapped to the seal positions.For each seal CTD-SRDL, a constant correction factor was thendetermined by a least squares regression that minimized thesquared difference between seal and Argo salinities. The salinitycorrections were all found to be negative (Table A1), in agreementwith earlier findings (Boehme et al., 2009). The calibrationprocedure also resulted in estimates of the over-all instrumenterror of CTD-SRDL salinities, and these turned out to lie in therange between 0.02 and 0.1.

2.2.3. The combined datasetFig. 7 shows the position of all of the hydrographic profiles

collected by Argo floats and seals in 20072008. Most of the NordicSeas are sampled, and the seal observations are responsible for thebulk of the coverage, especially over the traditionally poorly coveredshelf areas and ice-covered areas in the west (compare with Fig. 4).The coverage is higher in the summer months, but even in winterthere are seal observations along the MIZ in the west. As notedpreviously, the Argo data are gathered primarily from the topogra-phically guided boundary currents and the central basins. The twoobservation sources are thus highly complementary in achievingbroad regional coverage.

Histograms of the bottom depths at the location of the profiles,the measurement depths and the observation months are show inFig. 8. The bottom depth data show how the Argo observations areconcentrated over the deep ocean while the seal dataset alsocovers shallow regions. The histogram of measurement depthsshows that seals primarily sample the top 500 m of the watercolumn, regardless of their geographic position, while the Argofloats also obtain data from greater depths. Finally, the time stampdata show moderate seasonal biases in both datasets; seal andArgo observations are more abundant in early summer and late

summer and fall, respectively. Because of these observationalbiases, we will primarily be looking at summer and winter monthsseparately. But first, we do a preliminary exploration of all of the20072008 data combined.

Fig. 9 shows hydrographic conditions at 10 m depth for all ofthe available Argo and seal data. There is considerable scatter, asexpected from synoptic observations of ocean fields containingdiurnal, mesoscale and seasonal variability. But the large-scaletemperature and salinity patterns nonetheless agree well with theclimatological mean distributions shown in Fig. 2; we see warmand salty Atlantic Water to the east of the JMK Ridge and cold andfresh water to the west. So 15 months of combined seal and Argodata are able to map nearly all important large-scale features ofthe Nordic Seas surface hydrography.

3. Results

3.1. Temperature and salinity

3.1.1. Regionally averaged differencesFor a statistical comparison of the new data with the WOA09

climatology, the combined dataset of Argo and seal temperaturesand salinities were interpolated vertically onto four standard depths(10 m, 50 m, 100 m and 200 m) and then mapped, month-by-month, onto the WOA09 grid. For each month the profile data weregrouped, or binned, into the 1111 grid cells. Then, for cells thathad at least four observations from at least two different observingplatforms (different seals or Argo floats) we calculated the samplemean, standard deviation and standard error within the cells. Thetotal temperature and salinity errors for each such grid cell werethen estimated as the sum of the squared standard errors and themean squared instrumental errors (described above).

Temperature and salinity differences were calculated month-by-month for each grid cell and each of the four depths bysubtracting the WOA09 values from the newly gridded data. Thesegridded differences were then averaged over two regions, theregion dominated by Atlantic Water east of the JMK Ridge and theregion dominated by Arctic water west of the ridge. Student'st-tests were set up to check whether these regionally averageddifferences were significant. For each of the two regions we rantwo-sided t-tests on the temperature and salinity differences andrejected the null hypothesis (no difference) for p-values smallerthan 0.05. To incorporate our information on temperature andsalinity errors within each grid point, we repeated the tests 10,000times in a Monte Carlo simulation, drawing both 20072008values and climatological values randomly from normal distribu-tions defined by the errors statistics in each grid cell. Finally, wejudged regionally averaged temperature and salinity differences tobe statistically significant if 95% of the repeated t-tests rejected thenull hypothesis. These tests were made for both the summer andwinter seasons and for each of the four depths (10 m, 50 m, 100 mand 200 m).

Results are summarized in Table 1. Summer temperatures in20072008 were significantly higher than climatological condi-tions both east and west of the ridge and at all depths (except forat 200 m in the east). Temperatures east of the central ridgesystem were also significantly higher during winter at all depths.In the west, the domain-averaged winter temperature differenceswere all negative but indistinguishable from zero. Domain-averaged salinity differences were all positive but small. Summer-time differences in the east were statistically significant down to100 m.

Seal and Argo observations thus give unequivocal evidence that20072008 temperatures were higher than the climatological


mean east of the JMK ridge system, down to 100200 m depth.Waters west of the ridge were also anomalously warm inthe summertime but not during winter. There is also someindication of significant salinity differences, but the magnitudeof the differences are small and comparable to our estimates ofthe instrumental errors. The spread of the data within each of the

sub-regions, here represented by the sample standard deviation, isalso large. This is particularly true west of the JMK ridge where, aswe see, there is indication of consistently negative (but notstatistically significant) temperature differences during winter. Toinvestigate these signals in more geographic detail we thereforeturn to maps of temperature and salinity differences.

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80 oNFig. 7. Position of all Argo (blue) and seal (red) hydrographic profiles obtained between July 2007 and November 2008: (left) MayOctober profiles and (right) NovemberApril profiles. The black dotted lines show the 25% contour for the seasonal-mean ice concentrations in 2008 (ice data from http://osisaf.met.no). (For interpretation of thereferences to color in this figure caption, the reader is referred to the web version of this paper.)

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Fig. 8. Histogram of (left) bottom depths, (middle) observation depths and (right) observation months for (red) seal and (blue) Argo observations. (For interpretation of thereferences to color in this figure caption, the reader is referred to the web version of this paper.)

2 0 2 4 6 8 10Temp. [C]

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Fig. 9. Hydrographic conditions at 10 m depth, as observed by all instrumented seals and Argo floats between July 2007 and November 2008: temperature (left) and salinity(right).


http://www.argos-system.org

3.1.2. Maps of differencesMaps of the temperature and salinity differences on the

WOA09 grid are provided in Appendix B. These show the detaileddistribution of the data that have gone into the regional averagescalculated above. But here we study differences at the highestpossible resolution, namely at the position of each individualseal and Argo profile. For this analysis, temperature and salinityanomalies were calculated by subtracting climatological valuesinterpolated onto each Argo and seal data point. This operationwas done month-by-month and, as before, summer (MayOcto-ber) and winter (NovemberApril) data were grouped. The resultsare shown in Figs. 1013.

Near the surface, at 10 m depth, there is again striking evidencethat summer waters were anomalously warm over most of theNordic Seas in 2007 and 2008. There are also large regions ofanomalously salty surface waters during summer, especially in thenortheast, but some negative differences are also seen. In parti-cular, 20072008 surface waters within the East GreenlandCurrent appear to be both colder and fresher than the climatology.The same patterns are generally also seen during winter monthsalthough the lower data density makes these signals less clear,especially when some data points are rejected to make the griddedfields (see Fig. B1 in Appendix B). The overall picture deeper in thewater column is generally similar. But anomalies at depth areprogressively weaker, and salinity anomalies at 200 m are notdetectable from the error estimates.

The East Greenland Current stands out from the rest of theregions, and data from there are likely the reason why regionallyaveraged winter temperature differences west of the JMK Ridgewere negative (although statistically insignificant). The contrastbetween hydrographic differences in waters in the EGC and thosein the central Greenland Sea is also likely the reason behind thehigher standard deviations of the regionally averaged statisticswest of the JMK ridge seen in Table A1. The new data hencesuggest that the Polar Water exported from the Arctic Ocean nearthe surface was colder and fresher in 20072008 than the averagefor 19562006. But any comparison with climatology in this regionmust be treated with care. Not only are the number of new datapoints relatively low in this region, but the number of historicalobservations and the robustness of the gridded means in theclimatology are also low (Fig. 4).

Further east, there is indication that waters along the westernflanks of the JMK ridge were colder than the climatology. Thissignal does not show up in the gridded maps in Appendix B due tothe relatively strict criteria for assigning values to grid cells. Butsince the signal is consistent over a larger region in the scatterplots seen here, it is likely real. These are the so-called inter-mediate waters, which are either formed locally or imported from

the Arctic Ocean and advected cyclonically around the GreenlandSea (Mauritzen, 1996). So the new data suggest that the inter-mediate waters, which ultimately will return to the North Atlantic,were colder than normal in 20072008. But another possibilityalso exists. Considering the joint effect of the warm anomaly onthe other side of the JMK Ridge, the signal may also simply reflectchanges in the frontal structure that separates the eastern andwestern basins. The front is set up by the topographic constraintsof the ridge system itself, so the signal seen here is likely notassociated with a lateral movement of the whole front. But it mayindicate a steepening of the front, i.e. an enhanced thermal windtransport along the ridge. This possibility is discussed below.

3.2. Dynamic height

The hydrographic differences described above can have directdynamic implications via their modification of the density fieldand the thermal wind shear. But compensating temperature andsalinity effects on density may be important in some regions,especially in the transition zone between waters of Atlantic versusArctic origin. So not all hydrographic differences discussed aboveneed have dynamic consequences. Actual dynamic effects may befound by calculating changes in dynamic height between twopressure (or depth) levels:

Dp1; p2 Z p2p1

T ; S;p dp;

where T ; S; p is the specific volume anomaly, which is a functionof temperature, salinity and pressure (see e.g. Pond and Pickard,1983). Horizontal gradients in Dp1; p2 give the thermal windshear between pressures p1 and p2.

We calculated dynamic height differences, i.e. the differencebetween the 20072008 dynamic height field and the climatology,from 200 dbar (approximately 200 m depth) to the sea surface, i.e.

D200 Z 0200

S;A dpZ 0200

clim dp;

where S;A and clim are the specific volume anomalies calculatedfrom the combined seal and Argo data and from the climatology,respectively. To obtain a picture that was as extensive as possible,the calculations were done on the position of the seal and Argoprofiles, and data from all months of the year were used. Consi-dering the possibility of introducing seasonal biases, the resultsshould be viewed qualitatively rather than quantitatively.

Fig. 14 shows the estimated dynamic height difference. Itsuggests that the dynamic height relative to 200 dbar was lowerthan the climatological mean in the central Greenland and Icelandseas, especially in the west, close to the continental slope east of

Table 1Sample statistics (mean and standard deviation) of gridded temperature and salinity differences integrated over the eastern and western sub-domains of the Nordic Seas.Statistically significant means, as judged from Monte Carlo simulations of Student's t-tests, are shown in bold.

Depth (m) West East

T (1C) stdT (1C) S () stdS () T (1C) stdT (1C) S () stdS ()

MayOctober10 0.52 1.29 0.05 0.52 0.74 0.72 0.06 0.1550 0.73 1.13 0.03 0.28 0.41 0.61 0.04 0.06

100 0.52 0.88 0.02 0.17 0.19 0.57 0.02 0.05200 0.21 0.73 0.02 0.06 0.05 0.71 0.02 0.05

OctoberApril10 0.36 1.82 0.06 0.38 0.40 0.73 0.02 0.1650 0.09 1.68 0.13 0.27 0.38 0.68 0.03 0.08

100 0.06 1.55 0.08 0.18 0.32 0.68 0.02 0.07200 0.11 1.17 0.02 0.06 0.23 0.82 0.02 0.05


Greenland. Dynamic height was higher than the mean immedi-ately to the east of the JMK Ridge as well as along parts of theeastern continental slope along the Barents Sea opening and westof Svalbard. There is also an indication of higher than normaldynamic height along the EGC; this is linked to the anomalous lowsalinity there. The large-scale horizontal gradients correspondingto these dynamic height changes imply enhanced northwardupper ocean thermal wind shear along the JMK Ridge as well asalong northern parts of the eastern continental slope. They alsoimply enhanced southward upper ocean thermal wind shear inthe EGC.

So it appears that the upper-ocean baroclinic circulation in20072008 period can be characterized by enhanced upper-oceanpoleward transport along the JMK Ridge and in the easternAtlantic Water domain. There is also suggestion of an enhancedsouthward transport in the East Greenland Current. The implica-tions for heat and freshwater transports are quite uncertain sincethey depend on delicate correlations between the circulationchanges and the property changes that, in part, drive these samecirculation changes. But, if we consider the circulation changes inisolation, and also neglect the barotropic component of the flow

due to sea level gradients, the implication will be an anomalouslyhigh northward transport of sensible heat and southward trans-port of fresh water.

4. Discussion and conclusions

The hydrographic observations studied here, from instrumen-ted seals and from Argo floats, give systematic indications thatmost of the Nordic Seas region was warmer in 20072008compared to the long-term average for 19562006. The statisticalevaluation of regionally averaged trends, via mapping of the newobservations onto the 1111 grid of the WOA09 climatologyfollowed by Monte Carlo-based Student's t-tests, showed thatthe warming signal is significant down to 100200 m in the east.In west of the JMK Ridge there is also evidence of warmer watersin 20072008 during summer, but not during winter. Inspection ofthe geographic distribution of the new observationsand theirdifference from the climatological meansuggests that waters inthe East Greenland Current were in fact colder in 20072008compared to the climatology.

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Fig. 10. Temperature differences (left) and salinity differences (right) between the 20072008 data and the WOA09 climatology at 10 m depth. Climatological values havebeen interpolated onto the seal and Argo profiles. MayOctober differences are shown on the top and NovemberApril difference on the bottom. Thick black lines show thelocations of three repeated hydrographic transects along the eastern domain, the Sviny, Gimsy and Srkapp sections (from south to north). (For interpretation of thereferences to color in this figure caption, the reader is referred to the web version of this paper.)


These findings are in general agreement with other indepen-dent ocean temperature observations, at least in the east. Forexample, the temperature data from the Sviny Section off thesouthwestern Norwegian coast (see Fig. 10) were the highest everin 2007, being 0.9 1C warmer than the climatology (Huges et al.,2008). In 2008 ocean temperatures were 0.2 1C above the long-term average (Holliday et al., 2009). In the Gimsy Section furthernorth along the Norwegian coast the measured temperatureanomalies were 0.4 1C and 0.11C higher in 2007 and 2008,respectively, and in the Srkapp Section off the west Svalbardcoast they were 0.6 1C and 0.3 1C higher than the norm. Finally,satellite SST observations, e.g. the OSTIA reanalysis and operationalanalysis (Donlon et al., 2011), indicate a gradual warming of thesurface layers of the Nordic Seas over the last decades.

The satellite SST observations in particular help put thesein situ observations into a broader perspective. Fig. 15 showsremote-sensed SSTs averaged over the entire Nordic Seas duringthe last 25 years. The estimates are based on the OSTIA (Opera-tional Surface Temperature and 20 Sea Ice Analysis) reanalysisfrom 1985 to 2007 and on the OSTIA operational analysis sincethen (Donlon et al., 2011). Both the five-year running mean and aleast-squares linear fit across the whole period indicate a gradualwarming of the surface layers over the last 25 years. This

independent dataset thus supports the conclusion that surfacewaters over most the Nordic Seas were anomalously warm in200708, and also gives an indication that this signal is not due toan isolated event but rather is part of a longer-scale trend.Conversely, the results found in this study can be taken asindication that the longer-term warming seen at the surface bysatellites likely penetrates down to 100200 m in this oceanregion.

The seal and Argo observations also suggest elevated salinitiesof the Atlantic Water in 20072008. These signals were statisti-cally significant down to 100 m during summer, but not so duringwinter (when the observational density was lower). These findingsalso largely agree with independent observations from the easternNordic Seas. Salinities measured in the same hydrographic sec-tions mentioned above were above the long-term average in both2007 and 2008. In the Sviny Section salinity values were 0.05 and0.04 above the climatological mean, in the Gimsy Sectionsalinities were 0.04 and 0.01 above the norm, and in the SrkappSection salinities were 0.07 and 0.05 above (Huges et al., 2008;Holliday et al., 2009).

Finally, we made an estimate of dynamic height differencesbetween the 20072008 and the 19562006 climatological mean.These calculations suggest enhanced northward upper-ocean

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Fig. 11. Same as Fig. 10 but at 50 m depth.


thermal wind transport in 20072008 along the JMK Ridge andalong the north-eastern continental slope. If the observationalbasis from the East Greenland Current can be trusted, there is alsoan indication of enhanced southward baroclinic transport in thisregion. The map of dynamic height differences was producedby grouping all of the data (from different seasons) together, sothe inferences made should be considered qualitative ratherthan quantitative. But, the differences seen in Fig. 14 are largelyconsistent with recent trends in the sea surface height fieldmeasured by satellites. Fig. 16 shows the sea surface heightdifferences between the 20062009 average and the 19932010average, estimated from gridded sea level anomaly data dissemi-nated by AVISO (http://www.aviso.altimetry.fr). The large-scalepatterns of enhanced sea levels in the Atlantic Water domainand lowered levels in the Arctic Water domain agree with Fig. 14and so do many of the large-scale spatial gradients. The differencesusing 20072008 values instead of 20062009 values are similarbut more noisy.

There is of course little reason to expect a perfect matchbetween the two estimates of circulation changes. One givesindication of changes in the thermal wind shear between 200 mdepth and the surface while the other gives indication of changes

in the absolute geostrophic flow at the surface. The sea surfaceheight differences are also relative to a much shorter referenceperiod than the dynamic height difference is. Perhaps the mostnotable discrepancy between Figs. 14 and 16 is found in the centralGreenland Sea where Fig. 14 indicates lowered dynamic height dueto hydrographic differences but where no corresponding depres-sion in the sea surface is seen. The lack of agreement here is nottoo surprising since this is the region that has the weakest densitystratification in the entire Nordic Seas, i.e. the region wheresurface height variations are governed less by baroclinic dynamicsthan anywhere else. There are also other discrepancies, notablyalong the East Greenland Shelf and Slope where the altimeter datadoes not give unequivocal indication of enhanced southward flowalong the entire stretch. It is worth noting, though, that thealtimeter observations in this region are also hampered by thepresence of sea ice (see Kaczmarska, 2011, for a study that usesreprocessed data from this region that also give information oversea-covered regions).

Clearly the suggestion brought forward by the new hydro-graphic observations studied here, that the East Greenland Currentwas colder and fresher in 20072008 than climatology, requiresscrutiny. The new observations here were relatively few in

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Fig. 12. Same as Fig. 10 but at 100 m depth. Note the different color scales. (For interpretation of the references to color in this figure caption, the reader is referred to the webversion of this paper.)


http://www.aviso.altimetry.fr

numbers compared to other regions. And, as Fig. 4 indicates, thisregion is also seriously under-sampled by traditional methods, sothe quality of the gridded climatology itself comes into question.Grist et al. (2011) studied hydrographic observations in the NorthWest Atlantic from instrumented seals and Argo floats and found asimilar result, namely anomalously cold temperatures in shelfareas in the Labrador Sea. These authors attributed this signal tosampling errors in existing climatological analyses. This may verywell be the situation along the western flanks of the Nordic Seas,so a more careful evaluation of the actual meaning of the EGCsignal seen here is clearly needed in the future.

There remains little doubt that waters in the eastern parts ofthe study area, especially in the Atlantic Water domain east of theJMK Ridge, were both warmer and saltier in 20072008 than theclimatological mean. The observational coverage by the seal andArgo data in 20072008 is unrivaled by any other in situ observa-tion system, and we have been able to study the spatial extent ofthe warming signal with unprecedented detail down to 100200 m. The extensive coverage has even allowed us to makeinferences about region-wide baroclinic circulation changes. SoArgo and seal-borne hydrographic observations can undoubtedlygive very useful information about the ocean state at highlatitudes, as also demonstrated in a number of earlier studies.

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Fig. 13. Same as Fig. 10 but at 200 m depth. Note the different color scales. (For interpretation of the references to color in this figure caption, the reader is referred to the webversion of this paper.)

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Fig. 14. The dynamic height (from 200 dbar to the sea surface) difference betweenthe 20072008 conditions and the climatological mean.


We are convinced that climate monitoring, and also short-termoperational forecasting, can benefit greatly from the use of theserelatively inexpensive observation platforms. The global Argoprogram has already been an unprecedented success, and it isclear to us that the Nordic Seas Argo program should be strength-ened. Nations bordering the Arctic high-latitude oceans shouldalso initiate wider use of seal-borne instrumentation for regularmonitoring purposes, similar to monitoring programs in theSouthern Oceans that use marine mammal platforms broadly(e.g. The Southern Ocean Observing System, SOOS; Rintoul et al.,2012).

Acknowledgments

The hooded seal data was collected as part of the InternationalPolar Year program: Marine mammals Exploring the Oceans Poleto pole (MEOP), financed by the Norwegian Research Council(Grant number: 176477). We would like to thank Lutz Bachmann,Jrgen Berge, Martin Biuw, Mike Fedak, Bjrn Munro Jenssen, ReneSwift, ystein Wiig, Hans Wolkers and the crew on RV Lance forhelp during the fieldwork. We also thank Abdelkader Mezghani forhelpful discussions on the statistical treatment and three anon-ymous reviewers for very useful comments on the paper.

Appendix A. Calibration and error estimates for the CTD-SRDLsalinity data

Salinities from the CTD-SRDLs were corrected by comparisonwith nearby Argo observations. Since property variability generallydecays with depth, seal-to-Argo comparisons were done for thedeepest observation of every seal profile that reached deeper than50 m. For each such seal profile, nearby Argo salinities wereinterpolated vertically to the depth of the deepest seal observa-tion. A weighted mean Argo salinity at the bottom of seal profile kwas then estimated as

SAk lwk;lS

Ak;l

lwk;lA:1

where SAk;l is the interpolated salinity from Argo profile l and wk;l isthe relative weight assigned to that Argo observation.

The weights were taken as the inverse of a squared uncertaintyof the interpolated Argo salinities, or

wk;l SAk;l2: A:2

We took these uncertainties to be the instrumental error reportedfor each Argo float scaled by exponentially increasing functions ofgeneralized distances between the Argo and the seal observation.Specifically, the uncertainty of any one Argo observation was madeto increase exponentially with the horizontal distance from theseal profile, the minimum vertical distance to the lowest sealobservation, the difference in time and, finally, also the climato-logical mean salinity difference between the Argo and seal posi-tions. Thus, the error assigned to Argo observation l whencompared to seal observation k was

SAk;l SAl ejrk;lj=r ejzk;lj=z ejtk;lj=t ejSCk;l j=S : A:3

Here, SA is the instrumental uncertainty of the Argo data andr; z; t and SC are the differences in horizontal position, theminimum difference in depth, the time difference and the clima-tological salinity difference between the observations in seal andArgo profiles k and l. Finally, decay parameters r, z, t and S areused to scale the various differences (see below).

The above procedure resulted in a set of seal vs. Argo observa-tion pairs for each CTD-SRDL that could be used to calibrate theCTD-SRDL salinity observations. A simplest possible regressionmodel was applied, assuming salinity biases that are constant foreach instrument (sensor drift over time can be considered small;Roquet et al., 2011). Hence, for any given CTD-SRDL we have a setof linear equations:

SAk SSkbnk; A:4where SkS is the CTD-SRDL salinity recorded at the bottom of sealprofile k, SAk is the corresponding objectively mapped mean Argo

Fig. 15. SSTs from the OSTIA analysis, spatially averaged over the entire NordicSeas. Blue lines show daily values, the red dashed line shows five-year low-passedvalues, and the black line shows a linear fit. The time period covered by the seal andArgo data studied here is shaded. (For interpretation of the references to color inthis figure caption, the reader is referred to the web version of this paper.)

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Fig. 16. The sea surface height differences between the 20062009 average and the19932010 average. Data from http://www.aviso.altimetry.fr.

Table A1Calibration offsets with errors b and b, as well as an estimate for the total salinityerror of seal CTD-SRDLs (seals 12, 19 and 20 did not produce useful data for thisstudy).

Seal b b S Seal b b S

1 0.13 0.01 0.02 11 0.09 0.02 0.032 0.04 0.01 0.02 12 3 0.06 0.01 0.02 13 0.06 0.01 0.024 0.24 0.04 0.04 14 0.08 0.02 0.035 0.39 0.02 0.03 15 0.17 0.01 0.026 0.11 0.02 0.03 16 0.12 0.02 0.037 0.06 0.01 0.02 17 0.12 0.01 0.028 0.13 0.07 0.08 18 0.15 0.03 0.039 0.10 0.01 0.02 19

10 0.15 0.10 0.10 20


http://www.argo.ucsd.edu/FAQ.html

salinity, b is the salinity correction (to be determined) for this CTD-SRDL and, finally, nk is the error in applying the linear model toprofile k.

Applying this model to multiple seal profiles of the same CTD-SRDL gives an overdetermined system of linear equations for eachseal instrument. We then find the correction b that minimized thesum of weighted squared errors

J kWkn2k ; A:5

where Wk is a mean weight for each seal profile:

Wk Ll wk;l

L; A:6

for a total of L Argo profiles objectively mapped onto seal profile k.The solution to (A.5), given (A.6), is

bkWkSAk SSk

kWk; A:7

and the estimate of the error is

b kWk=K 1

K

" #1=2; A:8

where K is the total number of SSk; SAk seal-vs-Argo regressionpairs for this particular CTD-SRDL.

The solution to the inverse problem depends on the decayparameters used to assign relative weights wk;l to Argo observa-tions. These parameters should reflect expected decorrelationscales of the ocean environment sampled by the instruments.For the horizontal, vertical and temporal decay parameters it isnatural to use mesoscale decorrelation scales, i.e. horizontal scalesof a few tens of kilometers, vertical scales of a few tens of meters(for the mesoscale heaving of the vertical stratification) andtemporal scales of a few weeks. Finally, the scale for the climato-logical salinity difference should reflect a typical salinity jumpacross semi-permanent fronts (as represented in the WOA09fields).

We tested the regression model using decay parameters r15,20, 25, 50 km; z5, 10, 25, 50 m; t15, 30, 60 days and S0.025,0.05. In addition, to reduce the chance of influence from clearlyinappropriate observations, only Argo data that fell within two suchdecay scales (e.g. jrk;ljo2r) were used. These tests demonstratedthat the results were not too sensitive to the exact values chosen.But smaller values often admitted only a very few data points forcomparison. In the end we therefore chose the smallest decayparameters from the above lists that met two sampling criteria:(1) a minimum of two Argo profiles were required to form anobjectively mapped Argo salinity SAk at the bottom of each sealprofile, and (2) a minimum of four profiles where seal and Argo datacould be compared were required to correct salinities of a giveninstrument. With these criteria we ended up choosing r 25 km,z 25 m, t 30 days, and S 0:05. For this set of parameters, thenumber of sealArgo comparisons obtained for the various instru-ments ranged from 4 to 111, with an average count of 38.

Table A1 shows the estimates of b and b for the CTD-SRDLinstruments that produced data for this study. Also shown is anestimate of the total uncertainty for each instrument obtained byadding the reported instrumental error of the CTD-SRDL salinities(discussed in Section 2.2.2).

Appendix B. Maps of hydrographic differences on the WOA09grid

Figs. B1B4 show temperature and salinity differences betweenthe 20072008 data and the climatological mean on the 1111WOA09 grid. As outlined in Section 3.1.1, the new in situ data weregrouped into the WOA09 grid cells, month by month, and cellswere assigned sample mean temperatures and salinities if theyhad four or more observations collected by two or more differentinstruments. The figures below show mean temperature andsalinity differences averaged over summer months (MayOctober)and winter months (NovemberApril) at four depths (10 m, 50 m,100 m and 200 m).


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Fig. B1. Temperature differences (left) and salinity differences (right) at 10 m depth. The WOA09 data have been interpolated onto the seal and Argo profile positions. MayOctober differences are shown on the top and NovemberApril differences are shown on the bottom.


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2 1 0 1 2Temp. [C]

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Upper-ocean hydrography of the Nordic Seas during the International Polar Year (20072008) as observed by instrumented...IntroductionThe hydrographic dataThe WOA09 climatologyThe new data from 2007 to 2008ArgoSeal-borne CTD-SRDLsThe combined dataset

ResultsTemperature and salinityRegionally averaged differencesMaps of differences

Dynamic height

Discussion and conclusionsAcknowledgmentsCalibration and error estimates for the CTD-SRDL salinity dataMaps of hydrographic differences on the WOA09 gridReferences

upper-ocean hydrography of the nordic seas during the international polar year (2007–2008) as...

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