JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. C3, PAGES 4833-4846, MAY 20, 1985

Thermohaline Circulation in the Mediterranean Seas

K. AAGAARD

School of Oceanography,University of Washington, Seattle

J. H. SWIFT

ScrippsInstitution of Oceanography,La Jolla, California

E. C. CARMACK

Departmentof the Environment,West Vancouver,British Columbia

The renewal of the deep North Atlantic by the various overflows of the Greenland-Scotlandridges is only one manifestation of the convective and mixing processeswhich occur in the various basins and shelf areas to the north: the and the Greenland, Iceland, and Norwegian seas,collectively called the Arctic Mediterranean. The traditional site of deep ventilation for these basinsis the Greenland Sea, but a growing body of evidence also points to the Arctic Ocean as a major source of deep water. This deep water is relatively warm and saline, and it appears to be a mixture of dense, brine-enriched shelf water with intermediate strata in the Arctic Ocean. The deep water exits the Arctic Ocean along the Greenland slope to mix with the Greenland Sea deep water. Conversely, very cold low-salinity deep water from the Greenland Sea enters the Arctic Ocean west of Spitsbergen.Within the Arctic Ocean, the excludesthe Greenland Sea deep water from the Canadian Basin, leaving the latter warm, saline, and rich in silica. In general, the entire deep-water sphere of the Arctic Mediterranean is constrained by the Greenland-Scotland ridges to circulate internally. Therefore it is certain of the intermediate waters formed in the Greenland and Iceland seas which ventilate the North Atlantic. These waters have a very short residence time in their formation areas and are therefore able to rapidly transmit surface-inducedsignals into the deep North Atlantic.

INTRODUCTION relatively shallow, being only about 600-800 m at their deep- The primary northern hemisphere source of deep venti- est. lation for the World Ocean lies north of the Greenland- The hypsography of the AM (Figure 2) shows the total Scotland ridge system,over which dense water spills into the volume to be 17 x 106 km 3, or about 1.3% of the volume of deep North Atlantic (cf. Mantyla and Reid [1983] for a com- the World Ocean. The largestcomponents are the two major prehensivediscussion). The seas to the north of these ridges Arctic Ocean basins, which make up 75% of this volume' the Canadian Basin alone accounts for 43% of the total volume of consist of a series of interconnected basins, each with its own distinctive characteristics and contributions to the thermoha- the AM. Figure 2 also makes clear the large proportion of the Arctic Ocean which is continental shelf. The Greenland and line circulation. In keeping with earlier nomenclature, we denote these basins collectively as the Arctic Mediterranean Norwegian seas together account for only 22% of the total (AM) [cf. Sverdrupet al., 1942, p. 15], and in our discussionof volume, and they do not have unusually large shelves.The their ventilation, we shall pay particular attention to the role Iceland Sea contains only about 2% of the volume, but it is of of the Arctic Ocean. Far from being simply a passiverecipient major importance in ventilating the North Atlantic [Swift et of ventilated water from the south, we shall show that the al., 1980]. Arctic Ocean is itself an important source of dense water, a Figure 2 also gives the mean depth of selected isopycnals portion of which is exported southward through . within each basin of the AM (cfi the appendix for discussionof This is a role in the thermohaline circulation distinctly differ- notation and units used in this paper and a comparison at ent from the estuarine one which in the past has received the different pressuresof the isopycnals of Figure 2). The surface principal attention [e.g., Sti•tebrandt,1981]. ao = 27.9 separatesthe upper water masses,including those of Figure 1 shows that the Arctic Ocean constitutesby far the the pycnocline,from the intermediate ones in the next density largest portion of the AM. There are two major basins, the range. Although it is theseuppermost waters which have been Canadian and Eurasian, bordered by extensive shelf seas. most heavily studied, they constitute only 17% of the total volume of the AM. South of 2600-m-deep Fram Strait, the basin complex west of the mid-ocean ridge is defined as the Greenland Sea and that The intermediate waters have densities as great as a• = to the east is the Norwegian Sea. The latter leads into Fram 32.785, which value is found at the sea surface in the cyclonic Strait through a long trough extending northward. The area gyres of the Greenland and Iceland seasduring winter, while between Iceland, Greenland, and the island of Jan Mayen has waters denser than this outcrop much more rarely. The divi- its own distinctive circulation and hydrography, and it is usu- sion at this density value was in large part chosenbecause no ally referred to as the Iceland Sea. The ridges from Greenland denser water is directly exported to the North Atlantic. The to Scotland which confine the AM at its southern end are mean lower boundary of these intermediate overflow waters is found at very shallow levels in both the Greenland and Ice- Copyright 1985 by the American GeophysicalUnion. land seas.The intermediate waters comprise 29% of the total Paper number 5C0002. volume, so that 46% of the total volume of the AM is poten- 0148-0227/85/005C-0002 $05.00 tially in communication with the remainder of the World 4833 4834 AAGAARDET AL.' THERMOHALINECIRCULATION IN THE ARCTIC MEDITERRANEAN

180 ø

90 ø 90 ø W E

BARENTS SEA

Fig. 1. The Arctic Mediterranean. Depths in meters.The long line extendingfrom the southernNorwegian Sea to the southernCanadian Basin locatesthe sectionof Figure 3. The line PL representsthe Point Lay section(Figure 6), and ME the Meteor section(Figure 10). The triangle CS locatesmooring CS-2A (Figure 7).

Ocean, while 54% is effectivelyisolated. Becausethe tradition- In Figure 2 we have distinguishedbetween deep waters in al deep-water boundary of 0øC lies mostly above the a• = the densityrange a• - 32.785to 0'2 -- 37.457and thosedenser 32.785 surface, the estimate that 54% of the total volume is than 0'2 --37.457. The latter owe their high density to the made up of deep water is by traditional standards an under- relatively low temperatures,near or below -IøC. This is par- estimate. ticularly important at elevated pressures,because of the tem-

ICELAND ARCTIC OCEAN SEA CANADIAN EURASIAN GREENLAND NORWEGIAN BASIN BASIN SEA SEA • 0'o=279

iooo

2OOO _ .457.785 0.4'I06 km 3 I

Ld 3000 _

4000 7'3'106km3 •l o I 2

5OOO • xI0 6 KM;' 5.9' 106 kms

Fig. 2. Hypsography of the Arctic Mediterranean,based principally on the GEBCO (General Bathymetric Chart of the Oceans).Horizontal coordinate is area of each depth interval, planimetered at the following isobaths: 200, 500, 1000, 1500, 2000, 2500, 3000, 3500, and 4000 m. The total volume for each basin is given below the individual hypsographic curves.The horizontal bars representthe mean depths within each basin of isopycnalsurfaces separating upper, intermedi- ate, and two categoriesof deep waters. AAGAARD ET AL.: THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN 4835

perature dependence of the compressibility. In the Arctic meridional section of Swift, Reid, and Clarke through the Ocean the Lomonosov Ridge rises above the 37.457 Greenland and Norwegian seas (Figure 3, section location in a2-surface,and the densestwaters in the Eurasian Basin are Figure 1). Some of the Arctic Ocean stations are actually care- therefore excluded from the Canadian Basin. Furthermore, fully chosenstatistical reductionsfrom a larger, noisier data deep-water formation in the Canadian Basin apparently does set assembledfrom the ice camp stations,such as Alpha in not yield a product cold enough to raise the density to the 1957-1958, and from shipbornework near Fram Strait, such highest values found in the other major basins.The seeming as the 1980 Ymer cruise. Other Arctic Ocean stations, such as absenceof the denser forms of deep water from the Iceland the 1979 Lomonosov Ridge Experiment (LOREX)profiles, Sea (Figure 2) is due to the very small amounts of such water were used intact. The Norwegian and Greenland Sea stations presentin this shallow basin. were occupied by the Hudson in March 1982. Because of The effectivenessof surface-drivenconvection in ventilating sparse data and uncertainties in accuracy, the Arctic Ocean the deep ocean depends critically on the prevailing stratifi- portion of this section is not resolved and defined at a level cation and associated ice conditions. The Arctic Ocean is in commensuratewith that of the southern portion (cfi the ap- fact so stably stratified by low-salinity surface water that pendix for discussionof data quality and uncertainties). winter cooling does not drive convection over the deep basins Figure 3a shows that surface waters are warmest in the below about 50 m. Conversely, ice formation is suppressed Norwegian Sea, becoming much cooler in the central Green- when saline water is supplied to a region of intense cooling, land Sea. The small area warmer than IøC at the surface in destabilizingthe water column. This appears to be the casain Fram Strait representswater from the Norwegian Sea passing the Greenland Sea, which not only receivescold, low-salinity westward across the section as it recirculates and moves surface water from the Arctic Ocean via the southward flow- southward with the East Greenland Current. Relatively warm ing East Greenland Current, but also is fed by warm and water also enters the Eurasian Basin, where it sinks beneath saline water from the south via the Norwegian Atlantic Cur- the much lessdense, cold, but low-salinity polar waters and is rent. The surfacewater entering the Norwegian Sea from the gradually modified. In this manner, an intermediate temper- Atlantic is amply saline to form deep water instead of freezing, ature maximum is maintained throughout the Arctic Ocean. but it is initially much too warm to do so west of Norway. This water is normally referred to as the Atlantic layer of the Only when it enters the cyclonic gyre of the Greenland Sea Arctic Ocean, but within the overall context of the water has it been sufficientlycooled to allow its transformation into massesof the AM it most nearly resemblesthe intermediate deep water. waters of the cyclonicgyres farther south (the arctic domain in The traditional deep-water circulation scheme [Helland- the Swift and Aagaard [1981] terminology),and in the Green- Hansen and Nansen, 1909] holds that deep water originates in land and Iceland seasit would in fact be defined as a compo- winter in the Greenland Sea, where the densestisopycnals are nent of Arctic Intermediate Water. In general, when found closeto the sea surface.This sourcemay to some extent intermediate-depth temperature or salinity extrema are found be supplementedby cooling and freezing in the in water less dense than a• = 32.785 (our division between [Nansen, 1906]. The deep waters then spread throughout the intermediate and deep waters), they are properly referred to as AM, constrainedin doing so only by bathymetry. As the deep Arctic Intermediate Water, sincethermohaline processesin the waters circulate, they may be modified, especiallyin passages arctic domain play a major role in their formation. For his- or over ridges. The principal observational basis for this torical reasons, we are comfortable in continuing to refer to schemeis that the winter properties of the surface and deep the warmest water in the Arctic Ocean as the Atlantic layer, waters are nearly the same in the Greenland Sea, and that the but we recommend that analogous terminology (e.g., Atlantic deep waters are colder there than anywhere else in the AM. Water) not be used in water massclassifications. Present understandingis that the Greenland Sea is indeed a Figure 3a also shows that while deep water is much the major deep-water formation area, with a renewal time of order coldest in the Greenland Sea, it is warmest in the Canadian 30-40 years [Carmack and Aagaard, 1973; Peterson and Basin becausethe Lomonosov Ridge excludesthe coldestdeep Rooth, 1976]. The details of the renewal process are only water. Note, however, that even the Eurasian Basin deep speculative,with leading candidates being intense convection water is warmer than that of the Norwegian Sea, which has in small regions,the so-calledchimneys [Killworth, 1979], and been supposedto be its source. subsurfaceformation involving double-diffusive mixing [Car- The salinity distribution in Figure 3b suggestsmany of the mack and Aagaard, 1973; McDougall, 1983]. Regardlessof the same circulation features as does that of temperature. For formation mechanism, the end product, viz., Greenland Sea example, the salinity maximum in Fram Strait is a product of Deep Water (GSDW), is the coldest and least saline (T < the recirculation of warm and saline water westward. How- - 1øC,34.88 < S < 34.90) of the AM deep-water masses.In ever, our chief interest is in the deep salinities, which are the traditional deep circulation scheme,the other basinsof the lowest in the Greenland Sea. Since this is also the coldestdeep AM do not contribute to the deep ventilation. water, so that the GSDW represents an extremum in water The first serious modification of this scheme came with the mass characteristics, the Greenland Sea must be a source of discovery that the dense overflow through Denmark Strait deep water. However, this source accounts for the deep into the North Atlantic consists primarily of intermediate characteristicsonly within the Greenland Sea, which contains water formed in the Iceland Sea [Peterson and Rooth, 1976; lessthan 12% of the deep water of the AM. Deep salinities are Swift et al., 1980]. However, it has recently become apparent of intermediate value in the Norwegian Sea, increase in the from the salinity distribution that deep-water renewal must Eurasian Basin, and reach a maximum in the Canadian Basin. also occur within the Arctic Ocean [Aagaard, 1981], and it is While the Arctic Ocean salinity values are probably not uni- upon this theme that we focus our attention. formly of the same quality as those to the south, we believe that measurementuncertainties are not such as to significantly DEEP-WATER FORMATION IN THE ARCTIC OCEAN distort the distribution shown in Figure 3b (cfi the appendix To provide an overviewof the water characteristicsof the for a detailed discussionof data quality). AM, we have extended into the Arctic Ocean the unpublished We therefore distinguishbetween four deep-water massesin 4836 AAGAARD ET AL.' THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN

IOOO

2000

3000 -055

4000 -(a) I I

IOOO

2000

3000

4000

0 ,---=i--•-•/-,,, I I I I1 I I I ill I Ill11127•llillil I I I I I I I I I till rn 27.9------,.,,.,,., ,.., It I'•0'• • Ii •%.-,"'_•,-CY.--•-•--. 2805 oooz8o5 - II - -_ /'/ hl

3000 oool,o, I I I I I I I I I ,I I I I I I I I I I I, I I • I I, I I I I I

IOOO

2000 41.98

3OOO

4000 -(d) I I I $, iooo '''' 3000''' 'o'oo' ' i' 5000' ''Km ' ALASKA LOMONOSOV RIDGE FRAM STRAIT JAN MAYEN ,,• CANADIAN BASIN =t---EURASIANBASIN =J..-GREENLAND-..-•-NORWEGIAN BASIN BASIN Fig. 3. Distributionsof potentialtemperature, salinity, and densityalong the sectionshown in Figure 1. the AM: cold, relatively fresh Greenland Sea Deep Water creasewith depth in the Canadian Basin. The deep salinity is (GSDW), somewhat warmer and saltier Norwegian Sea Deep therefore a maximum in both the horizontal and vertical Water (NSDW), still warmer and saltier Eurasian Basin Deep planes,and as pointed out by Aagaard [1981], the conclusion Water (EBDW), and the most saline,but much warmer Cana- must be that the salt source for the deep water lies neither dian Basin Deep Water (CBDW). higher in the water column nor in the nearby deep basins. The upper-layerdensities are most easilycompared using a0 This contrasts with the hypothesized origin of the Arctic (Figure 3c). However, becauseof the temperature dependence Ocean deep water in the Norwegian Sea [Metcalf, 1960] or as of the compressibility,the deep waters should be compared at involving a mixing near Fram Strait with water from the At- some greater pressure.In Figure 3d we therefore show the lantic layer [Timofeyev, 1960]. There is in fact only one likely density relative to 3000 dbar. When the relative compress- salt source, viz., the adjacent continental shelf seas, where ibilities are thus taken into account, EBDW and NSDW are brine expulsion during freezing produces cold and saline nearly equally dense but are slightly less densethan GSDW. water. This processand its likely importance to the regional At this pressure,CBDW is by far the least densedeep water. salt budget have recently been documented for the northern Conversely,at sea surface pressure, GSDW would be the least Bering Sea [Schumacheret al., 1983]. To date the primary denseform of deep water. interest in the effects of brine formation on the shelves has We now consider the LOREX data [Moore et al., 1983] attached to its role in maintaining the Arctic Ocean halocline from the Canadian Basin, together with the deepest values [Aagaard et al., 1981; Moore et al., 1983], but we shall show from the Eurasian Basin, as shown in Figure 4. It is clear that here that the processis probably also in part responsiblefor not only does the Canadian Basin contain the most saline ventilatingthe deepwaters of the Arctic Ocean. deep water in the AM (Figure 3b), but also that within the In the following discussionwe implicitly assumethe deep Canadian Basin the salinity is significantlygreater below 2000 salinity distributions to be steady over the basin renewal m than anywhereelse in the water column, includingthe core times. If the renewal times of the Canadian and Eurasian of the Atlantic layer. Examination of other data sets,generally basins are substantiallydifferent (as is probable), we cannot shallower, confirms the existenceof a monotonic salinity in- discount the possibility that the higher salinity of the Cana- AAGAARD ET AL.' THERMOHALINECIRCULATION IN THE ARCTIC MEDITERRANEAN 4837

0.6

0.4

ß 0.2 2000 m

ß 800 m ./ 1500 m ß 0.0 /.

GSDWA ,,,, •2100-2450 m I000 rn ß I '• / ß -0.2 /

800 rn

1500 m - t IOOO m ß ./ 2000m -- 6.0

-0.6 I ' / 25 4.0

-0.8 2O 2200- 2505 m/./•"•'x o •

t•,o._oj/ ..o -15 • 2500m -I.0 0.5 - /"1500m -IO NSDW ß 800 m 0.0 _ •00rn -5 q•

-I.2 -0.5 _

GSDWo - \

-I.4 -I.5 - \ -- ß CANADIAN BASIN, 8/S o EURASIAN BASIN, 8/S ß CANADIAN BASIN, S•/S -2.0 --FREEZING ---7-- -- POINT .8 .9 35,0 "EURASIAN BASIN, Si/S s I 1 34.82 .84 .86 .88 .90 .92 .94 .96 .98 :55.00 SALINITY

Fig. 4. Correlationsof O/Sand Si/S near the LomonosovRidge. Data from LOREX [Moore et al., 1983]. Inset shows hypotheticalmixing lines for intermediateand cold shelf waters to producedeep water; circled points in inset are estimatedshelf values. Characteristics of Greenlandand NorwegianSea deepwaters are shownby the large solid circle (GSDW O/S),large solidtriangle (GSDW $i/S), and cross(NSDW O/S). dian Basin in part reflects the different deep conditions of an working hypothesisand examine it both for consistencyand earlier period. We are, however, able to make an argument implications. consistentwith all available data, based on the simpler steady Figure 4 also showsthe silicate/salinitycorrelation for the state assumption.Furthermore, there is at present no realistic samedata set. If we extrapolatethe correlationto the freezing basis for dealing with the nonequilibrium case (for further point salinity suggestedby the O/S correlation, viz., 35.1, we remarks on this, cf. the discussionsection). project the shelfsource to have a silicatevalue of about 25 HM Based on our inability to find other sourcesof salt for main- L-X. Furthermore,and perhapsmore important, the silicate taining the salinity maximum of the deep Canadian Basin, we values on the mixing line are appropriate to a 1:2 mixture of begin with the assumption that shelf water at the freezing freezing point water with water from about 800 m; i.e., the point constitutesthe primary salt source for the deep water. mixing hypothesis based on O/S properties is also consistent (We shall later show evidencefor the existenceof such water with the silicate data. Aagaard et al. [1981] have argued that on the shelf.)Examination of the O/S correlation in Figure 4 the Chukchi and northern Bering seas are likely sourcesof then suggeststhat CBDW is a nearly linear mixture of the cold and saline water for the Canadian basin, and although freezing point water with water at intermediate depths (near their attention was directed to the halocline, their case can 800 m) in the volume ratio 1:2. The cold shelf water is in- also be extended to deep-water sources.The projected silicate ferred to have a salinity near 35.1. We shall use this as our concentration of the shelf source for deep water (,--25 HM 4838 AAGAARD ET AL.' THERMOHALINECIRCULATION IN THE ARCTIC MEDITERRANEAN

Si jzg at ,•_.-1 is indeedthe case,then its mixing into the CBDW represents 5 IO 0 I •,e-w i eq i ß i ß I I I the final stagein the transformationof watersfrom the upper ß ß 0 • North Atlantic into abyssalwaters. Beforewe examinethe evidencefor the existenceof appro- o..oc. priately saline shelf water, we shall point out some conse- quencesand implicationsof our hypothesis.An important fea- .•o•O ture to be explainedis the differencebetween the deepwaters tooo - : o o of the Canadianand Eurasianbasins, the latter beingcolder, ß .ø.o ø less saline, and lower in silicate. Now, there is no reason to o ß o 0 believethat very salinewater at the freezingpoint capableof ß ßo g, feedingthe abyssis formedonly on the shelvesadjoining the o CanadianBasin, rather than in a more circumpolarfashion. Oß

0 Indeed, Aagaardet al. [1981] and Swift et al. [1983] have all 2000 argued that the Barents and Kara seashave a particularly o o great potentialfor formingvery salineshelf water. Why, then, o ß

ß is the EBDW not more saline? The answerappears to lie in its ß connectionto the Greenland Sea, whencecold, low-salinity, EURASIAN ß BASIN ß low-silicate deep water has been observed to enter the Arctic ß Oceanthrough the deepchannel in easternFram Strait [Swift 3000 ß et al., 1983]. Becauseof the temperatureeffect on compress- CANADIAN BASIN ibility, GSDW (0 =-1.28øC, S = 34.89) is denser below about 1800 dbar than EBDW (0 = -0.85øC, S = 34.94 at the LOREX site, Figure 4). This pressureappears to lie below the Fig. 5. Vertical profilesof silicatenear the LomonosovRidge. Data Lomonosov Ridge sill depth, so that GSDW is excludedfrom from LOREX [Moore et al., 1983]. the CanadianBasin. The GSDW is thereforeable to keep the EBDW colder, less saline, and lower in silicate than it would L-x) is in fact within the rangetypical of the deeperparts of otherwisebe, but has little or no effecton the deep Canadian the water columnin the centralChukchi Sea (20-30 #M L-x; Basin. With respectto silicateconcentrations, note particu- Codispotiand Lowman [1973]) but somewhat lower than the larly that they are identicalin the deep water in the Canadian 35 #M L-x characteristicof the BeringStrait inflow [Codis- and Eurasianbasins down to about 1800 m (Figure 5), but poti and Owens,1975]. that below this depth the Eurasian Basin valuesfall off. This is If these ideas about the shelves feeding the deep Arctic consonantwith dilution by GSDW in the deeperwaters, but Ocean are combined with the halocline hypothesisof Aagaard unfortunatelyour knowledgeof the processesaffecting the et al. [1981], then the shelfseas can be representedas contain- silicate distribution in the Arctic Ocean is insufficient to take ing a seriesof salt sourcesnear the freezing point. The less the matter further. dense shelf waters (S < 34.7) mix with water lying above the We turn now to the evidencefor the formation of very temperature maximum to form the intermediate waters be- dense shelf waters adjacent to the Canadian Basin. Schurna- tween 100 and 500 m [Aagaard et al., 1981], while the denser chefet al. [1983] reporteda maximumdaily meansalinity of shelf waters (a large amount of which we have argued has a 35.1 at a mooring in the polynya south of Saint Lawrencein salinity near 35.1, compare Figure 4) mix with water from the northern Bering Sea during the 1980-1981 winter, but below the temperature maximum to make new deep water, monthly mean valueswere much lower (near 33). The forma- thus ventilating the abyss. The sharp change in slope of the tion of densewater was in fact stronglyepisodic, being forced Si/S correlation near 800 m and, to a lesserextent, the change by the local windsdriving a coastalice divergence. in the O/S correlation suggestthat it is appropriate to treat During winter 1982 we ran a seriesof conductivity,temper- this water as a parent type in our mixing scheme.We do not ature, and depth (CTD) sections normal to the Chukchi Sea know the exact origin of the water from below the temper- coastof Alaska. The sectionseaward from Point Lay (Figure ature maximum, but its low silicate concentration, and to 6, location shown in Figure 1) showed salinities within about some extent also its O/S properties, suggestthat this water is 20 km of the coast to exceed 36.5, and the 35 isohaline ex- closely related to the Atlantic layer core (a form of Arctic tendedseaward at least 40 km in a near-bottomlayer. How- Intermediate Water (AIW) in water mass terminology). If this ever,other sectionsshowed considerably lower salinities,sug-

,39 40 42 4J 44 3'7 3'6' g5, 3'4 ,3'3' g2 STA. NO. I I I I I I I I I I I 180 160 140 120 I00 80 60 40 20 0 KM I i i. i i i It i i i ii i • i ! i i I i / ii I /

• 20

• 40

•xx-x--x--' ' - ' PT. LAY 6o

Fig. 6. Salinity sectionacross the northeasternChukchi Sea, March 3-4, 1982.A data collectiongap of 1 day is indicatednear the 100-kmmark. Sectionlocation shown in Figure1. The waterwas everywhere at the freezingtemper- ature. AAGAARD ET AL.: THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN 4839 gesting strong temporal variability in the production and o-0=27.7 o-0=27.8 • ßWEST of 30øE northward flow of saline water. The salinity and temperature ß EAST of 30øE recorded at mooring CS-2A in Barrow Canyon (location •. ßß O'o=27.9 shown in Figure 1) show the variability very clearly (Figure 7), with the temperaturefollowing the freezingpoint as the salini- ß.eiß ø'0=28-0 ty of the water changed. Two other moorings were also in place in Barrow Canyon during the 1981-1982 winter, and together the set of three moorings provides a velocity section o-0=28.1 acrossthe canyon. Record-long means from these instruments rangedfrom more than 20 cm s- • on the easternside of the canyonto lessthan 10 cm s- • on the westernside, but during o-o=28.2 the last week in February the mean velocity in mid-canyon was about 45 cm s-•. Two winter CDT sections taken across the canyon suggestthat the highestsalinities were restricted to a layer about 15 m thick and 25 km wide. If we then consider ß ß ßß the last week in February, during which the mean salinity at -! ß ß ßß CS-2A was 35.5 (Figure 7), the annual rate of outflow this ß ß salinethrough Barrow Canyonwas 3.2 x 103 m3 s-• (0.003 -2 sverdrups(Sv)). If this water mixed with water below the tem- 34.6 34.7 34.8 34.9 35.0 35.1 perature maximum (S = 34.89, Figure 4), this shelf source SALINITY alone could renew the CBDW in about 2000 years. Apart from the fallacy of annualizing an event in this Fig. 8. Correlations of O/S from the Barents Sea near bottom [from manner, such calculations are obviously also sensitive to the Swift et al., 1983]. mixing ratio used, i.e., to the salinity both of the cold shelf water and of the ambient water. For example, if this same Sea is only 30-40 years [Carmack and Aa•taard, 1973; Peter- shelf water mixed with ambient water of 34.92 (found at about son and Rooth, 1976]. 1300 m in the LOREX data), a 500-year residencetime would In the Eurasian Basin an additional shelf source of dense requirean off-shelfflux of 10'• m3 s- 1 (0.01Sv), whereas shelf water is indicated,viz., the cooling of water already saline by water of 35.1 mixing with ambient water of 34.89 would re- virtue cf its beingderived from the northward flow of Atlantic quire a 0.06 Sv flow. The point is that extremelysaline water is water in the Norwegian Sea. Such water does not require its clearly found on the shelfduring winter, but that uncertainties salt augmented through freezing in order to become denser both in the formation rates (which might vary greatly from than EBDW and is in this respectdistinct in its origins from year to year) and in the subsequentmixing processesdo not the shelf waters discussedearlier, although obviously hybrids presently allow a meaningful residencetime to be calculated of theseprocesses are conceivable.The Barents Sea is a partic- for the CBDW. Such an estimate is probably most easily had ularly likely site for the cooling of saline water, since it re- from appropriatetracer measurements,e.g., •'•C. When such ceives a considerable inflow of Atlantic water from the Norwe- measurementsbecome available, they will likely indicate a gian Sea. Such a schemewas proposed by Swift et al. [1983], large difference in the residencetimes of the deep waters on but while they cited observationsof densewater in the Barents either side of the Lomonosov Ridge, since the deep Canadian Sea (Figure 8), the salinity of the water colder than 0øC was Basin appears primarily to be ventilated by relatively small lessthan 34.94. Certainly the potential for forming exceedingly volumes of shelf water (hence the relative warmth of the dense water in this area exists, and L. Midttun (unpublished CBDW), whereas the Eurasian Basin is also ventilated by the document, 1984) has recently reported water near -iøC and influx of GSDW, for which the renewal time in the Greenland more saline than 34.95 on the southern side of the Novaya Zemlya-Franz Josef Land passage. From there such water might enter the Arctic Ocean either via the northeastern ONE-DAY MEAN VALUES AT 76m, 6.5m ABOVE BOTTOM MOORING CS-2A Barents Sea or through the . FEB 1982 MAR 1982 Our conclusionis thereforethat deep water is formed within 10 20 '1 10 2O 36 the Arctic Ocean itself,including the Canadian Basin, but that the critical observationsregarding rates, sites,and the relative ß ß importance of the severalprobable mechanismsremain to be 3.5- ß ß ß ßß ß ß ß ß accomplished.

ß •- 34- ß ß ß ßß OUTFLOW OF DEEP WATER FROM ß THE ARCTIC OCEAN 33 - oo ß We have argued that shelf waters which primarily acquire 32' - their great density from the addition of brine during freezing, mix with water from below the temperature maximum in the Arctic Ocean to form new deep water. This deep water is more ~-•.801•t ß ß " ß ßßß0 ßßß ßßß ßßßß ß saline,warmer, and higher in silicate than the GSDW, and we ß ßßß ßß ß ß ßß ß ßß ß ß ß ß shall look for such a signaturein Fram Strait, where we might ßß expect a juxtaposition of the water massesif there is signifi- I I i I •-2O0 t cant outflow of deep water from the Arctic Ocean. Fig. 7. Daily mean salinity and temperature in Barrow Canyon, In this context, the higher-order effectsof pressureon den- February-March 1982. Mooring location shownin Figure 1. sity become important. In general, there exists some pressure 4840 AAGAARD ET AL.' THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN

BARRIER pp Ap.•,2 o \ '\•,\=w,s

4,000 • o. b. c. d. Fig. 9. Schematicrepresentation of the role of the compensationpressure, Pc- (a) Adjacenthomogeneous warm, saline and cold, fresherdeep-water columns, with (b) in situ densityequal at Pc,(c) are allowedto collapseto form a temperature and salinitymaximum around Pc.(d) Under rotation, a geostrophiccurrent core will alsoform around Pc,with the vertical variation in the horizontal pressuregradient due solely to the effect of temperatureon compressibility.The overbar in Figure 9d representsthe vertical average. at which the in situ densities of two water types of slightly the colder water to represent the resident GSDW, and at the differenttemperature and salinity are equal. We definethis as same time consider the effect of rotation, we should expect to the compensationpressure, Pc. For p < Pc,the in situ density find the outflowing core of EBDW trapped against the west- of the colder, less saline water is less than that of the warmer, ern boundary near the level Pc-The core is constrainedin the more saline water, but for p > Pc, the colder water is denser. vertical plane by the compressibilityterms in the equation of The importanceof this differentialcompressibility effect in the state and in the horizontal plane by rotational effects.For O/S stratification of the deep ocean was apparently first pointed values of the EBDW near Fram Strait of -0.95øC, 34.93, and out by Ekman [1934]. Considernow the situationin which of the GSDW of -1.28øC, 34.89, Pc is in the range 1900-2000 two very deep isothermal and isohaline water columns are dbar, equivalent to about 1900 m. separatedby a verticalpartition, one columnbeing colder and Recent data show that there is in fact an outflow of EBDW fresher than the other, and assumethat Pc exists within the to the Greenland Sea and that it behaves in the manner sug- range of pressureexhibited by thesecolumns (Figures 9a and gestedby the above arguments. Figure 10 shows a composite 9b). If the partition is removed,the warmer and saltiercolumn salinity section drawn from the 1982 Meteor observationsin will collapseabout the level of Pc to form a temperatureand the western Greenland Sea. The data were kindly made avail- salinity maximum at middepth (Figure 9c). If we take the able by K. P. Koltermann. The core of saline water (also warmer water to representEBDW entering Fram Strait and relatively warm and silica-rich) against the Greenland slope

W ~34.7 ß

>54.93 ß 34.85-----• >34.90 ß ß 34.89 •

IOOO ß ß 34.90

2000 34.905 / /

SALINITY '

3000 METEOR CR. 61 , 5-6 JUNE 1982

I I I I I I I I 0 20 40 60 80 DISTANCE FROM IOOOM ISOBATH (KM) Fig. 10. Compositesalinity section across the westernGreenland Sea. Data from the Meteor made availableby K. P. Koltermann. Compare Figure 9c. AAGAARD ET AL.: THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN 4841

near 1500 m can only representdeep outflow from the Arctic faces generally outcrop in winter, and the waters are thus Ocean. A similar situation is also suggestedby the Hudson relatively well ventilated. Their annual formation provides a data, and one can follow this outflow as it is incorporated into rapid replenishmentof a set of rather small reservoirs[Swift the periphery of the convectivelyrenewed cyclonic gyre of the and Aagaard, 1981], the contents of which are subsequently Greenland Sea and carried southward. In the process, the modifiedduring spreadingand mixing. Not only doesthis give salty outflow is freshenedby the convective products of the rise to a rather broad range of water properties,but the short Greenland gyre, and the modified deep water flows into the replacementtimes for the intermediatewaters make them very Norwegian Sea through gaps in the mid-ocean ridge northeast responsiveto environmentalchanges at the sea surface.These of Jan Mayen to ventilate the southern basins of the AM. In changescan be transmittedquickly (within 2-1•0 years) both this connection,it is probable that the compensationpressure withinthe AM and alsoto the North Atlantic[Ostlund et al., again plays a special role. Other factors being equal, we 1976; Brewer et al., 1983; Swift, 1984]. The connection be- should expect the most effectivemixing between the two water tweenthe ventilatingregions and the deepNOrth Atlanticis types to occur at the level of Pc,since the stratification disap- most nearly direct through Denmark Strait, and this path and pears there. the associatedtime scaleshave been discussed bY Swiftet al. A further aspectof this situationis that the compressibility [1980] and by Livingston et al. [1985]. The other primary effect alone could conceivably sustain a geostrophic shear. outflow to the North Atlantic, through the •Faeroe Bank Consideragain the two water columnsof Figure 9. The differ- Channel, is derived from a somewhat deeper level than that ence between the two columns of the vertical integral of pres- through Denmark Strait. The Hudson data suggestthat a sure,AP•.2 (Figure9d), increases with depthto a maximumat blend of intermediate water recently ventilated in the gyres Pcand then decreaseswith increasingdepth. The relativegeo- with deep water (which must mix upward into the intermedi- strophic motion associated with this horizontal pressure ate waters as it is displaced by new deep water) seems to gradienthas a maximum at Pcand is directedout of the page. account best for the properties of the outflow through the The appropriate length scalefor the pressuregradient is not Faeroe Bank Channel. This outflow is less sensitive than that clear. The internal deformation radius, with stratification through Denmark Strait to environmentalchanges induced at based solely on the differential compressibilityeffect, is very the sea surface [Brewer et al., 1983; Swift, 1984; Livingstonet small, about 1.4 km for the O/S values cited near Fram Strait. al., 1985], undoubtedlybecause it is more strongly mixed with This lengthscale gives a currentcore near 10 cm s-•. How- the deeper layers in the Norwegian Sea. ever, the data of Figure 10 suggestan actual core width of at least 10-20 km, requiring a proportionate reduction in baro- DISCUSSION clinic velocity. The intention of this paper has been to synthesizethe deep and intermediate circulation of the AM in a manner consistent ROLE OF THE INTERMEDIATE WATERS with recent data. We concludethat to explain property distri- We have focusedupon the formation and circulation of the butions adequately,there must be at least two major sources deep waters of the AM becausethese represent the most ex- of deep water: open-oceanconvection in the Greenland Sea, treme reaches of the ventilating processes.However, these and near-boundary convection around the perimeter of the deepwaters primarily circulateinternally within the AM, since Arctic Ocean. The various deep-watervarieties, with salinities they lie below the greatest densitiesin free communication between 34.89 and 34.96, all lie well below the southern sills with the North Atlantic. The only pathway of escapefor the which confine the AM system,and the deep waters thus are densestwaters is through vertical mixing with the overlying constrained to circulate internally. In the Iceland Sea in par- layers.In contrast, waters of lesserdensity are produced near ticular, open-oceanconvection penetrates only to middepth, the sea surface in all the basins and found at intermediate and the resultant intermediate water, together with similar depth, and certain of theseintermediate waters passover the water from the Greenland Sea, comprisesthe bulk of the over- Greenland-Scotlandridges into the North Atlantic. flow into the North Atlantic. The intermediate waters have their principal origin at the We have summarized our synthesisschematically in Figure surfaceof the Iceland and Greenland Sea gyres, where winter 11. In a plane view (Figure 11a) the major circulationfeatures convection creates thick layers of cold and well-oxygenated of at leastthe upper ocean,many of which are primarily wind water. This water has been called upper Arctic Intermediate driven, are the Bering Strait inflow (A), the Beaufort gyre (B), Water (upper AIW) by Swift and Aagaard [1981]. As this the transpolar drift (C), the East Greenland Current (D), the water intrudes into the Norwegian Sea, it forms an intermedi- Norwegian Atlantic Current (E), the West SpitsbergenCurrent ate salinity minimum (Figure 3b) and also an oxygen maxi- (F), the Greenland gyre (G), and the Iceland gyre (H). In a mum. The AIW is also carried northward through eastern vertical section along the 180ø-0ø meridians (Figure 11b), Fram Strait, where it mixes with the warm waters of the West open-oceanconvection is shown to reach the bottom in the SpitsbergenCurrent, thereby decreasingthe temperature of Greenland gyre, while in the Iceland Sea such convection the warm inflow to the Arctic Ocean. reachesonly to middepth.In the Arctic Ocean,near-boundary Within the Arctic Ocean, outflowsfrom the peripheral shelf convection renews water masses down to the bottom of the seasprobably mix with the Atlantic layer (AIW in water mass basin. The selectedisopycnals in Figure llb separatesurface, terms),continually increasing the densityof the Atlantic layer intermediate,and deep waters. The water mass classification as it circulates. This is entirely consistent with the modifi- appropriateto this circulationscheme (Figure 1l c) largelyfol- cation of the Atlantic core observedby Coachmanand Barnes lows Swift and Aagaard [1981] but further expandsthe deep- [1963], and in fact, Aagaard et al. [1981] calculatedthe shelf water classification. water outflow in such a schemeto have a mean salinity near We do not propose that the distributions of water proper- 34.7. ties representa strict equilibrium.Indeed, it is clear that the The intermediate waters are a critical component of the exchangeswith the North Atlantic exhibit O/S variations on thermohaline circulation in the AM, since their density sur- the decadal time scale [Swift, 1984]. There is also evidenceof 4842 AAGAARD ET AL.' THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN

90 ø

BAREN TS

180' CHUKC/-I/SEA Oø A E

BERING SEA BAY

BRINEFORMATION MID-GYRECONVECTION / • • / • MID-GYRECONVECTION 0

_ = ,

2,000 SEA • CANADIANBASIN -'"'/,,,• Oz EURASIAN

• • / •j •Oo• t BASIN,-• /• GRE•"•'-•ANDREENLAND • \l\j _ -.,,.-., v NORWEGIAN 4,000

6

INSET: DW CANADIANBASIN• ASW L••N•OAEURASIAI' BASIN RWEGIAN

o GREENLAND PWI n GYRE I DW: see •nset I I .88 .90 .92 .94 .96 ARCTICOCEAN I I I I I I

SALINITY

Fig. ! 1. Schematiccirculation and water massstructure in the Arctic Mediterranean.For key to (a) upper ocean circulation,see text. Note that a portion of the relatively warm water carried northward with the West Spitsbergen Current (arrowsmarked F) continuesinto the Arctic Ocean,where it sinksand spreads,filling both the Canadianand Eurasianbasins at intermediatedepths; this is the so-calledAtlantic layer.(c) The abbreviatedO/S curve marked with an asterisk is for the Greenland gyre. significanttemperature changes in the GSDW over only a few the data presentedin this paper and elsewherein the literature years [Aagaard, 1968]. We consider it likely, however, that [e.g., Aagaard, 1981; Swift et al., 1983]. In particular, we note while the balance of the thermohaline processesmay shift the role of the Lomonosov Ridge in restricting the GSDW from time to time, and thereby produce O/S variations in the from fresheningthe deep Canadian Basin. This suggeststhat various water masses, the basic features of the thermohaline the Canadian Basin was also in the past more salinethan the circulation portrayed in Figure 11 have not changed signifi- Eurasian Basin. However, most of the details of the pro- cantly in recent times. We further believe that the patterns of duction of deep and intermediatewaters required to drive the interbasinexchange shown in Figure 11 are well supportedby circulation suggestedby Figure 11 are not well established. AAGAARDETAL.' THERMOHALINE CIRCULATION INTHE ARCTIC MEDITERRANEAN 4843

BRINEWATER FORMATION

/ e/s a& ao.• a5 I = 0 "' PLUME <-J -1 DEEPSEACURVE -• PLUME •Dc• -2 :• 31 32 33 34 35

LLJ

__ ENTRAINMENT ISOPYCNAL/ - MIXING•,• / I-- \ \ RECIRCULATION

• /•//' OFPLUME i.u , J --'-'] -- i m--- SHAVING

P •

Fig.12. Schematicrepresentation ofa stratified sinking plume at the freezing temperature. Thesequential shaving off andinterleaving ofsuccessively denser layers modifies both the plume and the ambient O/S curves as in theinsets on the right-hand side.

For example,while there is generalagreement that deepcon- Antarcticaprior to theInternational Weddell Sea Expeditions vectionand ventilationoccurs at leastintermittently in the in the late 1960'sand early 1970's[-cf. Foster and Carmack, GreenlandSea [Carmackand Aagaard,1973; Swift and Aa- 1976].A similarobservational program is probablyrequired gaard, 1981], the overturn processitself has never been ob- for the Arctic. served[Killworth, 1979]. An essentialsupposition in the conceptualmodel we have IMPLICATIONS FOR MODELING suggestedis that densewater forms on the continental shelves A recentreview of modelingof theArctic Ocean [Killworth surrounding the Arctic Ocean and that this water sinks to andCarmack, 1983] included two areasparticularly relevant depthin near-boundaryconvection. It is furtherlikely that to the presentdiscussion: vertical TS structureand turbulent deep-waterventilation occursin this manner in both the Ca- plume dynamics. nadianand Eurasianbasins, although in the EurasianBasin Threerecent papers have attempted to explainthe basic TS renewalfrom the GreenlandSea is probablyalso an impor- structure.Stigebrandt [1981] useda two-layermodel to exam- tant process.While there is ample evidencethat shelfwater of ine the combinedeffects of riverrunoff, basin inflow, and ice sufficientlyhigh salinity(density) to displacedeep water is formationon surfacewater characteristics; because of the two- formedin winter,obviously missing are observationsof con- layerapproximation, no accountwas taken of deepwater. A vectionalong the continentalslope itself. The situationis anal- more detailedexamination of water massstructure and circu- ogous to that pertaining to bottom water renewal around lationwas made in Semtner's[1976] three-dimensionalsimu- 4844 AAGAARD ET AL.: THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN lation. However, this model used as boundary conditions in- thermohaline circulation in the Arctic is particularly timely, flow/outflow valuespublished prior to 1973. In particular, the since major observationalefforts are still in their infancy. model includedneither an outflowof deep water from the There is thereforeample opportunity for theoreticalconstruc- Arctic Ocean nor the production of denseshelf waters, which tions to guideexperimental design. we feel to be of critical importance. Killworth and Smith [1984] constructeda filling-box model APPENDIX of the Arctic Ocean to examine the role of shelf-derived plumesin maintainingthe verticalTS structureof the cold Notation halocline.In this model, horizontal mixing from the sideswas Our hydrographic notation is as follows. Salinity units, supposedto be instantaneous,and inflowing water masses whichare nominally 10-3 massmass-•, are expressed without were required to spread adiabatically at the depth at which the factor of 1000, as we generally use the Practical Salinity their in situ density matched that of ambient basin water. Scale (PSS 78). However, older salinities were not re- Shelf water itself was assumedto be horizontally uniform and determined with the new scale. In the salinity range of the to sink without mixingto its equilibriumdepth. Although this deepwaters the difference between the new and old scalesis model yieldedan adequatesimulation of the surfacewaters, it <0.001 and is not significant,nor do our conclusionsrest on failed to reproducethe TS structurebelow about 200 m. We the small differences between the new and old scales over the believe the discrepancymay be reconciledby the ideas we much greater surfacesalinity ranges.We have expressedden- havepresented. In a filling-boxmodel the key dynamicalvari- sitiesin sigma-notation,e.g., a2 = P2- 1000 kg m -s, where able is the verticalvelocity forced by continuityat the level at P2 is the in situ densityin kg m -3 the water would haveif which a plume entersthe interior and raisesoverlying water. moved adiabatically to the depth where the pressureis 2000 Indeed,the poor simulationbelow the haloclineled Killworth and Smith to wonder if their formulation had omitted an dbar (= 20 MPa). In computing density we have used the importantprocess, e.g., deeper plume penetration which would International Equation of State (EOS 80). Had the Knudsen equation of state been used, the "densities"would actually alter the temperature,salinity, and vertical velocityfields in have been specificgravities, and would have been higher, by the deep water. about 0.02 at the sea surface and 0.05 at 2000 dbar. We suggestthat the filling-box approachprovides an es- peciallyuseful framework for viewingthe water massstructure of the AM, and that means be found to include the ideas Comparisonof Density Surfaces presentedhere. Specifically, account should be taken of shelf The water found on a given isopycnal has some range in water, or more probably its derivatives,penetrating to the temperatureand salinity,and if moved adiabaticallyto some bottom, and provision should be made both for nonuniform different referencepressure it would exhibit a range of den- shelf water sourcesand for appropriate mixing with ambient sitiesthere, rather than a singlevalue. In Figure 2 we have waters during sinking. chosenan appropriatereference pressure for each isopycnal The sinking of turbulent plumes along a slopingbottom in usedin our analysis.To assistthe readerin comparingFigures polar seashas beentreated by Killworth [1977], Carmackand 2 and 3, we have calculatedthe approximate range of in situ Killworth [1978], and Melling and Lewis [1982], using the densities(in a-notation) at 0, 1000, 2000, and 3000 dbar, ap- streamtlabeformulation of Smith [1975]. While such models propriate to the actual O/S rangeson each of the isopycnals yield goodinsight into the relativeimportance of buoyancy, usedin Figure 2 (Table A1). entrainment, and bottom friction, they are limited in their ability to explainwater massformation by the assumption Data Sources and Errors that at any positionalong its path the plumeis homogeneous. Our interpretationsof the deep circulation are dependent Thereforein any given numericalexperiment, spreading into on small differencesin salinity and temperature, and so are the interior can only occurat a singledepth. sensitive to measurement errors. The data from the Greenland By contrast,there is evidencethat at least somesinking and Norwegian seasused in Figure 3 were collectedduring plumesare stratified[Foster and Middleton,1980; Thorpe, Hudsoncruise 82-001, February 28 to April 6, 1982, by J. H. 1984; Carmack, 1985]. Now, in a stratified plume a spectrum Swift, J. L. Reid, and R. A. Clarke. A data report is available of TS Pointsis being transporteddownward. Since a given from Swift. The deep temperaturesare thought to be accurate water type cannot be carriedbelow its in situ densitylevel within 0.01øC, and the deep salinities to about 0.002 with (exceptby penetrativeconvection), it followsthat for sucha respectto Wormley P90 standard seawater.The deep water plume,detachment into the interior will occurat a varietyof massessampled during the Hudsoncruise includednot only depthsalong the continentalmargin. We will tentativelyrefer NSDW and GSDW, but also EBDW in Fram Strait. This to this process,by whichthe buoyancyof ambientwaters lifts off successivelayers of the plume, as "shaving"(Figure 12), TABLE A1. In Situ Density Ranges(in a-Notation) at 0, 1000, and we suggestthat a meansof parameterizingthis process be 2000, and 3000 dbar of the Water Masses Found on the Three developed.We deemit likely that suchStratified plumes have CharacteristicDensity Surfaces Used in Figure2 both an uppei'stratified portion and a lower well-mixedpor- tion. At any levelit wouldbe the upperportion that actually Density Surface mixes with ambient water. This upper portion of the plume Pressure, must maintain a densitygreater than or equal to that of the dbar a o = 27.9 a• = 32.785 a 2 = 37.457 ambient water and would thereforecontinually be shavedoff to interleavealong isopycnalsurfaces. Shaving is thereforea 0 27.9 28.02-28.07 28.068-28.092 form of detrainment,in whichelements of a plumeare mixed 1000 32.52-32.67 32.785 32.816-32.828 2000 37.04-37.33 37.39-37.44 37.457 into an ambient fluid of reduced turbulence. The effect of such 3000 41.46-41.88 41.89-41.99 41.981-4 1.993 a processon the TS structureis shownin Figure 12. Finally, we state our belief that modelingrelated to the AAGAARD ET AL.' THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN 4845 latter sampling added much to our confidence in the EBDW deployment. Bottle calibrations in the field were extremely characteristics. difficultduring the Chukchi work in 1982 becausethe helicop- The Hudson section was extended north through Fram ter was nearly unheated, so that the sampling bottle could Strait with CTD-derived temperature and salinity data from rarely be kept from freezingand leaking. (The water sample the Yrner 1980 cruise [Anderson and Dyrssen, 1980]. Where was already at the freezing point, and the actual cast was Yrner coverageoverlapped with that of the Hudson(and of the made a short distanceaway from the helicopterin a unprotec- Transient Tracers in the Ocean North Atlantic Study in 1981), ted location.) On the Point Lay section (Figure 6), only one the deep salinities were compared. As a result, the Yrner sal- successfulbottle sample was obtained, and unfortunately,it inities reported by Anderson and Dyrssen [1980] were uni- was located in the steep salinity gradient near bottom and is formly reduced by 0.013 to conform with the salinometer- therefore of limited usefulnessin calibration. However, on the derived values from the other data sets [Swift et al., 1983]. section immediately south of the Point Lay section, a good These adjustments in effect represent a recalibration of the calibration was obtained 2 days earlier, when a sample bottle Ymer data, assuming that the deep waters are relatively in- was kept from freezing and leaking and three subsamples variant spatially and temporally.Thus adjusted,the Yrnersal- agreedwith the CTD to within +0.002 in salinity. A review of inities show not only the same relative distribution as those the entire data set with respectto the laboratory calibrations, from the Hudson (e.g., an EBDW salinity increase to the the very few field calibrations,internal consistencyin the TS bottom and a nearly homohaline NSDW) but also nearly the distributions,and maintenanceof the freezingtemperature as same numerical values. We note also that unpublished data a function of salinity suggestthat the salinity determinations collectedin 1984 north of Fram Strait by Swift on the Polar- are accurateto within at least 0.02. This is ample to ascertain stern,using the samestandards and proceduresas thoseof the the presence of the anomalously saline water, which ap- Hudsoncruise, yield EBDW valuesconsistent with those cited proaches4 above the far-field salinity. here. With respectto the serial data of Figure 7, the only mean- The salinities from the Lomonosov Ridge Experiment ingful calibration was that performed in the laboratory. A (LOREX) were important in shaping our conclusions.These CTD station taken 11 km away, but in nearly the same water data span both sides of the Lomonosov Ridge and demon- depth, showed the identical temperature with that of the strate striking differencesbetween CBDW and EBDW (Moore moored sensor,but a salinity 0.37 lower. A second CTD sta- et al. [1983]; discussedin greater detail by Aagaard [1981]). tion 16 km away and somewhat shallower, recorded the same The LOREX profiles were collected over 38 days, and the temperature to within 0.015øC; the CTD salinity was only analysis environment was not ideal. This introduced noise into 0.04 lower than at the mooring, and 1 m from the bottom the the measurements,but the signalsbasic to our interpretation CTD showed a salinity 0.02 higher. The minimum temper- were well above the random errors. All indications are that ature recorded at the mooring during the winter was - 1.97øC, the LOREX deep salinities are sufficiently accurate for our which is identical with the freezingpoint at the salinity calcu- purposes:EBDW values are similar to those from the Fram lated for the simultaneouslyrecorded conductivity, viz., 35.84. Strait expeditions,and Canadian Basin values are very close We thereforeaccept the mooring data basedon the laboratory to those measured by R. M. Moore (unpublished data, 1983) calibration but recognizethat an absolute salinity error of as during the 1983 Cesar expedition near 86øN, 110øW. We were much as a few tenths is possible.We note, however, that the thus encouragedin our efforts to produce a basin-scalesalini- temperature in Figure 7 follows the freezingpoint very closely, ty intercomparisonextending over the whole of the AM. suggestingthat errors in the record are not large. We are not so confidentin the quality of the remaining data Acknowledgments. Among the many colleagueswhose assistance sets used in Figure 3, distilled from a search of National was indispensablewere R. A. Clarke and J. L. Reid in collectingthe OceanographicData Center (NODC) files. Individual profiles Hudson data' D. J. Hanzlick, S. D. Harding, and R. B. Tripp, the were often noisy and occasionallycontained glaring inaccura- Chukchi Sea data; K. P. Koltermann, the Meteor data' M. G. Low- cies. If these were our only guides, we would have been re- ings and R. M. Moore the LOREX data; and B. Rudels the Ymer data. E. A. Aagaard performed the hypsographiccalculations, and M. strictedto a speculativepaper. We do note that after eliminat- Mitchell and J. Washington constructedmany of the figures. The ing obvious errors and averaging horizontally and vertically, work was supported by the National ScienceFoundation through these data usually showed features consistentwith the results grant DPP-8100153 and the Office of Naval Research through con- from our principal data sets, and so were a useful aide in tracts N00014-75-C-0893 and N00014-84-C-0111 to the University of Washington' by the National Science Foundation through grant contouring the section across the deep basins. Becauseour OCE-8320410 and the Office of Naval Research through contract conclusionsdo not rest upon these subsidiary data, we will N00014-80-C-0440 to the Scripps Institution of Oceanography' and not discuss them further. by the Marine Minerals Servicethrough interagencyagreement with We also note that the coverage of Meteor cruise 61 (data the National Oceanic and AtmosphericAdministration, as part of the used in Figure 10) overlapped with that of Hudson cruise Outer Continental Shelf Environmental AssessmentProgram. This is contribution 1403 from the School of Oceanography, University of 82-001 (which took place 2 months earlier). An intercompari- Washington, and also constitutesa contribution from the Marine Life son of deep-water temperaturesand salinitiesshowed on the ResearchGroup, ScrippsInstitution of Oceanography. average the Meteor sampleswere warmer by 0.003øC and REFERENCES fresher by 0.002 than equivalent samplesfrom the Hudson. These systematic differences are much smaller than the Aagaard, K., Temperature variations in the Greenland Sea deep- water, Deep Sea Res., 15, 281-296, 1968. EBDW outflow signalshown in Figure 10. Aagaard, K., On the deep circulation in the Arctic Ocean, Deep Sea Finally, we consider the winter Chukchi Sea data and, in Res., 28, 251-268, 1981. particular, the salinities.The observationsfor Figure 6 were Aagaard, K., L. K. Coachman, and E. C. Carmack, On the halocline made with a Neil Brown Instrument SystemsMark IIIB CTD of the Arctic Ocean, Deep Sea Res., 28, 529-545, 1981. Anderson, L., and D. Dyrssen, Constituent data for Leg 2 of the Ymer system carried by helicopter; the underwater unit had been 80 Expedition, Rep. Chem. Seawater 24, Dep. Anal. Mar. Chem., reconfiguredto allow deployment through an 8-in. auger hole Chalmers Univ. of Technol. and Univ. of G6teborg, G6teborg, in the ice. All sensorswere laboratory calibrated prior to field Sweden, 1980. 4846 AAGAARD ET AL.' THERMOHALINE CIRCULATION IN THE ARCTIC MEDITERRANEAN

Brewer, P. G., W. S. Broecker, W. J. Jenkins, P. B. Rhines, C. G. Nansen, F., Northern waters: Captain Roald Amundsen's oceano- Rooth, J. H. Swift, and T. Takahashi, A climatic fresheningof the graphic observationsin the arctic seasin 1901, Nor Vidensk.Acad. deepNorth Atlantic (north of 50øN) over the past 20 years,Science, Kl. Skr. Mat. Naturvidensk.Kl., 1(3), 1906. 222, 1237-1239, 1983. Ostlund,H. G., H. G. Dorsey,and R. Brescher,GEOSECS Atlantic Carmack, E. C., Circulation and mixing in ice-covered waters, in radiocarbon and tritium results(Miami), Data Rep. 5, Tritium Lab., Air-Sea-Ice Interaction, edited by N. Untersteiner, NATO Ad- Rosenstiel Sch. of Mar. and Atmos. Sci., Univ. of Miami, Fla., Dec. vanced Study institute Series,in press,1985. 1976. Carmack, E. C., and K. Aagaard, On the deep water of the Greenland Peterson, W. H., and C. G. H. Rooth, Formation and exchangeof Sea, Deep Sea Res.,20, 687-715, 1973. deep water in the Greenland and Norwegian seas,Deep Sea Res., Carmack, E. C., and P. D. Killworth, Formation and interleaving of 23, 273-283, 1976. abyssalwater massesoff Wilkes Land, Antarctica,Deep Sea Res., Schumacher,J. D., K. Aagaard, C. H. Pease,and R. B. Tripp, Effects 25, 357-369, 1978. of a shelf polynya on flow and water properties in the Northern Coachman, L. K., and C. A. Barnes, The movement of Atlantic water Bering Sea, J. Geophys.Res., 88, 2723-2732, 1983. in the Arctic Ocean, Arctic, 16, 8-16, 1963. Semtner, A. J., Numerical simulation of the Arctic Ocean circulation, Codispoti, L. A., and D. Lowman, A reactive silicatebudget for the J. Phys. Oceanogr.,6, 409-425, 1976. Arctic Ocean, Limnol. Oceanogr.,18, 448-456, 1973. Smith, P. C., A streamtube model for bottom boundary currents in Codispoti,L. A., and T. G. Owens,Nutrient transportsthrough Lan- the ocean,Deep Sea Res.,22, 853-873, 1975. caster Sound in relation to the Arctic Ocean's reactive silicate Stigebrandt,A., A model for the thicknessand salinity of the upper budget and the outflow of Bering Strait waters,Limnol. Oceanogr., layer in the Arctic Ocean and the relationshipbetween ice thickness 20, 115-119, 1975. and some external parameters, J. Phys. Oceanogr.,11, 1407-1422, Ekman, V. W., Georg Wrist' Das Bodenwasserund die Gliederung 1981. der Atlantischen Tiefsee, $. Consell, 9, 102-104, 1934. Sverdrup, H. U., M. W. Johnson,and R. H. Fleming, The Oceans: Foster, T. D., and E. C. Carmack, Frontal zone mixing and Antarctic Their Physics, Chemistry,and General Biology, Prentice-Hall, En- Bottom Water formation in the southern Weddell Sea, Deep Sea glewoodCliffs, N.J., 1942. Res., 23, 301-318, 1976. Swift, J. H., A recent 0-S shift in the deep water of the northern North Foster, T. D., and J. H. Middleton, Bottom water formation in the Atlantic, in Climate Processesand Climate Sensitivity, Geophys. Weddell Sea, Deep Sea Res.,27, 367-382, 1980. Monogr. 29, Maurice Ewing Ser., vol. 5, edited by J. E. Hansen and Helland-Hansen, B., and F. Nansen, The Norwegian Sea: its physical T. Takahashi,pp. 39-47, AGU, Washington,D.C., 1984. oceanographybased upon the Norwegian researches1900-1904, Swift, J. H., and K. Aagaard, Seasonal transitions and water mass Rep. Norw. Fish. Mar. Invest.,2(1), 1909. formation in the Iceland and Greenland seas, Deep Sea Res., 28, Killworth, P. D., Mixing on the Weddell Sea continentalslope, Deep 1107-1129, 1981. Sea Res., 24, 427-448, 1977. Swift, J. H., K. Aagaard, and S.-A. Maimberg, The contribution of the Killworth, P. D., On "chimney" formations in the ocean, J. Phys. Denmark Strait overflow to the deep North Atlantic, Deep Sea Oceanogr.,9, 531-554, 1979. Res., 27, 29-42, 1980. Killworth, P., and E. C. Carmack, Proceedingsof the Arctic Ocean Swift, J. H., T. Takahashi, and H. D. Livingston,The contributionof Modelling Meeting, Cambridge,June 21-25, 1982, Rep. 57, Inst. of the Greenland and Barents seas to the deep water of the Arctic Geophys.,Univ. of Bergen,Bergen, Norway, June 1983. Ocean, J. Geophys.Res., 88, 5981-5986, 1983. Killworth, P. D., and J. M. Smith, A one-and-a-half dimensional Thorpe, S. A., A laboratory study of stratifiedaccelerating shear flow model for the Arctic halocline, Deep Sea Res.,31,271-293, 1984. over a rough boundary,J. Fluid Mech., 138, 185-196, 1984. Livingston,H. D., J. H. Swift,and H. G. C)stlund,Artificial radio- Timofeyev, V. T., Vodnye Massy ArkticheskogoBasseina, Gidrome- nuclidetracer supplyto the Denmark Strait overflowbetween 1972 teorol. Izdat., Leningrad, 1960. and 1981,J. Geophys.Res., in press,1985. Mantyla, A. W., and J. L. Reid, On the abyssalcirculation of the World Ocean, Deep Sea Res.,30, 805-833, 1983. K. Aagaard, School of Oceanography,University of Washington, McDougall, T. J., Greenland Sea bottom water formation: A balance Seattle, WA 98195. between advection and double-diffusion,Deep Sea Res., 30, 1109- E. C. Carmack, Department of the Environment, NWRI Branch, 1117, 1983. 4160 Marine Drive, West Vancouver, British Columbia, V7V 1N6 Melling, H., and E. L. Lewis, Shelfdrainage flows in the BeaufortSea Canada. and their effect on the Arctic Ocean pycnocline,Deep Sea Res.,29, J. H. Swift, Scripps Institution of Oceanography,MLR Group, 967-986, 1982. A-030, La Jolla, CA 92093. Metcalf, W. G., A note on water movement in the Greenland- Norwegian Sea,Deep Sea Res., 7, 190-200, 1960. Moore, R. M., M. G. Lowings,and F. C. Tan, Geochemicalprofiles in the Central Arctic Ocean' Their relation to freezing and shallow (ReceivedAugust 17, 1984; circulation,$. Geophys.Res., 88, 2667-2674, 1983. acceptedSeptember 13, 1984.)