Thermohaline Circulation in the Arctic Mediterranean Seas
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. C3, PAGES 4833-4846, MAY 20, 1985 Thermohaline Circulation in the Arctic 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 Arctic Ocean 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 Lomonosov Ridge 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 Fram Strait. 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.