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Evolution of North Atlantic Water Masses Inferred from Labrador Sea Salinity Series

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Evolution of North Atlantic Water Masses Inferred from Labrador Sea Salinity Series or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution article has This been published in S P E CI A L I SSU E On Sa LI N ITY Oceanography EVOLUTION OF NORTH ATLanTIC WATER MASSES journal of The 21, Number 1, a quarterly , Volume InFERRED FROM LaBRADOR Sea SaLINITY SeRIES B Y I G O R Ya SH AYaev an D A L LY N C L A RK E O ceanography ceanography S ociety. ociety. C opyright 2008 by The 2008 by opyright ABSTRACT ocean is approximately conserved, its local or regional signatures are affected The Labrador Sea is the coldest and freshest basin of the North Atlantic. Winter by planetary freshwater and salt fluxes. Because the net poleward atmospheric O cooling in this sea produces Labrador Sea Water. This intermediate water plays an ceanography O moisture flux is largely compensated by ceanography important role in the exchange of heat, freshwater, and other substances between the net oceanic freshwater transport, S the atmosphere and the abyssal ocean, affecting the water masses, circulation, and, any sustained change in regional salin- rights All reserved.ociety. S ociety. ociety. ultimately, climate of the subpolar North Atlantic basins. The subpolar gyre of the ity would likely reveal an important S tendency in the large-scale redistribution or Th e [email protected] to: correspondence all end North Atlantic has exhibited large changes in temperature, salinity, and volume of freshwater and sea salt that would over the past six decades, largely in response to changing winter conditions over P provide insight for understanding and article use for research. and this copy in teaching to granted is ermission the Labrador Sea. The signature of these changes can be seen in the lower limb of predicting possible climate change. The planetary cycle of freshwater the Meridional Overturning Circulation down into the North Atlantic tropics. includes evaporation, precipitation, continental runoff, atmospheric mois- ture flux, and, finally, the ocean’s own O InTRODUCTION joined together to form the lower limb of processes of freshwater transport and ceanography The subpolar sector of the North Atlantic the “ocean conveyor belt.” exchange, ice formation, and melting. Ocean is the region where the warm, The global ocean conveyor, by trans- These processes determine changes in S ociety, ociety, saline upper-layer waters from the low forming poleward flows of warm saline the storage of freshwater in the global PO (tropical and subtropical) latitudes meet upper waters to deep return flows of ocean, the main planetary reservoir of B ox 1931, ox the cold, less-saline outflows from the cold, less-saline waters, makes a signifi- this most vital substance. On the flip R high (polar) latitudes and interact to mix cant contribution to the global distribu- side, these same processes, along with ockville, M R epublication, systemmatic reproduction, reproduction, systemmatic epublication, and exchange their properties. It is also tion of heat, freshwater, and a variety advection and mixing of salt, control the region in which the major intermedi- of other substances carried by seawater salinity, which is a measure of (or, practi- D 20849-1931, ate and deep water masses of the North (e.g., salts, nutrients, gases, pollutants). cally, a proxy for) the concentration of Atlantic are created, modified, and Although the salt content of the world the salts dissolved in seawater. US A. 30 Oceanography Vol.21, No.1 Oceanography March 2008 31 The distribution of temperature and Sea (Figure 1)—exhibit unique ranges Figure 1 illustrates the Gulf Stream/ salinity in the subsurface waters of the of temperature and salinity that allow North Atlantic Current system, which subpolar gyre of the North Atlantic their waters to be distinguished from transports warm, saline water north- (Figure 1) results from four processes: each other. This characterization of eastward across the southern boundary 1. The Gulf Stream and then the North waters from their temperatures and of the subpolar gyre. This current is the Atlantic Current carry warm and salty salinities is an important tool of classical northern extremity of the upper limb water from southwest to northeast. oceanography that underlies water-mass of the ocean conveyor belt. The North 2. The Arctic Ocean outflow delivers analysis of any level of complexity. In Atlantic Current splits into a number of the cold, low-salinity water found physical oceanography, a “water mass” branches and eddies after it crosses the over the Greenland, Labrador, and denotes a large, relatively homogeneous Mid-Atlantic Ridge. The easternmost Newfoundland shelves. or uniform body or volume of water, branch carries some of this warm and 3. The subpolar gyre’s circulation carries formed in the same source or forma- salty water through the Faroe-Shetland the initially warm, salty surface and tion region and by the same process Channel into the Nordic seas. subsurface waters counterclockwise (Dobrovolskiy, 1961). Water masses are Another of these branches passes around the northern and western defined by their physical (and chemical) around the Iceland Basin and then boundaries of the deep basins. As properties. In this article, we use sea- around the Irminger Basin before the water moves westward, it loses water salinity as the principal identifier entering the Labrador Sea. As the waters heat to the atmosphere and becomes and tracer of major water masses of the move westward, winter cooling creates fresher because precipitation exceeds subpolar North Atlantic. deep, winter-mixed layers whose waters evaporation over these waters. In classical oceanography, water tend to become cooler and fresher along 4. As the upper-layer waters cool, they masses were thought to be constant the track (toward the Labrador Sea). become denser (heavier), and they in time. Modern, high-precision and These waters are called Subpolar Mode mix with the colder, fresher waters repeated measurements show that Waters (SPMW) (McCartney and Talley, found at deeper levels. This process of assumption to be false. Here, we use the 1982). In the Irminger and Labrador vertical mixing also contributes to the spatial and temporal distributions of Seas, the westward branch of the North freshening of the surface and subsur- seawater salinity to illustrate the produc- Atlantic Current is called the Irminger face waters as they move toward the tion and evolution of the intermediate Current. It delivers water originating Labrador Sea. and deep water masses of the subpolar from the SPMW to the Labrador Sea. The large-scale pattern of temperature North Atlantic. This relatively warm, salty water1 is an and salinity set by these processes means important part of the deep-convection that each of the three main subpolar UPPER-LaYER CIRCULATION story of the Labrador Sea, as discussed in basins—the Iceland Basin, the Irminger OF THE SUBPOLAR NORTH the next section. Sea, and, particularly, the Labrador ATLanTIC The subpolar gyre also receives cold, The subpolar North Atlantic is character- low-salinity waters from the Arctic Igor Yashayaev ([email protected] ized by an intense, upper-layer circula- Ocean, as illustrated by blue arrow- mpo.gc.ca) is Research Scientist, Ocean tion (Figure 1) that establishes “commu- headed lines in Figure 1. This circulation Circulation Section, Ocean Sciences nication” lines between the subtropical, is concentrated over the shelves and Division, Bedford Institute of Oceanography, subpolar, polar, and coastal regions, thus upper continental slopes and consists Fisheries and Oceans Canada, Dartmouth, providing, maintaining, and regulating of the East and West Greenland and NS, Canada. Allyn Clarke is Scientist their exchanges of heat, freshwater, salt, Labrador currents. Most of this Arctic Emeritus, Ocean Circulation Section, and other substances. outflow, restricted to the upper layer, Ocean Sciences Division, Bedford Institute of Oceanography, Fisheries and Oceans 1 The Irminger Current (containing SPMW) is, in fact, warmer and saltier than the rest of the waters in the Labrador Sea Canada, Dartmouth, NS, Canada. (Figures 3 and 5). 32 Oceanography Vol.21, No.1 Figure 1. Distributions of temperature (A) and salinity (B) at 100 m below the surface in the northern North Atlantic. The red arrows indicate the Gulf Stream and associated North Atlantic Current, which transport warm saline surface water. The blue arrows indicate the East and West Greenland and Labrador Currents, which carry relatively cold and fresh water southward. Red circles are the locations of the oceanographic stations on the Labrador Sea repeat hydrography line (AR7W). The star indicates the site that was chosen for Figure 3. Oceanography March 2008 33 is the return flow associated with the Because of their still-high salinity, these properties directly affect the strength of warm, saline inflow through the Faroe- cooled waters are the densest waters in the deep limb of the Atlantic meridional Shetland Channel between Iceland and the global ocean and form the deep and overturning circulation (MOC) and, Britain. However, a significant contribu- bottom waters of the Nordic seas. The consequently, the global ocean conveyor. tion comes from the inflow of water flow of these dense, deep waters directly Cold, fresh, and dense at its begin- through Bering Strait. The Arctic outflow into the North Atlantic is restricted ning, the ISOW crosses the sills of the attains its low salinity in part from this by the shallow ridge system joining Faroe Bank Channel in the Iceland- low-salinity North Pacific water and the Shetlands, the Faroes, Iceland, and Scotland Ridge and enters the Iceland also from the large river runoff into Greenland. This dense water flow is con- Basin where it quickly entrains large the Arctic Ocean. fined to three narrow passages: the Faroe amounts of the warm, saline Subpolar Bank Channel, some small gaps to the Mode Water.
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