Deep And Bottom Water Masses

Deep water masses How between the surface and intermediate water masses and the sea-bed: il the deepest water in contact with the sea-bed is distinguishable from overlying water, it is referred to as hallam waier.

North Atlantic Deep Water

As shown by Figure 6 19. the major deep water mass of the Atlantic is North Atlantic Deep Water (NADW). The main surface source region of North Atlantic Deep Water is believed to he the subpolar gyre in the Greenland Sea, As shown in Figure 5.26. there is a fairly free passage of surface water between the Norwegian and Greenland Seas and the North Atlantic, w ith water flowing into the Seas mainly between Scotland and Iceland, and out mainly beiween Iceland and Greenland. However, the irregular plateau extending from Scotland to Greenland tand passing through the Facroe Islands and Iceland) presents a major obstacle to flow at depths greaier than about 400 m and a complete barrier at depths greater than about 850m (Figure 6.20). Furthermore, the bottom topography isolates the Norwegian and Greenland Seas from water in the deepest basins of the Arctic Sea (Figure 5.261

Cold polar water and icc from the Arctic enter the region via the Fram Strait, and arc carried south in the Fast Greenland Current, which forms the western limb of (he cyclonic circulation. Forming the eastern limb is the north ward-(lowing Norwegian Current, carrying surface water that is exceptionally warm and saline for these latitudes, with temperature and salinity values generally in excess of 8 °C and 35,25 respectively.

Can you identify the origin of this water?

This is water from die North Atlantic Current, the downstream continuation ot the Gulf Stream. While circulating in the mixed surface layer of the Norwegian and Greenland Seas, this water is not only considerably cooled but also diluted« because - like the Labrador Sea - the region is generally one ol excess precipitation. Furthermore, in the summer ihere is addition of Iresbwatcr from melting ice carried in the East Greenland Current.

In the winter, surface water in the Greenland Sea may become sufficiently dense to sink As in the Labrador Sea. cyclonic winds lead to a surface divergence and a bowing up of the isorherins/isopycnals. circumstances which 'precondition' (he surface water to be readily desiahili/cd Deep convection seems to occur in very short-lived events, in well-defined regions, perhaps only a few kilometres across, sometimes referred to as 'chimneys'. These form in response lo particularly cold winds and/or ice-format ion. but exactly how is not clear. As mentioned above, ice is carried down the western side of the gyre in the East Greenland Current. Fddics form along the ice-edge, and mi some winters a 'tongue" of locally formed ice develops al about 72-76" N. sometimes extending well across the Greenland Sea (ef. Figure 5.27(b)). Development of this feature, which is known as the Odden (a Norwegian word meaning "headland'). seems to encourage deep convection: the brine left behind when freshwater is abstracted into sea-ice increases the salinity of surface seawater. so thai (given the low temperatures) it becomes sufficiently dense to sink into (he depths of the Greenland Sea. from where it may spread into (lie Norwegian Sea (Figure 6.20).

The salinity of the dense water that accumulates and circulates in the deep basins of the Norwegian and Greenland Seas is increased through the addition of high-salinity Arctic water which enters at depth via the Fram Strait (Figure 6.20). This cold, very saline water results mainly from the seasonal formation of shelf ice around the margins of the Arctic Sea. which leaves behind dense 'brine' that sinks into the deep Arctic basins. The cold deep water that results from the high-salinity Arctic outflow and the deep water formed in the Greenland Sea is the densest in the world ocean. It has a salinity of -34.9 and temperatures below 0°C, and fills up the basin -which in places is 3 km deep - to the level of the submarine ridge.

Figure 6.20 Schematic map showing the cyclonic gyres in the Greenland and Labrador Seas, which are the main areas of deep water formation in the Northern Hemisphere; the dashed cyclonic arrow is a less important region of deep-water formation, in the Irminger Sea. The dark grey arrows show the extremely saline Arctic water which enters the Greenland Sea through the Fram Strait, and which is an important component of North Atlantic Deep Water. Undiluted North Atlantic Deep Water, and the paths of the densest water that overflows the ridge between Greenland and Scotland, are shown in olive green; DWBC = deep western boundary current. The broad green arrows indicate the two types of North Atlantic Deep Water, North-East Atlantic Deep Water (darker green) and the less dense North-West Atlantic Deep Water, which overlies North-East Atlantic Deep Water in the eastern Atlantic. Note: The denser type of NADW is also formed from the Denmark Strait overflow.

From time to time, perhaps in response to storms overhead causing isobaric and isopycnic surfaces to tilt, the dense water accumulating in the Norwegian and Greenland basins flows over the submarine ridge. The overflows occur at various specific locations - notably through the Denmark Strait (between Iceland and Greenland), between Iceland and the Faeroe Islands, and through a narrow channel south of the Faeroe Islands (Figure 6.20). As the deep water overflows the plateau and flows down into the depths of the Atlantic, there is extensive turbulent mixing with overlying water, some of which is entrained. The characteristics of the water mass that results therefore depend not only upon the characteristics of Norwegian Sea and Greenland Sea deep waters, but also on the characteristics of the water above the various outflows and the degree of entrainment that occurs. The water mass that flows south from the overflow sites in the eastern basin of the Atlantic is sometimes known as North-East Atlantic Deep Water (or eastern North Atlantic Deep Water), and is shown as the darker of the broad green arrows in Figure 6.20. This is essentially a mixture of the original deep water and the overlying North Atlantic Central Water, and so has temperature-salinity characteristics of 2.5 °C and 35.03.


Greenland Sea

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