North Atlantic Drift depth (m| | ¡<2000 \ [>2 000
Figure 5.26 The bathymetry and surface currents ol tie Arctic Sea ana itie adjacent seas of the North Atlantic (The North Atlantic Drift is the old name for the North Atlantic Current, the downstream continuation of the Gull Stream )
Figure 5.27 (opposite) Seasonal changes in ice cover in northern and southern high latitudes, as determined using microwave measurements obtained from the /V/mZjus satellite programme during February and September, 1982. The different colours represent the percentage of sea-surface covered by ice: a purple tone Indicates 100% coverage, while a light blue tone represents 20% or less.
(a) and (b) Near-maximum and near-minimum ice cover in the Arctic region, (c) and (d) Near-minimum and near-maximum ice cover in the Antarctic region.
The enclosed nature of the Arctic region also greatly affects its ice cover. Figure 5.27 shows the seasonal variation in the ice cover of both the Arctic and the Antarctic.
Which of rhe two regions show s the lesser variability in ice cover?
The Arctic. Much of the Arctic ice remains 'locked' in the sea all year round; only about 10% leaves annually, via the Fram Straits between Greenland and Spitsbergen (see Figure 5.26). Much of the Arctic Sea is permanently covered with ice (Figure 5.27(a)), and most Arctic pack ice is several years old. This situation contrasts markedly with that in the Southern Ocean where the limits of ice cover shift over about 20° of latitude during the course of a year (Figure 5.27(c) and (d)), and most of the ice cover is renewed annually.
Even in those parts of the Arctic Sea where the ice cover is permanent, the pack ice does not form a solid mass. Under the influence of winds and currents it continually cracks and shifts, so that layers of ice raft over one another; in addition, pressure ridges form, locally increasing the ice thickness from 3-4 m to 40-50 m.
It so much of the Arctic Sea is permanently covered by a thick layer of ice.
how has the general surface circulation shown in Figure 5.26 been determined'.'
The general circulation in the upper layers of the Arctic Sea was initially deduced from the average motion of the ice. as revealed by the movement of ice-bound ships and camps on the ice: more recently, buoys fixed in the ice have been tracked by satellite. There are difficulties in separating packice motion caused by local winds from that related to general current patterns, but it seems that the shorter-period fluctuations are largely related to local winds, and longer-period motion to the surface circulation. In addition to direct current measurements, geostrophic calculations have been made using temperature and salinity data collected from oceanographic stations based on ships or on ice-islands and ice-floes.
The circulation that has been deduced is a clockwise (anticyclonic) gyre centred over the Canadian Basin, with the main surface outflow being the East Greenland Current (Figure 5.26). This current carries southwards not only the pack ice but also icebergs which have calved from glaciers reaching the east coast of Greenland. Off the southern tip of Greenland, the East Greenland Current converges with the warm Irminger Current and most of the ice melts. Some, however, may be carried around to the west coast of Greenland where it is supplemented by large numbers of icebergs from glaciers reaching the west Greenland coast. The ice circulates in Baffin Bay and the Labrador Sea and eventually travels southwards in the Labrador Current. Off the Newfoundland Grand Banks, the Labrador Current converges with the Gulf Stream (Figure 4.31) and here even the largest icebergs gradually break up and melt.
Because it is thought that the effects of global warming should show up first at high northern latitudes, the extent and thickness of Arctic sea-ice has been closely monitored over recent decades. There is indeed evidence that Arctic sea-ice is thinning and penetrating less far south in winter, but it is still hard to be sure that this is part of a long-term trend rather than (at least in part) a climatic oscillation or anomaly.
One of the most extreme variations in 'ocean climate' observed during the last century was the event now known as the Great Salinity Anomaly. The low salinities were first noticed in the waters to the west of Scotland during 1973-79, with minimum values being recorded in 1975 (cf. Figure 5.28) but it was subsequently discovered that this was part of a larger and longer-lasting phenomenon. A pulse of low salinity water, which had been off eastern Greenland in 1968, travelled round to the Labrador Sea, where it was observed in 1972 (Figure 5.29); it then travelled cyclonically round to the north-east Atlantic and the Norwegian Sea, and was again detected off eastern Greenland in 1981-82. The cause of the event is still not known for sure - one possibility is that it was triggered by anomalous northerly winds over eastern Greenland in the 1960s; another that it may have begun with an unusually large export of ice from the Arctic Sea. Whatever the cause, for more than a decade, the layer of unusually low salinity water occupying the uppermost 500-800 m of the water column had a marked effect on the deep circulation of the North Atlantic, for reasons that will become clear in Chapter 6.
Figure 5.28 Monthly salinity anomalies (i.e. departures from monthly means) for the central Rockall Channel (also known as the Rockall Trough) between January 1972 and December 1982. The pulse of low salinity water is clearly seen to have passed through the Channel in autumn 1975. Salinity values are given in parts per thousand (%<>), now omitted, by convention. The minimum salinity of 0.2 below the monthly mean may not seem remarkable, but as you will see in Chapter 6, this is a significant decrease (it is about five times the normal variability in this area). (Similar, though less marked, salinity anomalies have been detected in data for the North Atlantic for the 1950s, 1980s and early 1990s.)
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