FIGURE 4

Catch-at-age curves for the 1944-46 year classes of cod at Iceland (from Schopka, 1991; reprinted with permission of the Northwest Atlantic Fisheries Organisation), but with the likely contribution of returning adults from Greenland shown hatched.

Greenland is an economic or socioeconomic one, acting to augment recruitment to the West Greenland cod population via larval drift from Iceland during years and decades when the wind field in winter and spring acted to strengthen the Irminger/West Greenland current system. A proportion of these fish may return as adults to Iceland.

Physically mediated exchanges of a similar type may also occur farther downstream along the same current system. As Templeman (1981) points out: ''Cod occasionally migrate from the Newfoundland area to West Greenland and vice versa (Templeman, 1974, 1979). Also in some years, especially 1957, there was considerable drift of cod larvae from Greenland toward the coasts of Baffin Island and Labrador (Hansen, 1958; Hermann et al., 1965)." (See also Templeman, 1965, and Danke, 1967.)

THE REGIONAL SCALE: THE "GREAT SALINITY ANOMALY" AND ITS LOCAL AND REMOTE CONTROLS

The widespread freshening of the upper 500 to 800 m of the northern North Atlantic, which has come to be known as the "Great Salinity Anomaly" (GSA), represents one of the most persistent and extreme variations in global ocean climate yet observed anywhere in this century. Dickson et al. (1988) describe it as largely an advective event, originating in the accumulation and preservation of an anomalously fresh near-surface layer in the seas north of Iceland in the middle to late 1960s, which was transferred through the Denmark Strait to the open North Atlantic in the late 1960s and was subsequently traceable around the subpolar gyre for over 14 years until its return to the Greenland Sea in 1981-1982. It is the first direct measure we have of the strength of the gyre circulation (about 3 cm s-1 on average). Figure 5 dates the passage of the GSA from its point of origin, through the Denmark Strait and around the Northern Gyre, while Figure 6 presents best-guess estimates of the salt deficit associated with the GSA at a series of points around this circuit.

The extreme nature of this event can be judged from the facts that as it passed south along the Labrador coast in 1971-1973 it represented a total salt deficit of some 72 × 109 tons; that in the eastern Atlantic, the observed freshening of about 0.1 psu to 500 m depth was the equivalent of adding an "extra" 1.4 m of fresh-water at the ocean surface (Pollard and Pu, 1985); that as it passed through the Faeroe-Shetland Channel in 1976 or so, the salinities in the North Atlantic and Arctic Intermediate Water masses were at their lowest since records began in 1902 (see Figure 1b); and that as it passed by northern Norway and west Spitsbergen in 1978-1979, the Norwegian Atlantic Current, by any conventional definition, ostensibly contained no Atlantic Water (i.e., water of salinity of 35 or more).

The chronology of this great event (Dickson et al., 1988) and its biological effects (e.g., Blindheim and Skjoldal,

FIGURE 5

Transport scheme for the 0-1000 m layer of the northern North Atlantic, with dates of the GSA salinity minimum superimposed. (From Dickson et al., 1988; reprinted with permission of Pergamon Press.)



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement