FIGURE 12

Change in the total number of cyclones between the "warm SE" and "cold SE" groups of winter months in the southeastern USA. (From Dickson and Namias, 1976; reprinted with permission of the American Meteorological Society.)

even at the freezing point, to mix with the underlying, more saline water (Figure 15); instead, ice will tend to form (Malmberg, 1969). Thus, not only was more fresh-water transported south toward Iceland during the 1960s (from whatever source) under the stress of an anomalous northerly airflow, but in the years of peak polar influence (1965-1971), the extremely cool, fresh character of the upper 200-300 m was preserved since it was not mixed out by convective overturn (Dickson et al., 1988).

Downstream Preservation of the Fresh-water Layer

Once the GSA has passed through the Denmark Strait and is undergoing its long circuit of the subpolar gyre, we are forced to be even more conjectural about the forces that might have maintained its integrity. One suggestion that is perhaps better based than most comes from Clarke (1984); it posits an interaction between the GSA and local environmental conditions as it passed south along the Labrador coast (see Figure 9 of Dickson et al., 1988). From the late 1960s until 1971-1972, and possibly for unconnected reasons, there was progressively less deep winter convection at OWS Bravo in the central Labrador Sea (Lazier, 1980). As a result, said Lazier, the surface layer at Bravo freshened and the offshore salinity gradient weakened, so that the offshore fresh-water flux by eddy-diffusive processes would have weakened also. For this reason, Clarke suggests that the GSA fresh-water signal would have been maintained rather than mixed offshore during its long circuit of the Labrador shelf and slope. Although convection at Bravo and the offshore salinity gradient were rapidly restored following the severe winter of 1972, the GSA signal had largely passed.

Periodicity

Our present patchy understanding of the GSA event suggests that the Arctic Ocean provided the fresh-water, the northerlies of the Greenland High brought it south, and conditions both north of Iceland and downstream prevented it from being mixed out (vertically and laterally). However, we have only an inconclusive answer to its likely periodicity. If the strength of the winter high-pressure cell at Greenland in the 1960s was a critical factor, as Dickson et al. (1988) claim, then that circumstance was a secular event—the product of a slow evolution over most of this century. If the maxima in Koch's sea-ice index indicate the same process of accumulation of fresh-water and suppression of convection north of Iceland as we describe in Figure 15, then there may be grounds for belief that an earlier, if lesser, salinity minimum in 1910-1914 (see, for example, Figure 1b) reflected a previous iteration of the GSA set of processes (Dickson et al., 1984). However, numerous authors have pointed to the scope for multiannual or decadal periodicity



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