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Natural Climate Variability on Decade-to-Century Time Scales
Normalized winter-mean pressure anomaly for the region 60-70°N, 30-65°W from 1900-1979. (From Rogers, 1984; reprinted with permission of the American Meteorological Society.)
The two most relevant questions in the present context are how unusual the Greenland Ridge of the 1960s was, and what factors might have contributed to its development. The first of these is addressed in Figures 8 and 9, which provide both a long-term and a hemispheric perspective. Figure 8 shows that the ridging that developed over Greenland in the late 1960s was unprecedented in a record of some 75 years' duration and, more remarkably, that it formed the end point of a steadily increasing trend in winter pressure there, which has been under way almost since the turn of the century. (The subsequent collapse of the Ridge in the 1970s is the event reported by Dickson et al. (1975).)
Even more remarkable is the spatial isolation of this change. In illustrating the difference in winter mean SLP between 1904-1925 and 1955-1971 (i.e., across the two endpoints of the above trend), Rogers shows (Figure 9 above) that the principal pressure change is confined to a restricted cell centered over Greenland, paired with a lesser
Difference in winter mean SLP between 1904-25 and 1955-71. Stippled areas are significant at the 95% level. (From Rogers, 1984; reprinted with permission of the American Meteorological Society.)
change of opposite sign near the Azores. Not surprisingly, then, the North Atlantic Oscillation (NAO = Azores - Iceland pressure difference) was at its century-long minimum also during the late 1960s (Figure 10).
Part of the reason for the shorter-term intensification of ridging at Greenland and decrease in the NAO between the mid-1940s and the late 1960s may be the processes that promote winter storm development in the zone of maximal land-sea temperature contrast along the U.S. eastern seaboard half a wavelength upstream. The reason for supposing such a connection lies in the notion that a steady strengthening of pressure at Greenland may more properly be regarded as a weakening of the statistical Iceland low. (A weakening Iceland low in twentieth-century Januaries has been noted by van Loon and Madden (1983).)
Dickson and Namias (1976) offer some support to this link by showing that a decadal change from a regime of warm winters (1948-1957) over the southeastern United States to a regime of cold winters there (1958-1969; Figure 11) was accompanied by a steepening of the thermal gradient at the coast, a sharp coastwise increase in the mean winter cyclone frequency (Figure 12) and a significant (approximately 350 nmi) southwestward retraction of the zone of maximum storm frequency as storms matured faster offshore. Thus, it remains a possibility that the record intensification of the Greenland High at this time (the 1960s) might in part be a reflection of a southwestward withdrawal of the statistical Iceland low.
The Upstream Supply of Ice and Fresh Water
The total salt deficit of the GSA as it passed the Labrador coast (72 × 109 tons—see Figure 6) is equivalent to a freshwater excess of 2000 km3. As Aagaard and Carmack (1989) point out, this quantity of fresh-water is too great to have its source in the Iceland Sea itself, but as the equivalent of about one-half of the annual fresh-water transport of the East Greenland Current where it enters the Greenland Sea from the Arctic Ocean, it can be accounted for by only a moderate perturbation of that outflow (e.g., a 2-year period of a fresh-water flux 25 percent above normal) and repre-