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Natural Climate Variability on Decade-to-Century Time Scales
Winter index of the North Atlantic Oscillation (1895-1983) based on the mean normalized pressure difference between Ponta Delgada, Azores and Akureyri, Iceland. (From Rogers, 1984; reprinted with permission of the American Meteorological Society.)
Winter mean air temperatures at Nashville, Charleston, and New Orleans, from 1947-48 to 1974-75. (From Dickson and Namias, 1976; reprinted with permission of the American Meteorological Society.)
sents an insignificant drain on the 100,000 km3 or so of the Arctic fresh-water reservoir. Thus, the fresh-water supply to the GSA is likely to have its origins upstream, in the processes of ice production and supply within the Arctic Ocean.
By comparing the timing of sea-ice variations in seven subregions that encircle the polar ocean over a 32-year period, Mysak and Manak (1989) were able to confirm a significant out-of-phase relationship between the ice-concentration anomalies of the Beaufort and Chukchi seas and the Greenland Sea. Later, Manak and Mysak (1989) and Mysak and Power (1991) extended and added detail to this sequence of relationships by showing, first, that fluctuations of the Mackenzie River discharge promote sea-ice anomalies of the same sign in the Beaufort and Chukchi seas about one year later and, second, that the export of large build-ups of sea-ice from the western Arctic via the Beaufort Gyre and the Transpolar Drift Stream would contribute to anomalies of ice extent and (from ice melt) freshwater supply in the Greenland Sea some 3-4 years after that.
Thus, annual runoff from the North American continent into the western Arctic precedes Koch's sea-ice severity index for the coast of Iceland by around 5 years (Figures 13a, b, c), and Mysak and Power (1991) conclude that the record freshets to the Arctic Basin in 1964-1966 (2 or more standard deviations from the mean; see Figure 13a) were significant precursors and suppliers of fresh-water to the GSA, as it developed in the Iceland and Greenland seas during the middle to late 1960s.
Local Preservation of the Fresh-water Layer in the Iceland Sea
The record extent of sea ice that developed in the Greenland and Iceland seas during the late 1960s (maximum in 1968) was partly the result of local as well as remote forcing, and was in fact a "telltale" of a local process that was fundamental to the preservation of an accumulating anomaly as large as the GSA: namely, by the mid-1960s the proportion of Polar Water in the East Greenland Current was so great that surface salinities north of Iceland decreased below the critical value of 34.7 (Figure 14). Below this value the surface layers will not reach a sufficiently high density,