FIGURE 4

Potential temperature-salinity correlation below 4.5°C for four stations from 1962-1972 and from TTO/NAS (1981 data), all from a location northeast of the Grand Banks extending to about 3500 m. (From Swift, 1984b; reprinted with permission of Pergamon Press, Ltd.)

FIGURE 5

Salinity (psu) at the sea surface. (From Reid, 1979; reprinted with permission of Pergamon Press, Ltd.)

There may have been long-term changes in the atmospheric circulation coincident with this freshening. For an example of possible changes, consider Figure 6, which shows the difference between average wind-driven transport in the period 1973-1981 and that from 1955-1972. The changes are an average of about 30 percent of the mean at each grid point, and so may be significant. The negative values indicate a greater southward interior surface transport in recent years. Since the greatest negative values occur over the North Atlantic Drift, I wonder whether the high salts there might have been swept southward, partially shutting down the supply of salty water to the northeast Atlantic, and hence to the Labrador and Greenland, Iceland, and Norwegian seas. But Dickson et al. (1988) show that the freshening in the Faeroe-Shetland Channel took place late in an ordered sequence of events that began eight years earlier in the East Greenland Current, suggesting that it was a pulse-like increase in the volume of the fresh-water source that was at least in part responsible for the overall freshening.

We should remember that although these deep-water salinity changes (e.g., those in Figure 3) were the largest observed (at the time), they were still small in magnitude (only 20 parts per million). It may be that in the sinking zones only a small shift of the balance between precipitation and evaporation, or something similarly small, is at work here. In either event, this does illustrate the possible ties to large-scale atmospheric forcing of the deep thermohaline circulation, making the salinity shift, as we trace its propagation, one more tool with which to study the responsible processes.

FIGURE 6

Change in integrated Sverdrup transport, average from 1973-1981 minus the average from 1955-1972. The transports were calculated from multi-year average sea level pressures on a 5° grid. Data from the Scripps Institution of Oceanography Climate Research Group. Owing to a minor (but non-recoverable) computational error, the values shown are not in sverdrups, but in nontraditional units. The average changes were 30 percent of the 1955-1972 base-period values.



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