Figure 8

Correlations of the November to March NP index with 500 mb heights for 1948 to 1991 (upper panel) and the departure pattern corresponding to a unit standard deviation of NP. Negative values are dashed. Twenty-six percent of the variance is accounted for by the correlations over the 140°E to 60°W, 30°N to 65°N region.

through baroclinic instability, changes in the transient storm tracks result (van Loon, 1979; Lau, 1988).

When possible causes of changes are considered for the North Pacific, one prospect is in situ forcing through the influence of extratropical SST anomalies in the North Pacific on the circulation (Namias, 1959, 1963). It has been difficult to substantiate such influences either statistically (Davis, 1976, 1978) or with models (Ting, 1991; Kushnir and Lau, 1992). Recent modeling studies of SST anomalies in the Northern Hemisphere indicate that the changes in the storm tracks alter the eddy vorticity fluxes in the upper troposphere in such a way that they often reinforce and help maintain the circulation anomalies (Lau and Nath, 1990; Ting, 1991; Kushnir and Lau, 1992).

While the changes in eddy transports from the altered synoptic systems are one major complication, another is that the atmospheric heating effects may not be local. The sensible heat exchanged between the ocean and atmosphere is realized locally, but the latent heat lost by the ocean through evaporation is realized only as an increase in moisture, and the actual atmospheric heating is not realized until precipitation occurs, often far downstream. This latter aspect depends on the prevailing synoptic situation at the time, and varies with location according to the prevailing winds and background climatological flow. These nonlocal effects are therefore a sensitive function of position, and they add a large nondeterministic component to any forcing. This means that it is much more difficult to detect any systematic effects in both the real atmosphere and models. It also helps account for differences in results from many different model experiments, because inserted SST anomalies vary in location and intensity and the model climatologies vary. Placing "super SST anomalies" into a model will enhance the local effects so that results are more likely to appear as significant, but they are also much more likely to be unrealistic and inappropriate for the real atmosphere.

Another prospective cause of changes in the North Pacific comes from changes in teleconnections. The best-known examples of global impacts of local forcing are those involving changes in tropical SSTs, like the El Niño/Southern Oscillation (ENSO) phenomenon. Such changes in the atmosphere and the underlying ocean in the tropical Pacific affect higher latitudes (Bjerknes, 1969; Horel and Wallace, 1981).

LINKS WITH THE TROPICAL PACIFIC

The period of the deeper Aleutian Low regime extends from 1977 to 1988; during it there were three El Niño (warm) events in the tropical Pacific but no compensating La Niña (cold) events. Because of the El Niños, the tropical Pacific experienced above-normal SSTs and a persistently negative Southern Oscillation index (SOI) for that period (Figure 9). Modeling studies (e.g., Blackmon et al., 1983; Alexander, 1992b) confirm the causal link between SSTs in the tropics and the North Pacific circulation, with a deeper Aleutian Low resulting from El Niño conditions. Alexander (1992a,b) further shows that the observed changes in the North Pacific Ocean SSTs can be accounted for largely by the atmospheric changes, by means of the associated changes in surface fluxes and mixing through the upper layers of the ocean, and by the deepening of the mixed



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