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Natural Climate Variability on Decade-to-Century Time Scales
Twelve-year (1977-to- 1988) average surface temperature or sea surface temperature anomalies as departures from the 1951-to-1980 mean. Contours every 0.25°C. Shown are the annual mean anomalies (upper panel) and the anomalies for the five winter months (November to March). Negative values are dashed.
to 1988, shown in Figure 5. The warming over Alaska and along the west coast of North America, together with the cooling in the central and western North Pacific, would be expected to accompany a stronger Aleutian Low, because of thermal advection (Rogers and Raphael, 1992) and increased ocean mixing and changes in the surface fluxes (Cayan, 1992; Alexander, 1992a,b). The increased southward gradient flow in the eastern North Pacific, revealed by the pressure pattern in Figure 1, would bring warmer and moister air into Alaska and along the west coast of North America, while anomalous northerly winds would give rise to colder-than-normal conditions in the central and western North Pacific. Lower SSTs are a consequence of large sensible and latent heat fluxes into the atmosphere, combined with increased mixing in the ocean (Cayan, 1992). Cayan and Peterson (1989) found that increased stream flow in the coastal region of the northern Gulf of Alaska results from increased coastal rainfall associated with a deepened Aleutian Low and changes in the PNA.
To further illustrate the nature of the surface temperature changes associated with the NP index, Figure 6 shows the correlations for the November-to-March five-month average over 1935 to 1990, along with the corresponding departure pattern in degrees Celsius associated with a unit standard deviation departure of the NP index. For each grid point, ''seasonal" values were computed only when data existed for at least three of the five months defining the season. Correlations between variables were not computed if the two variables had fewer than 75 percent of the total number of seasons in common. Across the North Pacific and North American regions, these patterns show that the anomaly featured in Figure 5 is consistent with the whole record: Below-normal NP values are associated with below-normal temperatures over the North Pacific and southeast United States and above-normal surface temperatures along the West Coast, extending throughout Alaska and across most of Canada.
We have also investigated these relationships as a function of various lags. To objectively decide how much variance is explained by the correlations across an area, we have averaged the correlation coefficient squared for the region 140°E to 60°W, 30° to 65°N. The largest surface temperature variance explained by the NP index for this region occurs with NP leading by I to 2 months (r2 values with NP leading by 3, 2, 1, and 0 months are 0.15. 0.19, 0.20, and 0.16). The pattern is similar to that at zero lag, but the magnitude of the correlation coefficients increase by about 0.1 (to >0.6 over the North Pacific, and to < -0.7 over British Columbia).
The above results are consistent with those of Davis (1976). The link between SST in the North Pacific and the overlying atmospheric circulation has become well established. The main relationship seems to be one where the changes in the atmospheric circulation are responsible for the SST changes, as shown by simultaneous and lagged correlations, for instance (Davis, 1976, 1978; Lanzante, 1984; Wallace et al., 1990). Nevertheless, there is the strong expectation that extratropical SST anomalies also influence and may reinforce the atmospheric circulation (Kushnir and Lau, 1992).
Further confirmation of the link between temperatures and the atmospheric circulation comes from correlations between the NP index time series in Figure 2 and tempera