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Natural Climate Variability on Decade-to-Century Time Scales
TABLE 1 Correlations between the NP November-to-March Index, Indices of SST, and the SOI*
*All values are five-month means. The period for the SOI and Niño 1 +2 regions is 1935 to 1990 inclusive, so the one-tailed 1 percent significance level is 0.32. For the other Niño regions the period is 1951 to 1990, and the 1 percent significance level is 0.38. The lead (in months) refers to the Niño SST or SOI index with respect to the NP index. To be included in the computation of the area-average SSTs, at least half the points in an area were required. Maximum values are in italics.
SST fields in the tropical Pacific as El Niño events develop (Trenberth, 1976; Rasmusson and Carpenter, 1982; Trenberth and Shea, 1987; Wright et al., 1988). Trenberth (1976), Trenberth and Shea (1987), and Wright et al. (1988) noted that pressures in the South Pacific (e.g., at Easter Island) respond about a season earlier than the SOI does, and Barnett (1985) suggested that changes can often be seen over the southeast Asian region before the SOI responds. Barnett et al. (1989) further suggested that this evolution might be linked to snow cover over Asia.
We have therefore examined in more detail the relationships between SSTs in the tropics and the NP index. Problems with data coverage are severe in the tropics prior to 1951. To help summarize the results, we have computed correlations between the area-averaged SST anomalies for the tropical Pacific Niño regions—Niño I and 2 (0 to 10°S, 90 to 80°W), Niño 3 (5°N to 5°S, 150 to 90°W), and Niño 4 (5°N to 5°S, 160°E to 150°W)—with NP at several leads and lags (see
Table 1). As larger areas are taken, the correlation coefficient increases in magnitude; for the Niño 3 and 4 regions combined, all correlations are larger, with maximum values of - 0.52 at a 3-month lead by the SSTs. This shows that the changes in SST throughout much of the tropical Pacific lead the NP index by about three months, although the cross-correlation is not sharply defined and values are only slightly smaller at zero lag. Nevertheless, these results emphasize the involvement of the tropical SST variations in the atmospheric and surface temperature variations over the North Pacific and North America.
DISCUSSION AND CONCLUSIONS
The picture emerging from these empirical and modeling studies is not yet fully clear, but the evidence suggests the following hypothesis. In the tropics, coupled ocean-atmosphere interactions result in coupled modes, of which ENSO is the most prominent. This coupling results in large interannual variability in the Pacific sector, with preferred time scales of 2 to 7 years, but with small-amplitude decadal variations. All these fluctuations have manifestations in higher latitudes through teleconnections within the atmosphere. In the North Pacific, ENSO variability is found in the PNA pattern (and the NP index), but is best seen when averages can be taken over the entire winter half-year, because the noise level associated with natural weather variability is high on monthly time scales. The deepened Aleutian low in ENSO events results in a characteristic SST anomaly pattern that, on average, is enhanced through positive feedback effects from effects of the extratropical SST anomaly itself and from changes in momentum (and vorticity) fluxes associated with changes in high-frequency storm tracks (see Kushnir and Lau, 1992). The same influences are present on long time scales, but whereas surface fluxes and mixed-layer processes are dominant in changing SSTs on interannual time scales, changes in ocean currents also become a factor on decadal time scales and would reinforce the SST changes. Moreover, the long time scale involved in changing the currents and the Sverdrup circulation adds further persistence to the extratropical system that, along with heat storage in the top 500 m of the ocean, serves to emphasize decadal over interannual time scales.
Aspects of the above hypothesis have appeared in the extensive works of Namias, but here we have emphasized much more the links with the tropics. A major but as yet unanswered question is whether either the intensity or the frequency of ENSO events might change as a result of global warming. A longer observational record than that given in Figure 9 reveals that the frequency and intensity of ENSO events have changed in the past (Trenberth and Shea, 1987), with strong ENSO fluctuations from about 1880 to 1920. Aside from the major event from 1939 to 1942, stronger and more regular ENSO events did not resume until the 1950s. However, the low-passed curve in Figure 9 indicates that the recent imbalance between the