Figure 9

Time series of the normalized Southern Oscillation Index (Tahiti minus Darwin sea level pressure anomalies) monthly, filtered with a five-month running mean and with a low-pass filter with 109 weights that removes periods less than 10 years.

layer by entrainment. But the results obtained here are not simply due to the 1982-1983 and 1986-1987 El Niños; the Aleutian Low was also much deeper than normal in several other years, especially in the winter of 1980-1981. Note, however, from Figure 4 that the previous time when comparably low values occurred over the North Pacific was during the major 1939-to-1942 El Niño event.

We have examined the correlation of the NP index with the SOI for the period 1935 to 1991. The Tahiti-minus-Darwin normalized surface pressure index has been used (Trenberth, 1984); it is used only from 1935 on because the Tahiti record prior to then is poor. Correlations of the SOI with the Northern Hemisphere sea level pressures for the November-to-March winter months combined show the link with the North Pacific and the extension across North America (Figure 10). Note the values of opposite sign over North America, which are very important as part of the overall pattern. The anomalous wind flow accompanying this pattern is indeed one where stronger southerlies along the west coast of North America accompany a negative SOI (i.e., El Niño conditions).

We have examined correlations of the SOI with NP at several lags, using five-month running mean values of the SOI (Table 1). This smoothing is needed to make the SOI representative of the Southern Oscillation (Trenberth, 1984) and its scale is compatible with the time scales of the NP index. The highest cross-correlations of 0.53 occur at zero lag ± 1 month. With the SOI leading by six months the correlation is 0.44, and with a lag of six months it is 0.16. An interpretation of this time/lag dependence is that the correlated changes are largely contemporaneous, and the

Figure 10

Correlations of the five-month mean (November to March) SOI with sea level pressures over the Northern Hemisphere for 1935 to 1991. The 1 percent significance level is 0.34.

persistence of the SOI is the factor contributing to the strong correlations when the SOI is leading. The lower values when the SOI is lagging are a reflection of the influence of the March-to-April time of year when the SOI tends to be weakest and preferentially changes sign (Trenberth, 1984).

However, this does not mean that there are not precursors in the tropics. On the contrary, it is well established that there is an evolution of the Southern Oscillation and the



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