changes in temperature and moisture advection over the oceans by anomalous winds, and from the associated changes in surface fluxes and vertical mixing within the ocean. Anomalous northerly winds over the ocean are typically not only cold but also dry, so that large increases in surface fluxes of both sensible and latent heat into the atmosphere can be expected, which in turn cools the ocean. Convection and mechanical mixing in the ocean can spread those influences through considerable depth and may also entrain water from below the thermocline, giving the anomalies a finite lifetime (see, e.g., Frankignoul, 1985).
In the SSTs, a very distinctive pattern (cf. Figure 5) emerges as the dominant mode of an empirical orthogonal function (EOF) analysis (Davis, 1976; Lau and Nath, 1990) that is linked with a preferred mode of variability in winter in the Northern Hemisphere and is similar to the PNA teleconnection pattern of Wallace and Gutzler (1981). The PNA consists of four centers of action in the mid-tropospheric height field; they are of one sign near Hawaii and along the west coast of North America, and of opposite sign over the North Pacific and southeast United States. Wallace and Gutzler (1981) defined a PNA index using single-point values of 500 mb monthly mean geopotential height at each of the centers. This index does not appropriately weight the four centers of the PNA, and it is sensitive to errors in the analyses. Also, Northern Hemisphere upper-air analyses are available only after 1947. Wallace and Gutzler show that the surface signature of the PNA is mostly confined to the Pacific. An evaluation of sea level pressure charts (Trenberth and Paolino, 1980) shows them to be most reliable after 1924, and we therefore choose the area-weighted mean sea level pressure over the region 30°N to 65°N, 160°E to 140°W as a robust but simple measure of the circulation in the North Pacific. We refer to this as the NP index (for North Pacific). This is slightly smaller than the area used in Trenberth (1990), but corresponds better to the area participating in the decadal time-scale variations (see Figure 1). Trenberth (1990) noted that the correlation of the NP index with a PNA index based on all four PNA teleconnected centers at 700 and 500 mb is —0.92 for 1947 to 1987.
Figure 2 shows the NP index for all months of the year from 1924 through 1991. The first panel gives the total value of the index, and thus includes the mean annual cycle for three-month (seasonal) means that have been smoothed in time using a low-pass filter (with weights [1, 4, 8, 10, 8, 4, 1]/36, for which the half-power point is 11 years), which emphasizes the interdecadal fluctuations. The second panel shows the individual monthly anomalies (with the mean annual cycle removed) and is rather noisy, with the month-to-month variations prominent. The third panel shows the anomalies from the first panel, so that it is a smoothed version of the second panel, to show the most persistent anomalies. The figures emphasize that the period