November through March corresponds to the wintertime regime, with low pressures in the Pacific, while for the rest of the year the Pacific tends to be dominated more by the subtropical anticyclone. Also, the main variability occurs only in November through March. Standard deviations of the monthly anomaly times series range from about 1 mb in the summer months to more than 4.5 mb in January and February. Values exceed 2.8 mb only from November through March. The variability from one month to the next in the same winter season stems in part from westward-propagating planetary-scale waves with a 20-day period (Branstator, 1987; Madden and Speth, 1989). Averaging over the five winter months removes much of this kind of noise.

Accordingly, Figures 3 and 4 show the NP time series for the five-month wintertime average. In Figure 3 the 1946 to 1991 time series is broken up to emphasize the regime from 1976 to 1988, while in Figure 4 the time series from 1925 is shown with the low-passed curve to reveal, without the arbitrariness of deciding on a start and end for the regime, just how unusual the 1977-to-1988 period is; the only previous time for which comparable values occurred was the much shorter interval from 1940 to 1941. Nevertheless, considerable interannual variability is present within the regime.

Figure 3

Time series of mean North Pacific sea level pressures averaged over 30°N to 65°N, 160°E to 140°W for the months November through March. Means for the combined periods 1946-to-1976 plus 1989-to-1991 and for 1977-to-1988 are indicated (1988 refers to the 1987-1988 winter).

Figure 4

Time series of mean North Pacific sea level pressures for November through March, as in Figure 3, but beginning in 1925 and smoothed with the low-pass filter.

In the Aleutian Low from 1977 to 1988, for November through March, pressures were lower by 3.0 mb when averaged over the NP area of the North Pacific. Lower pressures were present individually in all five winter months and are highly statistically significant (Figure 1). No such change is present in any of the other months of the year (Figure 2). The wintertime changes correspond to the center of the low farther east and deeper on average by 4.3 mb for the five winter months, being deeper by 7 to 9 mb in January (Trenberth, 1991; see also Figure 1).

A climatology for surface wind stress based on the years 1980 to 1986 reveals changes in the North Pacific relative to a climatology based on ship data prior to 1977 that help confirm the reality of the sea level pressure changes (Trenberth, 1991), and so do analyses with independent data sets (Nitta and Yamada, 1989). Moreover, the associated changes in the curl of the wind stress and the corresponding Sverdrup transport in the ocean (Trenberth, 1991) over such a long period imply significant changes in the North Pacific Ocean currents.

The corresponding changes in surface temperatures are shown in Figure 5. The surface temperatures are taken from the updated Intergovernmental Panel on Climate Change (IPCC) data set (IPCC, 1990, 1992), which consists of land surface data from the University of East Anglia (Jones, 1988) blended with sea surface temperature data from the U.K. Meteorological Office (Bottomley et al., 1990; see also Trenberth et al., 1992). The surface temperature anomalies, expressed as departures from the 1951-to-1980 means, are shown for both the whole year and the five winter months November through March averaged over the 1977-to-1988 period. The temperature anomalies are strongly regional, and have both positive and negative signs. The 12-year period features very large North Pacific basin temperature anomalies, with warming of more than 1.5°C in Alaska and cooling of more than 0.5°C in the central North Pacific. The pattern in Figure 5 over the North Pacific is similar to the first EOF of SSTs (Davis, 1976; Lau and Nath, 1990). The annual mean anomaly clearly arises from the wintertime atmospheric anomaly, but over the North Pacific it is sufficiently persistent throughout the year that there is little difference between the annual and the five-month means. The wintertime pattern also reveals below-normal temperatures over the southeastern part of the United States, illustrating the PNA teleconnection. This is reflected in a higher-than-usual incidence of major freezes affecting the Florida citrus crop after 1977 (Rogers and Rohli, 1991).

The most compelling argument that the changes in sea level pressure are real is the physical consistency with the very large regional Pacific temperature anomalies for 1977



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