for the winter half-year. These changes were associated with changes in the surface wind stress, and, by inference, in the Sverdrup transport in the North Pacific Ocean (Trenberth, 1991) that have been directly measured in the Gulf of Alaska (Royer, 1989). Bakun (1990) has also noted changes in the alongshore wind stress off California that are probably related. Changes in the monthly mean flow also imply substantial changes in the storm tracks and associated synoptic eddy activity (e.g., Lau, 1988; Rogers and Raphael, 1992) and in the sensible and latent heat fluxes at the ocean surface (Cayan, 1992). The circulation changes were accompanied by changes in heat and moisture advection due to the quasi-stationary flow (see, e.g., Rogers and Raphael, 1992), so that there were substantial changes in the temperatures along the west coast of North America and in sea surface temperatures (SSTs) over the North Pacific (Trenberth, 1990), as well as in coastal rainfall and streamflow (Cayan and Peterson, 1989), and in sea ice in the Bering Sea (Manak and Mysak, 1987).
In addition to the changes in the physical environment, Venrick et al. (1987) observed associated large changes in the epipelagic ecosystem in the North Pacific, with increases in total chlorophyll in the water column and thus in phytoplankton. Mysak (1986) earlier noted that the changes in ocean currents and temperatures had altered the migration patterns of fish, in particular tuna and salmon, in the northeast Pacific (see also Hamilton, 1987). Ebbesmeyer et al. (1991) found that the "step in Pacific climate" in 1976 had a profound effect on 40 environmental variables. They contrasted the periods 1968 to 1975 with 1977 to 1984 and noted that climate-related changes were found not only in the above parameters, but in the behavior of geese, salmon, and crabs in the Northeast Pacific, and in mollusk abundance, salinity, and water temperature in Puget Sound. Also accompanying the changes in the Pacific was a higher incidence of cold outbreaks across the plains of North America, which ultimately led to major freezes affecting the Florida citrus crop (Rogers and Rohli, 1991).
The change in 1976 is but one of several large changes that have occurred in the North Pacific. As noted above, Namias has documented decadal-scale variations in the past, with coherent signals in the atmospheric circulation and in the SSTs, and with teleconnections downstream across North America (see also Douglas et al., 1982; Namias et al., 1988). The fairly sluggish response of the mid-latitude ocean to changes in the ocean forcing through the surface momentum and heat fluxes effectively serves as a low-pass filter and emphasizes the longer time scales. More recent studies throw further light on these aspects and on the physical links between the atmosphere and ocean in the North Pacific, and will be discussed below. New evidence has also emerged on the teleconnections downstream across North America, particularly those associated with the Pacific-North American (PNA) teleconnection pattern.
Possible causes of the changes were discussed by Trenberth (1990); he noted the close link between North Pacific changes on the decadal time scale and those in the tropical Pacific and Indian Ocean, as well as the changes in frequency and intensity of El Niño versus La Niña events. That paper expanded on the similar link noted during El Niño events by Bjerknes (1969). These aspects will be pursued and quantified further below.
The time series in Trenberth (1990) ceased after the northern winter of 1987-1988, and it is quite interesting to see what the subsequent evolution has been. Accordingly, in this paper, we update the time series and carry out some more comprehensive correlation analyses with surface temperature and SST analyses, and examine further the links with the tropical Pacific and the Southern Oscillation. The time scales of the main circulation anomaly patterns contributing to the decadal variation and the links with the tropics and teleconnections elsewhere are also of interest. A more complete version of this paper is presented in Trenberth and Hurrell (1994).
In the North Pacific, a close association between SST anomalies and the atmospheric circulation has been well recognized. Changes in surface temperatures arise from