KEVIN E. TRENBERTH AND JAMES W. HURRELL1
Considerable evidence has emerged of a substantial change in the North Pacific atmosphere and ocean lasting from about 1976 to 1988. Significant changes observed in the atmospheric circulation throughout the troposphere during that period show that a deeper, eastward-shifted Aleutian Low pressure system in the winter half-year advected warmer and moister air along the west coast of North America and into Alaska and colder air over the North Pacific. This advection caused substantial changes in sea surface temperatures over the North Pacific, as well as in coastal rainfall and streamflow, and in sea ice in the Bering Sea. Associated changes occurred in the surface wind stress, and, by inference, in the Sverdrup transport in the North Pacific Ocean. Changes in the monthly mean flow also imply substantial changes in the storm tracks and associated synoptic eddy activity, and in the sensible and latent heat fluxes at the ocean surface. In addition to the changes in the physical environment, large changes are found in the biology in the Northeast Pacific and in fish and other animal behavior.
It is suggested here that clues to possible causes of these changes lie in the close link between North Pacific changes on the decadal time scale and changes in the tropical Pacific and Indian Ocean, as well as the changes in frequency and intensity of El Niño versus La Niña events. A hypothesis is put forward outlining the tropical and extratropical relationships, which stresses the role of tropical forcing but includes important feedbacks in the extratropics that serve to emphasize the decadal more than interannual time scales. Whether the observed decadal variations are linked to "global warming" issues is an open question.
Climate variations over the North Pacific and teleconnections downstream across North America have long been of interest; they have been particularly highlighted by the work of Namias (1959, 1963, 1969). Recently, considerable evidence has emerged of a substantial decade-long change in the North Pacific atmosphere and ocean that began about 1976. Changes in the atmospheric circulation throughout the troposphere at this time have been documented by Trenberth (1990; see Figure 1 below) and Nitta and Yamada (1989)