Atmospheric forcing of interannual variability in the northeast Pacific Ocean: Connections with El Niño

The nature and causes of the observed interannual variability of oceanic conditions in the Northeast Pacific were investigated using a variety of meteorological and oceanographic data sets. An examination of a long series of seasonal mean atmospheric sea level pressure (SLP) charts showed that when the winter average SLP pattern in the North Pacific varies significantly from climatology, it tends to do this in one of two characteristic ways. In particular, in some winters there is an anomalously strong circulation which differs from climatology mainly in having an intensified Aleutian low. In other winters there is an anomalously weak atmospheric circulation; this is almost invariably associated with the westward displacement of the Aleutian low and the production of a separate but much weaker low in the Gulf of Alaska. The North Pacific winter SLP appears to have some relation to the Southern Oscillation (SO) and oceanic temperatures in the tropics. In particular, El Niño/Southern Oscillation (ENSO) episodes are generally coincident with strong North Pacific SLP patterns and often follow winters with weak SLP patterns. However, this relationship is far from being perfectly regular and predictable. Interannual variations in the intensity of the North Pacific atmospheric surface circulation were found to be strongly correlated with the anomalies of both sea surface temperature (SST) and sea level height (SLH) at stations along the British Columbia coast. An intense Aleutian low is invariably associated with warm coastal waters and high coastal sea levels. By comparing a large number of Pacific SLP and SST maps, it was possible to verify that the relation between North Pacific SLP and oceanic surface temperature actually applies to the SST over much of the northeast Pacific (at least as far west as 150°W). The relationship of North Pacific SLP with the SO, together with the relationship between North Pacific SLP and oceanic conditions in the Northeast Pacific, can account for the observed general tendency for SST and SLH along the west coast of North America to be anomalously high during tropical ENSO episodes. On the other hand, the lack of a truly predictable relation between the Southern Oscillation and North Pacific SLP explains the fact that not all ENSO events are associated with wanning of the northeast Pacific.