Interdecadal variability in a numerical model of the northeast Pacific Ocean: 1970-89

Variations in the thermocline depth of the northeast Pacific Ocean during 1970-1989 are investigated using a reduced-gravity numerical model forced by the local surface wind stress and at the southern land-ocean boundary by a coastal Kelvin wave signal. Three experiments are presented with forcings by wind only, Kelvin wave only, and a combination of both. The wind forcing generates an anticyclonic gyre circulation with mostly annual variations. The Kelvin waves along the coast excite Rossby waves that propagate into the basin interior, producing changes in upper-layer thickness (related to changes in thermocline depth) that last for years after the Kelvin signal has passed. Two sequential upwelling Kelvin waves in 1973 and 1975 produce upwelling Rossby waves that reduce the mean upper-layer thickness by approximately 10-20 m during 1976. This shift is reinforced by later upwelling events lasting until the early 1980s. The authors present a new hypothesis that the previously known climate shift observed in winter sea surface temperature is influenced by changes in the depth of the permanent thermocline induced by remotely forced Rossby waves. Acoustic thermometry might be a sensitive means to detect Rossby waves, motivating a thermometric approach to studying interannual variations in the ocean. The role of the Rossby waves in the travel time variations of acoustic signals on geodesic paths from Hawaii to the North American coast are calculated using the scheme of Reed. Wind forcing produces mainly annual variations in the acoustic travel time anomalies, except for the high latitude paths above 40 degrees N that exhibit sudden shifts in travel times due to changes in the magnitude of the wind stress. The Rossby waves are primarily responsible for interannual variations.