Wind-forced Variations in Sea Surface Height in the Northeast Pacific Ocean

Abstract

Sea surface height (SSH) anomalies from the Geosat altimeter for the northeast Pacific Ocean were analysed to determine their annual and interannual fluctuations over a 2.5-year period. The interannual anomalies suggested large-scale changes in the intensity of the California and Alaska currents, with a weak California Current for the first year (1986?1987), which strengthened during the second year, partly by a diversion of flow from the Alaskan gyre into the California Current and partly by a decrease in SSH along the coast. In the California Current between about 36° and 46°N, the annual fluctuations in SSH showed westward phase propagation. These observations were modeled using a linearized potential vorticity equation with one active layer, forced by realistic wind stress curl, which resembled a standing wave. The annual fluctuations in SSH were produced primarily by Ekman pumping, because Rossby waves are coastally trapped poleward of about 37°N. The predicted response had excellent phase agreement with the data but underestimated the magnitude of the observations by nearly a factor of 2. This simple analysis suggests that the California Current core propagates offshore during the year not due to Rossby waves but rather due to a combination of spatial variations in the wind stress curl field and the meridional variation of the Coriolis parameter. Some qualifications to these conclusions are discussed along with an examination of errors in both SSH and wind stress.