Sea Level Differences across the Gulf Stream and Kuroshio Extension

Abstract

Geosat altimetric data for November 1986 to December 1988 are used to estimate sea level differences between the Sargasso Sea and the slope waters across the Gulf Stream region, averaged between 73° and 61°W, and comparable areas across the Kuroshio extension region, averaged between 143° and 156°E. Sea level differences (south minus north) in both regions are higher in the fall. Their projection onto an annual cosine peaks in late September-mid-October, with 9 cm (Gulf Stream region) and 6.9 cm (Kuroshio region) amplitudes, and accounts for about 60% of the variance. Residual altimetric errors due to water vapor, orbit, and sea state bias are shown to be minor or negligible contributors, as is uncertainty in the atmospheric pressure used in the inverse barometer correction and uncertainty in the tidal model removed. Ship drift data averaged in a comparable manner appear to respond to time changes in Ekman transport rather than the geostrophic transport measured by the sea level differences. Dynamic heights above 200 m computed from climatological (1940?75) water temperature and salinity data show that, while dynamic heights in both regions rise by 6 to 7 cm in September due to heating above the seasonal thermocline, the south?north difference in dynamic height has a somewhat different seasonal pattern: in the Gulf Stream region the annual cycle of climatological differential dynamic heights (south minus north) accounts for 86% of the variance, has 3 dyn cm amplitude and peaks in late November; in the Kuroshio region it is negligible.