SHIPBOARD AND ALTIMETRIC STUDIES OF RAPID GULF-STREAM VARIABILITY BETWEEN CAPE-COD AND BERMUDA

The temporal variability of the Gulf Stream between Cape Cod and Bermuda was examined using shipboard ADCP measurements, satellite infra-red (AVHRR) and altimeter (GEOSAT) data for a period of about 2 weeks in April 1989. Dominant changes were a southward shift and rotation of the Gulf Stream due to a meander, a westward translation of a warm-core ring and an eastward translation of a cold-core ring. Terms in the along-track (nearly cross-stream) and cross-track momentum balances from the ADCP data showed that horizontal advection of horizontal momentum was the principal ageostrophic term, exceeding both measured temporal changes and estimated vertical velocity terms by about a factor of five at a depth of 100 m. Cross-track pressure gradients (not measured) needed to be about 20% higher than expected geostrophically and, if not balanced by the horizontal advection terms, could force particles across and out of the Gulf Stream to the south at speeds of 0.1 m-1. Cross-track ADCP velocities agreed closely with those estimated from GEOSAT: both gave peak velocities in the Gulf Stream of about 2.0 ± 0.1 m s-1. Errors were of the same order as the ageostrophic terms in the along-track momentum balance. Except near the Bermuda Rise, GEOSAT-derived absolute velocities could be quantitatively related to major oceanographic signals, such as the Gulf Stream and rings. © 1990.