Translational Velocity of Warm Core Rings Relative to the Slope Water

Abstract

Five warm Core rings were examined during short time intervals using thermal infrared satellite imagery. A total of 12 pairs of observations of these rings were made, all east of 72°W. The observations concentrated on the translational velocity of the rings and the mean velocity of the surrounding slope water. The mean translational velocity of the rings was found to be 8.5 ± 3.1 cm s−1 at 282° from north. The mean velocity of the surrounding slope water, determined from a combination of in situ observations with a depth weighting based on theoretical arguments, was 5.2 ± 0.3 cm s&−1 at 258°. The difference, i.e., the velocity of the ring relative to the slope water, was 4.6 ± 3.0 cm s−1 at 208°, or, in component form, these rings were found to move relative to the surrounding slope water with a mean northward component of 2.8 ± 1.7 cm s−1 and a mean westward component of 3.2 ± 2,3 cm s−1. The observations presented here are in disagreement with previous observations of ring displacements which show a general southerly trend, as well as with most theoretical analyses of eddy propagation which show either westward or southwestward velocities. Previous observational studies, however, examined warm core rings over long periods of time, periods during which the rings invariably interacted with the continental shelf, with the Gulf Stream and/or with other rings. Because the continental shelf constrains rings to move generally in a southwesterly direction it is not surprising that estimates derived over long periods show a southerly trend. Only rings free of such interactions were considered in this study. This resulted in the observation intervals being short (12–36 h) and in a limited number of observations (12), hence in a relatively high uncertainty in the estimates. Nevertheless we note that 8 of the 12 observations showed a northward component. Finally, previous observational results dealt only with the rings' absolute velocities, not their relative velocities.