Observations of a cyclonic ring‐Gulf Stream coalescence event over the Blake Plateau

Abstract

Hydrographic data collected in September 1980 over the Blake Plateau were analyzed using a combination of empirical search and inverse techniques. Five sections, extending from the continental shelf break eastward across the Blake Plateau, were positioned at approximately 1 ø intervals between 28 ø and 32øN. The empirical search procedure was applied to four closed regions (boxes), constructed from adjacent sections, where each region was assumed to consist of two conservative layers. Five geostrophic velocity sections were obtained using the average optimum reference level for the four boxes. The inverse technique provided barotropic correction velocities that caused all layers to conserve mass. The sections and the resulting transport streamline fields revealed the presence of a cyclonic feature whose meridional and zonal extent was about 200 km in both directions. This feature was shown to be a coalescing Gulf Stream ring in a late stage of decay. Water mass analyses, current meter data, and earlier studies were used to support this hypothesis. The cyclonic advection of Gulf Stream water around the ring formed a meander in the eastern Gulf Stream wall. Topography appeared to be affecting this flow, suggesting that other rings entering the region would be similarly influenced. Evidence of recurring rings at or near this location and hence, meanders of the eastern Gulf Stream wall, was found in hydrographic data collected by NOAA over a 12-month period from 1965 to 1966. These data suggested that two such events, lasting about 4 weeks each, occurred during this period. The frequency of these events was in agreement with earlier findings, while the duration of these events was supported by current meter observations.