Separation of Warm-Core Rings in the Gulf of Mexico

Abstract

The separation of anticyclonic rings is studied using a 12-level primitive equation numerical model of the western North Atlantic. The ?Gulf Stream Formation Region? model is based on the Bryan-Cox-Semtner code, and uses ¼ degree horizontal resolution. The eastern boundary of the model, near the mid-Atlantic Ridge, is forced by a ?pumps and baffles? region to have the appropriate temperature and salinity structure, vertical shear, and total transport. The model is closed by a solid northern wall at 36°N and is forced by steady winds. In the results presented here, large rings separate from the Loop Current in the Gulf of Mexico at periods near 30 weeks. The separation of a single typical ring is shown in detail. The most striking feature is that the separation is not a single spectacular event but a long, gradual process involving recirculation between the ring and the main flow for many weeks after the time at which one would, on the basis of standard observational evidence, normally believe the ring to be completely separated. There is no clear point during the separation sequence at which one can point to the horizontal velocity pattern and say ?the ring has just separated.? This is the first modeling study focusing on the Gulf of Mexico that resolves the vertical structure of the currents with more than two degrees of freedom and the first that includes the sills at the Yucatan and Florida straits in a realistic way. The model velocities are lower than those observed in the ocean, but the fundamental idea of the ring-shedding process seems realistic. These results suggest an unexpected complexity in the circulation patterns. The flow in the deeper levels of the model consists of a rich field of vortexlike and wavelike features that travel in company with the upper anticyclone. They travel to the west at a greater speed than the upper anticyclone, and they have substantial north-south motions. They fill the deep basin and interact with the bottom topography. The ring behavior is completely consistent with the observations of Lewis and Kirwan; the deep flow is in keeping with the analysis of Hamilton.