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Regional Primitive Equation Studies of the Gulf Stream Meander and Ring Formation Region

Michael A. SpallDivision of Applied Sciences, Harvard University, Cambridge, Massachusetts

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Allan R. RobinsonDivision of Applied Sciences, Harvard University, Cambridge, Massachusetts

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Abstract

Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales of one month. A local primitive equation energy and vorticity analysis (PRE-EVA) routine is used to determine the dominant processes of simulated warm and cold ring formation events. The warm ring formation is achieved by differential horizontal advection of a developed meander system. The cold ring formation involves geostrophic and ageostrophic horizontal advection, vertical advection, and baroclinic conversion. Ageostrophic horizontal and vertical advections and stronger baroclinic conversion are believed to be responsible for the more realistic structure of the rings produced by the primitive equation model.

Abstract

Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales of one month. A local primitive equation energy and vorticity analysis (PRE-EVA) routine is used to determine the dominant processes of simulated warm and cold ring formation events. The warm ring formation is achieved by differential horizontal advection of a developed meander system. The cold ring formation involves geostrophic and ageostrophic horizontal advection, vertical advection, and baroclinic conversion. Ageostrophic horizontal and vertical advections and stronger baroclinic conversion are believed to be responsible for the more realistic structure of the rings produced by the primitive equation model.

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