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Initial Testing of a Numerical Ocean Circulation Model Using a Hybrid (Quasi-Isopycnic) Vertical Coordinate

Rainer BleckRosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149

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Douglas B. BoudraRosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149

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Abstract

An ocean circulation model, developed for the study of mesoscale to gyre-scale circulation and heat transport, is described and tested. The model employs density as vertical coordinate except in the immediate vicinity of possible coordinate surface intersections with the ocean surface. Test simulations are carried out for a double-gyre ocean basin of the type used by Holland (1978), but with better resolution in the upper 1000 m of the model ocean. The final state of a 5-year coarse-mesh (50 km resolution on a 2400 km × 1200 km domain) spin-up is used as the initial state for mesoscale experimentation on a 25 km mesh. Basin energetics, as well as mean mass transport streamfunction and mean potential vorticity fields are given for three mesoscale experiments, in which the magnitude and formulation of lateral dissipation are varied. Each of these experiments, run for approximately five years, exhibits vigorous mesoscale activity associated with western boundary current separation and the free jet. The mean gyre patterns and exchange of potential vorticity between the northern and southern gyres are considerably less symmetric with respect to the wind forcing pattern than those shown in Holland and Lin (1975b, case 10) and Holland (1978). The asymmetry results from a pronounced southward shift of the separation point of the free jet relative to the position of maximum wind stress, and from the tendency of the jet to form strong quasi-steady meanders in the western part of the basin. A reason for the difference between these results and those mentioned above is suggested, and the proposed experimentation to determine its validity is outlined.

Abstract

An ocean circulation model, developed for the study of mesoscale to gyre-scale circulation and heat transport, is described and tested. The model employs density as vertical coordinate except in the immediate vicinity of possible coordinate surface intersections with the ocean surface. Test simulations are carried out for a double-gyre ocean basin of the type used by Holland (1978), but with better resolution in the upper 1000 m of the model ocean. The final state of a 5-year coarse-mesh (50 km resolution on a 2400 km × 1200 km domain) spin-up is used as the initial state for mesoscale experimentation on a 25 km mesh. Basin energetics, as well as mean mass transport streamfunction and mean potential vorticity fields are given for three mesoscale experiments, in which the magnitude and formulation of lateral dissipation are varied. Each of these experiments, run for approximately five years, exhibits vigorous mesoscale activity associated with western boundary current separation and the free jet. The mean gyre patterns and exchange of potential vorticity between the northern and southern gyres are considerably less symmetric with respect to the wind forcing pattern than those shown in Holland and Lin (1975b, case 10) and Holland (1978). The asymmetry results from a pronounced southward shift of the separation point of the free jet relative to the position of maximum wind stress, and from the tendency of the jet to form strong quasi-steady meanders in the western part of the basin. A reason for the difference between these results and those mentioned above is suggested, and the proposed experimentation to determine its validity is outlined.

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