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Sensitivity of the Thermohaline Circulation to Surface Buoyancy Forcing in a Two-Dimensional Ocean Model

A. CapotondiNational Center for Atmospheric Research, Boulder, Colorado

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R. SaravananNational Center for Atmospheric Research, Boulder, Colorado

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Abstract

The performance of thermal surface boundary conditions based on energy balance models for the atmosphere is tested using a two-dimensional (meridional plane) ocean model. The results are compared to those from an idealized ocean – atmosphere coupled system. The latter consists of a two-dimensional Boussinesq ocean model coupled to a two-layer global atmospheric model. The various thermal boundary conditions are applied to the same ocean model used in the coupled system, and their ability to capture the essential atmospheric feedbacks is investigated. Some of the effects associated with the atmospheric eddy moisture transport are also incorporated by empirically relating variations in the surface freshwater flux to variations in the surface heat flux based on the coupled model results. Comparisons with the coupled results show a considerable improvement in the characteristics of the equilibria of the ocean thermohaline circulation when the alternative thermohaline boundary conditions are used instead of the so-called “mixed boundary conditions” commonly used in ocean-only integrations. Furthermore, the response of the pole-to-pole equilibrium to a freshening of the high northern latitudes is in remarkably good agreement with the one observed in the coupled model. However, a tendency for the “energy balance” boundary conditions to overstabilize the circulation is detected, and limitations in the present treatment of the eddy moisture transport effects are found, especially in the presence of convective adjustment.

Abstract

The performance of thermal surface boundary conditions based on energy balance models for the atmosphere is tested using a two-dimensional (meridional plane) ocean model. The results are compared to those from an idealized ocean – atmosphere coupled system. The latter consists of a two-dimensional Boussinesq ocean model coupled to a two-layer global atmospheric model. The various thermal boundary conditions are applied to the same ocean model used in the coupled system, and their ability to capture the essential atmospheric feedbacks is investigated. Some of the effects associated with the atmospheric eddy moisture transport are also incorporated by empirically relating variations in the surface freshwater flux to variations in the surface heat flux based on the coupled model results. Comparisons with the coupled results show a considerable improvement in the characteristics of the equilibria of the ocean thermohaline circulation when the alternative thermohaline boundary conditions are used instead of the so-called “mixed boundary conditions” commonly used in ocean-only integrations. Furthermore, the response of the pole-to-pole equilibrium to a freshening of the high northern latitudes is in remarkably good agreement with the one observed in the coupled model. However, a tendency for the “energy balance” boundary conditions to overstabilize the circulation is detected, and limitations in the present treatment of the eddy moisture transport effects are found, especially in the presence of convective adjustment.

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