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Sensitivity of a World Ocean GCM to Changes in Subsurface Mixing Parameterization

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  • 1 CSIRO Division of atmospheric Research, Aspendale, Victoria, Australia
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Abstract

The sensitivity of a coarse-resolution model of the World Ocean to parameterization of subgrid-scale mixing is examined. The model is based on the GFDL code. Results are presented from a series of model runs where the subsurface mixing parameterization is sequentially upgraded toward a more physical representation. The surface forcing is the same for all principal model runs and features a strong relaxation of surface temperature and salinity toward perpetual wintertime observed values. One model version is rerun with a full annual cycle of surface forcing and verifies that use of the perpetual winter surface relaxation introduces only minor biases in the essential characteristics of the solution.

Runs 1 and 2 feature the diffusivity tensor in the traditional horizontal/vertical orientation, and examines the effect of different vertical diffusivity profiles on the solution. Results are compared with those of previous studies. In both cases, the water mass properties (espcially the salinity Acids) are rather poor. In runs 3–5, a standard parameterization is introduced that allows for enhanced diffusion along the isopycnal surfaces. Each of these runs feature a different prescribed profile of isopycnal diffusivity, though with the same profile of vertical diffusivity as for run 2. Introduction of isopycnal mixing considerably improves the water mass structure, in particular by freshening and cooling water at intermediate depths toward realistic levels. However, the vertical stratification and density fields are little changed from run 2. likewise. the current structure and meridional overturning are little changed. Thus isopycnal mixing has a major effect upon the temperature and salinity fields, but very minor effect on the ocean dynamics. Isopycnal mixing is found to modestly increase poleward oceanic heat transport in the midlatitudes via enhanced quasi-horizontal mixing of warm salty subtropical and cold fresh subpolar waters.

In run 6, the isopycnal diffusivity of run 4 is retained, but the vertical diffusivity is instead allowed to vary as the inverse of the local Brunt-Väisälä frequency. However, the resulting solution is little changed from that of run 4. Reasons for this small change are discussed. Also discussed are the impact of numerical problems associated with the use of realistically small vertical diffusivity, and problems inherent in deep water formation in coarse-resolution models.

Abstract

The sensitivity of a coarse-resolution model of the World Ocean to parameterization of subgrid-scale mixing is examined. The model is based on the GFDL code. Results are presented from a series of model runs where the subsurface mixing parameterization is sequentially upgraded toward a more physical representation. The surface forcing is the same for all principal model runs and features a strong relaxation of surface temperature and salinity toward perpetual wintertime observed values. One model version is rerun with a full annual cycle of surface forcing and verifies that use of the perpetual winter surface relaxation introduces only minor biases in the essential characteristics of the solution.

Runs 1 and 2 feature the diffusivity tensor in the traditional horizontal/vertical orientation, and examines the effect of different vertical diffusivity profiles on the solution. Results are compared with those of previous studies. In both cases, the water mass properties (espcially the salinity Acids) are rather poor. In runs 3–5, a standard parameterization is introduced that allows for enhanced diffusion along the isopycnal surfaces. Each of these runs feature a different prescribed profile of isopycnal diffusivity, though with the same profile of vertical diffusivity as for run 2. Introduction of isopycnal mixing considerably improves the water mass structure, in particular by freshening and cooling water at intermediate depths toward realistic levels. However, the vertical stratification and density fields are little changed from run 2. likewise. the current structure and meridional overturning are little changed. Thus isopycnal mixing has a major effect upon the temperature and salinity fields, but very minor effect on the ocean dynamics. Isopycnal mixing is found to modestly increase poleward oceanic heat transport in the midlatitudes via enhanced quasi-horizontal mixing of warm salty subtropical and cold fresh subpolar waters.

In run 6, the isopycnal diffusivity of run 4 is retained, but the vertical diffusivity is instead allowed to vary as the inverse of the local Brunt-Väisälä frequency. However, the resulting solution is little changed from that of run 4. Reasons for this small change are discussed. Also discussed are the impact of numerical problems associated with the use of realistically small vertical diffusivity, and problems inherent in deep water formation in coarse-resolution models.

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