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Oceanic Angular Momentum and Torques in a General Circulation Model

Rui M. PonteAtmospheric and Environmental Research, Inc., Cambridge, Massachusetts

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Richard D. RosenAtmospheric and Environmental Research, Inc., Cambridge, Massachusetts

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Abstract

The ocean's angular momentum (M) and torques about the Polar axis are analyzed using output from the global, eddy-resolving model of Semtner and Chervin. Seasonal variability in M is dominated by the annual cycle, whose magnitude appears capable of helping explain the residual in the solid earth-atmosphere annual momentum budget. Planetary (MΩ) and relative (Mr) ocean angular momentum components have comparable seasonal amplitudes. Most of the mean signal in Mr, results from flows in the Antarctic Circumpolar Current region, but flows as far north as approximately 30°S am needed to explain the seasonal cycle. Local1y, the strongest variability in relative angular momentum is found in the Tropics at all depths, a manifestation of the zonal, recirculating character of the tropical circulation. The time rate of change of M is very small compared to the applied wind torque. Calculation of bottom pressure torques using the geostrophic relation reveals a dominant balance between them and the surface wind torques in the model, implying a rapid transfer of angular momentum between the atmosphere and the solid earth through the ocean. The torque balance holds for latitudes totally blocked by continental boundaries as well as for latitudes that are only partially blocked (e.g., Drake Passage), suggesting the same angular momentum transfer mechanism for closed basin and Antarctic Circumpolar Current regions. Implications of the results for future ocean modeling efforts are discussed.

Abstract

The ocean's angular momentum (M) and torques about the Polar axis are analyzed using output from the global, eddy-resolving model of Semtner and Chervin. Seasonal variability in M is dominated by the annual cycle, whose magnitude appears capable of helping explain the residual in the solid earth-atmosphere annual momentum budget. Planetary (MΩ) and relative (Mr) ocean angular momentum components have comparable seasonal amplitudes. Most of the mean signal in Mr, results from flows in the Antarctic Circumpolar Current region, but flows as far north as approximately 30°S am needed to explain the seasonal cycle. Local1y, the strongest variability in relative angular momentum is found in the Tropics at all depths, a manifestation of the zonal, recirculating character of the tropical circulation. The time rate of change of M is very small compared to the applied wind torque. Calculation of bottom pressure torques using the geostrophic relation reveals a dominant balance between them and the surface wind torques in the model, implying a rapid transfer of angular momentum between the atmosphere and the solid earth through the ocean. The torque balance holds for latitudes totally blocked by continental boundaries as well as for latitudes that are only partially blocked (e.g., Drake Passage), suggesting the same angular momentum transfer mechanism for closed basin and Antarctic Circumpolar Current regions. Implications of the results for future ocean modeling efforts are discussed.

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