Simulation of the Atlantic Circulation with a Coupled Sea Ice-Mixed Layer-Isopycnal General Circulation Model. Part II: Model Experiment

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  • 1 Meteocrologisches Institut der Universität Hamburg, Hamburg, Germany
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Abstract

An ocean general circulation model (OGCM) formulated on isopycnal coordinates is used to model the circulation of the Atlantic. The model domain is bounded meridionally at 30°S and the North Pole and extends zonally from 100°W to 50°E with cyclic boundary conditions in the Arctic basin. The Atlantic sector of the Arctic basin is included in order to obtain a more realistic exchange of water masses between the North Atlantic and the Arctic. For the purpose of achieving a sufficient resolution of the high-latitude current systems, the grid spacing is made variable with a 2° × 2° resolution in the entire equatorial and North Atlantic and a steadily increasing resolution in the Greenland-Iceland-Norwegian seas towards about 1° zonally and 0.5° meridionally. The model is integrated over 100 years, with acceleration of the deeper layers during the fist 50 years. Initial conditions are observed annual mean temperature and salinity. After the adjustment period the ocean approaches a cyclo-stationary state, except for the deep ocean where temperature and salinity have not become stationary.

The model yields realistic equatorial currents and also simulates the separation of the Gulf Stream and the complex current structure in the Greenland-Iceland-Norwegian seas. Heat and freshwater fluxes and the seasonal variation of the mixed-layer depth agree reasonably with observations. The stratification of the upper ocean demonstrates the capability of an isopycnal model with thermodynamics to reproduce the thermohaline circulation. Finally, the simulated sea ice cover is considered in order to discuss the coupling among sea ice, mixed layer, and the deep ocean.

Abstract

An ocean general circulation model (OGCM) formulated on isopycnal coordinates is used to model the circulation of the Atlantic. The model domain is bounded meridionally at 30°S and the North Pole and extends zonally from 100°W to 50°E with cyclic boundary conditions in the Arctic basin. The Atlantic sector of the Arctic basin is included in order to obtain a more realistic exchange of water masses between the North Atlantic and the Arctic. For the purpose of achieving a sufficient resolution of the high-latitude current systems, the grid spacing is made variable with a 2° × 2° resolution in the entire equatorial and North Atlantic and a steadily increasing resolution in the Greenland-Iceland-Norwegian seas towards about 1° zonally and 0.5° meridionally. The model is integrated over 100 years, with acceleration of the deeper layers during the fist 50 years. Initial conditions are observed annual mean temperature and salinity. After the adjustment period the ocean approaches a cyclo-stationary state, except for the deep ocean where temperature and salinity have not become stationary.

The model yields realistic equatorial currents and also simulates the separation of the Gulf Stream and the complex current structure in the Greenland-Iceland-Norwegian seas. Heat and freshwater fluxes and the seasonal variation of the mixed-layer depth agree reasonably with observations. The stratification of the upper ocean demonstrates the capability of an isopycnal model with thermodynamics to reproduce the thermohaline circulation. Finally, the simulated sea ice cover is considered in order to discuss the coupling among sea ice, mixed layer, and the deep ocean.

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