Adjustment of the Ocean under Buoyancy Forces. Part I: The Role of Kelvin Waves

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  • 1 Geophysical Fluid Dynamics Program, Princeton University, Princeton, NJ 08542
  • | 2 Meteorological Office Research Unit, Hooke Institute, Clarendon Laboratory, Oxford OX1 3PU, United Kingdom
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Abstract

The early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model. The initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the North Atlantic. The main adjustment in the time examined here (the first few months) is by coastal Kelvin waves. The first baroclinic mode dominates, so the behavior proves to be very similar to that in a shallow-water model. Also, the dynamics is locally close to that for an f-plane, but account must be taken of the way f varies with latitude. The essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin, especially at the level where the first mode has the maximum effect.

This study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations, namely in a model with too coarse a horizontal resolution and an artificially high lateral friction needed for computational stability. In the present case, the distortions can be quantitatively assessed and understood, and it is felt there are lessons to be learned about the use of general circulation models.

Abstract

The early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model. The initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the North Atlantic. The main adjustment in the time examined here (the first few months) is by coastal Kelvin waves. The first baroclinic mode dominates, so the behavior proves to be very similar to that in a shallow-water model. Also, the dynamics is locally close to that for an f-plane, but account must be taken of the way f varies with latitude. The essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin, especially at the level where the first mode has the maximum effect.

This study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations, namely in a model with too coarse a horizontal resolution and an artificially high lateral friction needed for computational stability. In the present case, the distortions can be quantitatively assessed and understood, and it is felt there are lessons to be learned about the use of general circulation models.

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