Numerical simulations of the response of intense ocean currents to atmospheric forcing.
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The two and three-dimensional response of strong ocean currents to atmospheric forcing is studied using numerical simulations. In particular, surface cooling is explored as a possible mechanism for explaining an observed 100 km southward shift in the mean position of the Gulf Stream during winter. The magnitude and direction of the cross-stream circulation is highly dependent on whether or not a vertical mixing of momentum occurs when the water column convectively adjusts in response to the surface cooling. A weak cross-stream flow toward the higher sea-surface temperatures occurs in the surface layer if momentum mixing does not occur, whereas a stronger flow toward lower sea-surface temperatures results if momentum mixing does take place. The response of three-dimensional simulations is very similar to the two-dimensional simulations in the immediate vicinity of the front. The response due to horizontal cooling gradients is not large enough to displace the Gulf Stream appreciably southward in any of the numerical simulations. By contrast, a moderate increase in the zonal wind stress is more effective in displacing the core of the current system than are strong gradients in the surface cooling. The position of the Gulf Stream has also been observed to change as it encounters changes in the bottom topography. The simulated adjustment of flow to a 200 m seamount is consistent with the conservation of potential vorticity. The inclusion of surface forcing does not affect the adjustment to the topography in any of the simulations, and does not steer the flow toward (away from) the topography so that the characteristics of the downstream flow are changed.
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