Response of an Intense Oceanic Current System to Cross-Stream Cooling Events

Abstract

Shifts in location and strength of an intense oceanic flow such as the Gulf Stream to a cross-stream gradient in cooling are studied using two-dimensional numerical simulations. The gradient in cooling is imposed by removing more heat from the warm side of the associated baroclinic zone than is removed from the cold side. The initial flow is assumed to be in geostrophic balance. When only a vertical heat exchange associated with the convective overturning induced by surface cooling is allowed, the magnitude of the horizontal pressure gradient is reduced and the flow becomes supergeostrophic. The resulting cross-stream velocity will tend to shift the front toward the region of larger upward surface heat fluxes. When a vertical exchange of momentum is also allowed in the convective adjustment, the reduction of the initial surface velocities due to turbulent momentum exchange is not balanced geostrophically by a reduction in the horizontal pressure gradient. The flow becomes subgeostrophic and a cross-stream flow is produced that shifts the front toward the region of smaller upward surface heat fluxes. Although the along-stream current decreases near the surface, the current below the mixed layer is strengthened due to the exchange with relatively high momentum from above. The additional response due to an increase in the southward and eastward wind stress is compared to the response due to cooling only. Small changes in the temperature and flow fields occur when a southward wind stress is included. An eastward wind stress of 0.2 N m has a greater effect on the position of the simulated Gulf Stream than does a very strong gradient in cooling.