On the response of ocean currents to atmospheric cooling

The response of steady ocean currents to forced atmospheric cooling is examined by a simplified two-layer model. The study focuses on currents, such as the Gulf Stream, which lose their heat to the atmosphere as they flow from one region to another. The model is inviscid and includes no coupling between the ocean and the atmosphere. Approximate solutions for specified cooling processes acting steadily on currents whose undisturbed speed is uniform are obtained analytically using a uniformly valid power series expansion.

It is found that upon encountering a region of cooling, the interface steepens and the whole current is displaced horizontally. The streamlines in the upper portion of the light layer are displaced to the right (looking downstream) whereas the streamlines in the lower portion of the upper layer are displaced to the left. These movements result from a combined effect of advection and “thermal wind” motion. For actual currents and heat losses in the ocean, the predicted interface steepening is of the same order as the slope upstream and the predicted horizontal displacements during strong cooling processes can be as high as ∼ 100 km.

Possible application of this theory to the separated Gulf Stream which loses heat to the atmosphere as it flows from Cape Hatteras toward the northeast is discussed. The model predicts that during the late winter the position of the Gulf Stream front will be farther to the south than it is during the summer and that the slope of the interface will be larger in winter. Both processes agree qualitatively with the observed seasonal variability of the Stream.