Cooling Spirals and Recirculation in the Subtropical Gyre

Abstract

The influence of cooling in a western boundary current on the recirculation of parcels in the subtropical gyre and their eventual transfer into the subpolar gyre is investigated. It is shown that heat loss to the atmosphere and the resulting vertical convection of dense water in the Gulf Stream of a general circulation primitive equation model forces a counterclockwise spiral of the velocity vector with depth (a cooling spiral). Parcels that pass through this region of cooling are forced to cross under the upper-level trajectories from south to north, in opposite sense to the beta spiral experienced in the interior of the subtropical gyre. This crossing of trajectories is an important consequence of the cooling in the western boundary current as it influences both scale and structure of the subtropical gyre recirculation. A simple expression is derived that relates the spatial scale of the recirculation to the cooling rate in the western boundary current and the wind forcing in the subtropical gyre.