Modification of the Gulf Stream through Strong Air–Sea Interactions in Winter: Observations and Numerical Simulations

Abstract

The greatest fluxes of heat and moisture from the ocean to the atmosphere occur off the east coast of North America during winter when the Gulf Stream is vigorously cooled by strong cold air outbreaks that move off the continent. In this paper observational and numerical modeling methods are employed to investigate the response of the Gulf Stream to such strong cooling events. Both methods show that the surface mixed layer can deepen several tens of meters during a single strong outbreak and that the heat decrease within the upper layer of the Gulf Stream, 2.9 ? 1013 J in the model and 3.2(±0.7) ? 1013 J in observations (per meter alongstream) for one case study, is balanced closely by the amount of oceanic heat released to the atmosphere. Computations also show that the cross-stream circulation is dominated by Ekman-like, wind-driven motion with velocities on the order of 20 cm s?1. A vertical circulation cell within the Gulf Stream, with vertical velocities on the order of 0.1 cm s?1, is found to be a result of convergence/divergence of the Ekman transport due to the altered inertial frequency caused by the horizontal velocity shear of the Gulf Stream jet.