Formation of the Gulf Stream Convergence Zone

The Gulf Stream convergence zone is a dipole of long-term averaged, near-surface wind divergence, positive over the cold and negative over the warm flanks of the Gulf Stream. We show that this climatological feature results from the boundary layer response to the varying large-scale winds of the storm track and the Gulf Stream sea surface temperatures. Using daily satellite observations of surface winds and sea surface temperature, we estimate the atmospheric boundary layer response of surface wind divergence as a convolution of ocean-mesoscale sea surface temperatures and a fitted impulse response function aligned with the direction and depending on the speed of the large-scale winds. Averaging daily reconstructions of the surface wind divergence in the northwest Atlantic recovers the notable features of the Gulf Stream convergence zone. In addition to the overall mean dipole, these include, west of 508W, its collocation with zonal bands of the sea surface temperature Laplacian, and the enhancement of the upwind and reduction of the downwind pole of the surface wind divergence dipole for averages conditioned on the meridional direction of large-scale winds. East of 508W, the largest averaged surface wind divergences are collocated with the maximum eastward gradient of the meridional sea surface temperature front. The distribution of daily large-scale winds in the storm track results in anisotropic, averaged impulse response functions. Convolutions with the varying geometry and orientation of the Gulf Stream sea surface temperature field explain the diversity of surface wind divergence features.