Processes Affecting the Time-Averaged Surface Divergence and Vorticity in the Northwest Atlantic

Long time averages of surface divergence and vorticity over the Northwest Atlantic show a band of convergence and cyclonic vorticity near the Gulf Stream SST front. These bands are coincident with maxima in both mean rain rate and rain frequency; satellite estimates of rain frequency suggest that rain occurs about 20% of the time over the core of the Gulf Stream. Recent work has suggested that the band of surface convergence forces a deep atmospheric response anchored to SST front, resulting in the band of collocated precipitation maxima over the Gulf Stream. This study uses scatterometer surface winds and high-resolution mesoscale model simulations to further study the factors responsible for this band of convergence and cyclonic vorticity in the Northwest Atlantic.

Our results show that the band of surface convergence and cyclonic vorticity owes much of its existence to the location of the storm track. The band of convergence and cyclonic vorticity in time averages disappears when the divergence and vorticity are averaged in only rain-free conditions. Variance in the surface meridional winds is collocated with the bands of convergence and cyclonic vorticity. The divergence (vorticity) has a strong negative (positive) skewness caused by intense but intermittent storm variability. Thus, the time mean and median surface divergence differs substantially, with the median divergence and vorticity differing in sign from the means.

Finally, the divergence and vorticity exhibits dependence on the wind direction relative to the SST gradient vector. This dependence is consistent with the Booth et al. (2010) hypothesis that the surface storm track over the Atlantic consists of the storm track aloft modified by the SST influence on the atmospheric boundary layer.