Drifters in the Gulf Stream

In the past, the Gulf Stream has frequently been viewed as essentially a barrier between the Slope and Sargasso seas. On the other hand, surface drifters have often been observed to leave the stream to the south. To gain a better understanding of surface flow of the Gulf Stream, we used drifter trajectory data to study their drift east and patterns of loss from the current. Two sets of drifter data were used, one from the 1995-1999 Georges Bank GLOBEC program and the other from the Global Drifter Program. We also made use of atmospheric wind stress estimates from a reanalysis data set to evaluate the effect of wind forcing on their movements. Without fail, all drifters that enter into the stream eventually detrain out of the current to the south, indicating significant cross-frontal transport at the surface. A first explanation of these detrainments relies on the Ekman drift to the south due to the westerly winds over the study area. However, the exits to the south are not uniformly distributed, but aggregate in three areas where the meandering is particularly sharp: the New England Seamount Chain, the Southeast Newfoundland Rise, and the 44°N trough in the North Atlantic Current. Although, intuitively, it would seem that the largest Ekman drift losses would occur in the winter, this study shows that the detrainment to the south occurs more effectively in the summer due to a minimum in the mixed layer depth.