Lagrangian Studies of Fluid Exchange between the Gulf Stream and Surrounding Waters

Abstract

Between spring of 1988 and winter of 1990, 75 RAFOS floats were released east of Cape Hatteras near the center of the Gulf stream on the 26.8 σt surface, O (15°C). The purpose of this sequential deployment was to investigate the spatial and temporal characteristics of the meandering stream and the Lagrangian properties of fluid motion in the upper thermocline. These new observations were also intended to provide a database for comparative studies of fluid motion in the upper thermocline with similar observations made earlier (1984?85) in the middle and lower thermocline. Sixty-one of the 75 RAFOS float trajectories in the 13°?16°C layers have been analyzed from both a Lagrangian and Eulerian point of view. The results reconfirm the strong baroclinic structure of the Gulf Stream established earlier and reveal enhanced cross-stream motion with increasing depth and over the New England Seamounts, Float trajectories tend to be more convoluted east of the seamounts reflecting the large amplitude meandering in that region. The lateral exchange of fluid between the current and surrounding waters has been grouped into three major, categories: ring formation, ring?stream interaction, and meandering. Of these, ring formation is responsible for less than 16% of the losses, while the other two contribute about equally. The loss of water to both sides of the stream is approximately symmmetric as evidenced by the statistics of float loss: 50% to the north and 54% to the south relative to the total numbers of floats that were launched on the same side with respect to the velocity maximum. These statistics are consistent with the results from earlier observations in the lower thermocline (1984?85). Curvature of the flow has a slight but measurable effect on the velocity structure of the current, such that between troughs and crests the locus of maximum velocity shoals about 100 m or in terms of the tilt of the density structure displace about 10 km to the north. The maximum itself is remarkably independent of curvature, about 1.10 m s?1. A narrow band of nearly nonexistent shear is observed to be embedded within the anticyclonic side of the current. Its magnitude is more pronounced at crests than at troughs. A comparative study of the surface and subsurface north walls reveals strong lateral diaplacements, O (15 km), of one relative to the other such that the surface front (i.e., the maximum thermal contrast in the SST field) extends farther north in meander crests and south in the troughs than the subsurface north wall defined by 15°C at 200 m. A longitudinal dependence of the north wall offset is also evident but its cause is unknown. The influence of topography on the Gulf Stream can be seen in two ways. First; analysis of pseudo-Eulerian statistics based on the float data reveals a striking northward shift of the current as it passes over the New England Seamounts, a signature not reflected by individual floats but clearly discernible in the ensemble of float trajectories. Second, more than 60% of the float losses occur in the seamount region although this encompasses only 20% of the total distance spanned by float trajectories Changes in the ratio of MKE to EKE along the mean path of the current are consistent with varying meander envelope of a well-defined baroclinic jet.