Gulf Stream Kinematics along an Isopycnal Float Trajectory

Abstract

An isopycnal-following float was deployed near the 400 m depth level in the high speed jet region of the Gulf Stream and tracked for approximately 300 km from the Blake Plateau towards Cape Hatteras during 16–19 May 1983. During its transit through a 400% change in bottom depth, the layer tagged by the float was surveyed with XBTs and hydrostations, and the surface mesoscale structure was determined from NOAA-6, 7 and 8 IR imagery. These data provide the first three-dimensional observations of water parcel motion along a Gulf Stream trajectory in the upper main thermocline. Approaching path segments with anticyclonic curvature, flow in the surrounding Stream volume was divergent and vertically compressed. Approaching path segments with cyclonic curvature, the flow in the surrounding Stream volume was divergent and vertically compressed. Approaching path segments with cyclonic curvature, the flow in the surrounding Steam volume was convergent and vertically stretched. The float was upwelled approaching anticyclonic path segments and downwelled approaching cyclonic path segments. Potential vorticity estimates along the float trajectory agreed within 10%, primarily through compensating changes in curvature vorticity and vortex stretching. Estimates of individual terms in the horizontal divergence from float and hydrographic data indicate that the cross-stream gradient of cross-stream velocity was approximately 7.0(±1.4) × 10−6 s−1 and the downstream gradient of downstream velocity was approximately 2.0(±0.4) × 10−6 s−1. An independent estimate of the vertical gradient of vertical velocity showed it to have been approximately 2.5 × 10−6 s−1. The downstream volume flux divergence in the local volume surrounding the float was approximately 2 × 10−5 s−1, comparable with previous calculations involving the entire GulfStream. A quantitative estimate of the mean horizontal divergence between a track crest and trough based on a wave model of Palmen and Newton using our float, hydrographic data, and satellite data results in a value of 3.1 (±1.9) × 10−6 s−1, comparable with previous estimates in the Gulf Stream.