A tomographic view of the Gulf Stream southern recirculation gyre at 38 degrees N, 55 degrees W

Abstract

Reciprocal acoustic transmissions made in a region just south of the Gulf Stream are analyzed to determine the structure and variability of temperature, current velocity, and vorticity fields at the northern extent of the southern recirculation gyre. For ten months (November, 1988 through August, 1989), a pentagonal array of tomographic transceivers was situated in a region centered at 38°N, 55°W as part of the eastern array of the SYNOP (SYNoptic Ocean Prediction) Experiment. The region of focus is one rich in mesoscale energy, with the influence of local Gulf Stream meandering and cold-core ring activity strikingly evident. Daily-averaged acoustic transmissions yielded travel times which were inverted to obtain estimates of range-averaged temperature and current velocity fields, and area-averaged relative vorticity fields. The acoustically determined estimates are consistent with nearby current meter measurements and satellite infrared imagery. The signature of cold-core rings is clearly evident in the sections. Spectral estimates of the fields are dominated by motions with periodicities ranging from 32-128 days. Secondorder statistics, such as eddy kinetic energies, and heat and momentum fluxes, are also estimated. The integrating nature of the tomographic measurement has been exploited to shed some light on the radiation of eddy energy from the Gulf Stream. The Eliassen-Palm flux diagnostic has been applied to an investigation of wave radiation from the Gulf Stream. Results of the diagnosis suggest that the Gulf Stream itself is the source of wave energy radiating into the far field and found in the interior of the North Atlantic subtropical gyre.

Estimating open-ocean boundary conditions : sensitivity studies

“…The real data t hat would be used in an assimilation to calculate the potential vorticity fields, would come from the lower part of the SYNOP East array that is situated in the recircula-tion. Also, the acoustic tomographic transceivers are situated only in the lower part of the SYNOP East array (Chester, 1993).…”

“…This can be obtained by integrating tomographic reciprocal travel times around the edges of the polygon defined by the locations of the acoustic transceivers (see Chester, 1993). Using the adjoint method with a quasigeostrophic model, a vorticity observation would provide much more influence on the Lagrange multipliers than the streamfunction observations used here (see appendix, also Tziperman & Thacker, 1989).…”