Ageostrophy and pressure work in the Gulf Stream at 73°W

Abstract

During 1984, velocity, bottom pressure and thermocline depth data were obtained from deployments of tall current meter moorings, bottom pressure gauges and inverted echo sounders at five sites on the anticyclonic side of the Gulf Stream at 73° W. An additional number of inverted echo sounders, deployed in several nearby sites, allowed us to construct daily regional maps of the thermocline topography. Using this data base we examined the momentum balance and the pressure work in the eddy kinetic energy balance at 500 m above the seafloor and at a depth of 400 m. At 500 m off the bottom, the correlations between fluctuating geostrophic currents, inferred from pressure differences between sites, and suitably averaged observed currents provide observational confirmation of the geostrophic balance in the deep Gulf Stream, within our ability to measure it. At 400 m depth, the ageostrophic current was computed using velocity measurements, curvature estimates from the thermocline maps, and the momentum equation in natural coordinates. Comparison of observed and ageostrophic velocities showed occasional departures from geostrophy of O(30% to 60%) with duration from 2 to 5 days. These ageostrophic events were caused by tangential and centripetal accelerations, mostly during the coalescence of two cold‐core rings with the stream. The mean departure from geostrophy at 400 m during the deployment period was 7%. The mean effect of the pressure work, on the anticyclonic side of the stream at 400 m, was to decrease the eddy kinetic energy at a rate of −3×10−3 ergs cm−3 s−1; a value with magnitude as large as that of the barotropic instability term. The pressure work record is very episodic with peaks, during the ageostrophic events, close to 30 times larger than the mean estimate and with both positive and negative values. At 500 m off the bottom, the pressure work was found to be insignificantly different from zero.