Subsurface Energetics of the Gulf Stream near the Charleston Bump

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Abstract
The energy budgets of the eddies and the mean flow in the Gulf Stream near a topographic feature known as the Charleston bump are computed. First, we consider these results in the context of the amplification hypothesis for the development of Gulf Stream meanders. According to this hypothesis, the finite amplitude Gulf Stream fluctuations observed offshore of Onslow Bay are the result of the destabilizing effect of the bump on the Stream. The present dataset was obtained both immediately upstream and downstream of the bump, and the results of our analysis suggest: 1) Immediately south of the Charleston bump, the eddies perform net work on the Gulf Stream at a rate of (1.02 ± .66) × 10−2 ergs cm−3 s−1 by transporting momentum offshore; 2) The net work performed by the eddies south of the bump is not used locally to accelerate the mean; rather, it is exported to the rest of the ocean at a rate of (1.58 ± 1.39) × 10−2 ergs cm−3 s−1; 3) In spite of the net work performed by the eddies south of the bump, eddy kinetic energy apparently does not decrease; 4) Immediately north of the Charleston bump, the flow appears to be both barotropically and baroclinically unstable. These results support the amplification hypothesis by demonstrating the destabilizing effect of the bump on the eddies (points 1 and 4) and that upstream perturbations may survive to encounter the bump topography (point 3). Other results of our analysis are that the mean of mean kinetic energy by the eddies constitutes the dominant form of energy conversion and that eddy pressure work may be an important factor in the fluctuation energy budget. The second application of our calculations is to a characterization of the mean Gulf Stream in the South Atlantic Bight (SAB). The results of this analysis indicate the following: 1) The mean Gulf Stream kinetic energy flux increases downstream at a rate of (2.17 ± .98) × 10−2 ergs cm−3 s−1; 2) The eddies tend to decelerate the mean flow at a rate of (-0.57 ± 1.3) × 10−2 ergs cm−3 s−1; 3) In order that the mean energy equation be balanced, the Gulf Stream in the SAB must be releasing mean potential energy by flowing down a mean pressure gradient. Thus we have evidence suggesting the existence of a component of the pressure gradient associated with the Gulf Stream which is not geostrophically balanced. The downstream pressure gradient inferred at our array site is consistent with published estimates of mean alongshore pressure gradients in the SAB; however, the partitioning of the pressure force between mean acceleration and eddy Reynolds stress most likely holds only near the bump. We also estimate the net loss from the mean potential energy in the SAB using our measured conversion rate and demonstrate that it compares in magnitude but is opposite in sign to that thought to occur downstream of Cape Hatteras. Thus we argue that the Gulf Stream in the SAB is exhibiting some of the properties of the inflow regions of western boundary layers in inviscid inertial models of the general ocean circulation. Our measurements, however, also indicate the presence of vigorous eddies whose effects in the mean energy equation are potentially sizeable. Such eddies are, of course, not contained in strictly inviscid, inertial models of the western boundary layer.

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