Available Potential Energy for the Tourbillon Eddy: A Tentative Energy Budget from the CTD Mesoscale Arrays

Abstract

Using the technique developed by Bray and Fofonoff, an estimation is made of the available potential energy (APE) stored in the North Atlantic domain which was intensively explored during the TOURBILLON experiment in September?October 1979. This technique allows an accurate determination of the topography of the material surfaces along which adiabatic displacements of fluid elements are assumed to take place. An illustration is given where the scale of the surveyed eddy, and the advection of Mediterranean water at mid-depth around it, appear clearly. Vertical profiles of APE were computed for each of the four occasions when the CTD army was surveyed. The shape of the profiles is very similar to the MODE one, except for a pressure shift of the main maximum, and an additional peak above the main thermocline which is interpreted as the signature of the eddy at this level (300?400 db). The total APE computed through an integration of the APE profiles over the depth range 300?3000 db is compared with the total geostrophic kinetic energy of the same fluid man as inferred from dynamic computations. Possible conversion between mesoscale baroclinic APE and kinetic energy is examined: the two quantities exhibit a parallel time evolution. This tends to prove that the surveyed domain has to extract some potential energy from the surrounding fluid at scales larger than the CTD array size, or to undergo local-scale transfers of potential energy by mass rearrangement, in order that conversion process from APE to kinetic energy at the mesoscale be realized effectively. The hypothesis of a noninsulated fluid system seems to be corroborated by the time evolution of the potential energy of the reference leveled field, whose changes are linked, at least from survey 1 to survey 3, to an extension of the eddy-core anomaly characterized by a relatively light water mass below the mean core depth, namely 500 db.