Moored observations of the current and temperature structure over the continental slope off central California: 2. The energetics of the flow off Point Sur

Abstract

The data from two current meter moorings off Point Sur, California, were analyzed to examine the energy transfers over the continental slope in the California Current system (CCS). The method used was to calculate terms in the heat equation at intermediate depths between instruments, using the thermal wind relation to estimate the horizontal temperature gradients from the vertical shear. Time series of u, v, and T were collected for 17 months (May 1989 to October 1990) at site P2 on the 800‐m isobath and for 12 months (May 1990 to May 1991) at site P3, 25 km farther offshore on the 1800‐m isobath. Instrument depths were 100, 350, and 500 m at both moorings plus 1000 m at site P3. Mean values of the energy conversion terms were computed at sites P2 and P3 at 225 m depth and at site P2 at 425 m depth. The local change of eddy potential energy (EPE) was approximately balanced (∂EPE/∂t ≈ 0) for each case but the dynamics were quite different. At P2 225, the sources of EPE were advection by the mean flow and baroclinic instability, which converted mean potential energy (MPE) to EPE. The losses came from eddy advection out of the region and the vertical eddy heat flux (VEHF), which converted EPE to eddy kinetic energy (EKE). At P2 425, baroclinic instability provided the major source of EPE. This, plus a small positive contribution from the VEHF, was balanced by advection of EPE out of the region by both the mean flow and the eddies. At P3 225, mean advection and baroclinic instability were negligible, and the downward eddy heat flux (VEHF) was balanced by advection of heat out of the region by the eddies. Time series of terms in the EPE equation were used to examine high energy transfer events at both moorings. Events at P2 involved both horizontal and vertical processes and had longer timescales (several days to weeks) than those at P3, which had much shorter timescales (2–5 days) and were dominated by vertical processes. Compared to the Gulf Stream, the mean baroclinic instability at P2 (both depths) was smaller by at least a factor of 10. A term‐by‐term comparison between the mean values off Point Sur and the Kuroshio Extension showed different processes, with the eddies gaining strength at the expense of the mean flow off Point Sur and the opposite occurring in the Kuroshio Extension.