Eddy Energetics in the Upper Equatorial Pacific during the Hawaii-to-Tahiti Shuttle Experiment

Abstract

Eddy energetics in the central equatorial Pacific Ocean is examined using Acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979?80. Three distinct sources of eddy energy are identified with varying degrees of statistical reliability, and are interpreted as evidence for three separate instabilities of the mean flow field. An instability at and just north of the equator occurs primarily in boreal summer and fall. It arises from the cyclonic shear between the Equatorial Undercurrent and the South Equatorial Current (SEC) north of the equator. The instability is present only when and where both currents are well developed, and there is little involvement of the shear between the SEC and the North Equatorial Countercurrent (NECC). The instability is characterized by local maxima in zonal and meridional eddy velocity variance, strong U*V* Reynolds stress, and large mean flow to eddy kinetic energy conversion. Despite seasonal variability of the eddy kinetic energy production, no annual cycle energy is converted to eddy energy. A second instability occurs at the equatorial front at 3°N to 6°N, primarily during boreal winter. The instability is identified by large mean-to-eddy potential energy conversion. Finally, a third instability is evidenced by strong downgradient (northward) eddy heat flux and large mean flow to eddy potential energy conversion, in the thermocline of the NECC during boreal spring. Both features are confined below 60 m at 5°N?9°N. While the eddies gain potential energy from these last two instabilities, they are losing kinetic energy to the mean flow at a somewhat slower rate. Nonlinear advection appears to be unimportant in the total eddy energy balance, but the meridional diffusion of eddy energy represented by the meridional divergence of eddy pressure work is large and significant The latter redistributes eddy energy into (not out of) the region of the barotropic instability just north of the equator.