Horizontal Reynolds stresses in the central equatorial Pacific

Abstract

Seven 13‐month‐long current records from the North Pacific Experiment Hawaii‐to‐Tahiti Shuttle Experiment during 1979–1980 were analyzed to investigate eddy energetics of the upper ocean within 75 km of the equator. Profiling current meter data from the experiment were also used in an attempt to relate the eddy motions to the mean zonal flow. The current fluctuations are highly anisotropic, with zonal current dominating. After removal of a linear trend plus annual and semiannual harmonics, the residual high‐frequency current fluctuations are much more nearly isotropic. The high‐frequency eddy kinetic energy decreases by a factor of 2 over the upper 250 m and at 100 m has a relative minimum on the equator. The seasonal time scale eddy kinetic energy shows a much more dramatic decrease below the thermocline. With the exception of tidal peaks, the spectra of zonal current are red, while meridional current spectra show a flattening at the lowest resolvable frequencies. Meridional current was non‐stationary, with an energetic packet of 10‐ to 15‐day‐period oscillations occurring in early 1980. The off‐diagonal component of the Reynolds stress tensor (〈u′υ′〉) calculated from the high‐frequency residuals is everywhere positive, decreasing from north to south and with depth. The implied cross‐equatorial flux of zonal momentum is at odds with eddy viscosity parameterizations which relate 〈u′υ′〉 to the mean meridional shear of the zonal current. The observed distribution of Reynolds stresses might be explained by radiation of energy from an instability of the portion of the South Equatorial Current north of the equator. In all cases, the high‐frequency u′υ′ time series are significantly skewed toward positive values and show no evidence of periodicity, indicating that the process(es) responsible is(are) intermittent and broadband. This is consistent with the instability mechanism.