A convergent instability wave front in the central tropical Pacific

Abstract

A narrow front encountered at 2.1°N, 140°W was characterized by high productivity and a 2°C temperature drop. Satellite sea-surface temperature (SST) imagery showed this front to extend over 400 km from just north of the equator to 5°N along the western (leading) edge of a tropical instability wave cold cusp. During 3 days of observations the front propagated westward at 64 cm s −1 while oriented SW-NE. Water velocities and densities around the front were measured using a shipboard acoustic Doppler current profiler and hydrographic stations. These data showed warm, fresh northern water moving southward at speeds of up to 50 cm s −1 to encounter cold, salty equatorial water moving northward at up to 40 cm s −1; both slabs were embedded in the South Equatorial Current, here a uniform 100 m deep westward flow of 90 cm s −1. At their meeting a net convergence of 30 cm s −1 across the 1-km wide front drove intense downselling of up to 0.9 cm s −1. Both T- S relations and velocity fields indicate that cold water subducted beneath the warmer water and continued northward to beyond 3°N, deepening to 120 m. For analysis the data were gridded in a co-ordinate system centered on the front and moving with it. The dynamics near the front consisted of a balance between pressure gradient and non-linear advection; the Coriolis force was smaller and associated mostly with the broader scale flow. Thus, the front itself was not geostrophic, but rather the leading edge of a density-driven flow that can be accurately modeled as a dissipative lock-exchange. The subduction released potential energy at the rate of 5290 ± 230 W m −1 of front length, comparable to previous estimates of large-scale energy conversion. Nevertheless, the kinetic energy gain by the larger-scale instability was only 840 ± 31 W m −1, implying that the rest of the energy was lost to the large-scale flow. Turbulent dissination as derived from the rate of mixing accounted for only 500 ± 300 W m −1 of this loss. On the full 100 km-scale of the layered flow, the Coriolis force became important: the orientation of the surface front and the angle of the interface's northward deepening produced a pressure gradient that balanced the meridional flow geostrophically.