Buoyant gravity currents released from tropical instability waves

Abstract

Two extremely sharp fronts with changes in sea surface temperature >0.48°C over lateral distances of 1m were observed in the equatorial Pacific at 0°, 140°W and at 0.75°N, 110°W. In both cases, layers of relatively warm and fresh water extending to 30-m depth propagated to the southwest as gravity currents. Turbulent kinetic energy dissipation rates averaging 4.5×10-6Wkg-1 were measured with a microstructure profiler within the warm layer behind the leading edge of the fronts1000 times greater than dissipation in the ambient water ahead of the fronts. From satellite images, these fronts were observed to propagate ahead of the trailing edge of a tropical instability wave (TIW) cold cusp. Results from an ocean model with 6-km grid resolution suggest that TIW fronts may release gravity currents through frontogenesis and loss of balance as the fronts approach the equator and the Coriolis parameter weakens. Sharp frontal features appear to be ubiquitous in the eastern tropical Pacific, have an influence on the distribution of biogeochemical tracers and organisms, and play a role in transferring energy out of the TIW field toward smaller scales and dissipation.