BUOYANT GRAVITY CURRENTS RELEASED FROM TROPICAL INSTABILITY WAVES

Extremely sharp fronts with a 0.5°C sea surface temperature change over lateral distances of ~2m were observed in the equatorial Pacific on two occasions at 0°, 140°W and at 0.75°N, 110°W. In both cases, relatively warm and fresh water extending to 50m depth propagated to the southwest as a buoyant gravity current. This is confirmed by comparing estimates of front propagation velocity, calculated from a variety of shipboard and moored instruments, to theoretical gravity current models. Turbulent kinetic energy dissipation rates exceeding 10^-4 W kg^-1 were measured with a microstructure profiler on the warm/fresh side behind the leading edge of the front — 1000 times greater than dissipation rates found on the quiescent side. From satellite images, these gravity currents were observed to propagate ahead of the trailing edge of a tropical instability wave (TIW) cold cusp. The observations are compared to a numerical model of the equatorial Pacific with 6km horizontal resolution. The model results suggest that TIW fronts may release gravity currents through frontogenesis and loss-of-balance as the fronts approach the equator. While the lateral scale of the modeled fronts is many times larger than the observed fronts, they appear to obey a similar dynamical balance between pressure gradient and across-front acceleration.