Turbulence and Internal Waves at the Equator. Part II: Details of a Single Event

Abstract

In the low Richardson number shear flow above the Pacific Equatorial Undercurrent, a single vertical microstructure profile intersected the overturning crest of a packet of high horizontal wavenumber waves. The observed dissipation rates within the overturning wave were so high that if they were representative of the volume-averaged rate, the total wave energy would have been dissipated within a single buoyancy period. The chaotic structure (and temperature fluctuations with horizontal scales less than 2 m) of the two wave crests and troughs west of the overturning wave crest suggest that recent mixing had occurred there. Wave crests and troughs east of the overturning wave crest showed little or no sign of turbulent mixing. Similar high horizontal wavenumber waves, believed to be shear-instability waves, have been observed in low Richardson number regions of the midlatitude seasonal thermocline. Although the equatorial waves have a horizontal wavelength appropriate for shear-instability waves, their vertical scale is much larger than the vertical extent of the low Richardson region, unlike that found for simple shear-instability waves.