Off-equatorial deep-cycle turbulence forced by tropical instability waves in the equatorial Pacific

This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer-"deep-cycle turbulence"-that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri approximate to 0.25).

Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0 degrees, 140 degrees W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear u(z) is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear upsilon (z) off of the equator.