CGD Seminar Series - Deepak Cherian, Off-equatorial deep cycle turbulence forced by Tropical Instability Waves in the equatorial Pacific

Title: Off-equatorial deep cycle turbulence forced by Tropical Instability Waves in the equatorial Pacific

Speaker: Deepak Cherian, NCAR

Date: Tuesday 19 January 2021

Time: 11 am -12 pm

To join via Zoom, contact Tracy Baker,

Abstract: The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enabled by a daily cycle of 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 ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°N, 140°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 can also occur off the equator within the cold cusps of TIWs where the flow is marginally stable. In the cold cusp, pre-existing 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 $v_z$ off the equator. Turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon.Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which when combined with shear from shipboard ADCP data suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator.

For more information contact Tracy Baker,, 303-497-1366

Type of event:

Will this event be webcast to the public by NCAR|UCAR?: 
Announcement Timing: 
Monday, January 18, 2021 to Tuesday, January 19, 2021
Calendar Timing: 
Tuesday, January 19, 2021 - 11:00am to 12:00pm MST

Posted by Tracy Baker at ext. 1366,

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