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

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  • 1 National Center for Atmospheric Research, Boulder, Colorado, USA.
  • 2 National Center for Atmospheric Research, Boulder, Colorado, USA. Earth Sciences Division, NASA Ames Research Center, Moffett Field, California, USA.
  • 3 Climate Change Research Centre, School of Mathematics and Statistics and the ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW, Australia
  • 4 Applied Physics Laboratory, and School of Oceanography, University of Washington, Seattle, Washington
  • 5 National Center for Atmospheric Research, Boulder, Colorado, USA.
  • 6 National Center for Atmospheric Research, Boulder, Colorado, USA.
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Abstract

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. 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 ≈ 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 may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, pre-existing equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear vz off of the equator. Parameterized 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.

Corresponding author address: Deepak A. Cherian, NCAR, P.O. Box 3000, Boulder, CO 80307-3000. E-mail: deepak@cherian.net

Abstract

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. 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 ≈ 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 may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, pre-existing equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear vz off of the equator. Parameterized 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.

Corresponding author address: Deepak A. Cherian, NCAR, P.O. Box 3000, Boulder, CO 80307-3000. E-mail: deepak@cherian.net
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