The equatorial oceans not only dominate global heat uptake from the atmosphere but spawn global scale weather and climate phenomena like the El Nino Southern Oscillation and the Madden-Julian Oscillation. Mean flow structures range from energetic, highly-sheared currents in the eastern cold tongues of Atlantic and Pacific forced by steady trade winds to intermittent thin layers of high shear in the western Pacific and Indian Oceans where westerly wind bursts sporadically force subsurface turbulence. In boreal fall, tropical instability waves add a layer of complexity that includes not only additional wave shear but advection and mixing through frontogenesis. One distinction between midlatitude mixed layers and the upper waters of the cold tongues is the penetration of diurnally-forced mixing beneath the mixed layer. This is due to the balance between current shear, forced by the trade winds, and stratification maintained by the combination of surface heating and equatorial upwelling which acts to maintain near-critical gradient Richardson number.
The figure below shows a diurnal composite computed from the shipboard profiling measurements made at 0, 140W during the period 25 Oct 2008 to 02 Nov 2008. The abscissa is the local time, which lags UTC by nine hours. The solid white line shows the evening descent of the near-surface sheared layer into the stratified fluid below the upper mixed layer, a unique feature of upper equatorial ocean dynamics.
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Feedback of mixing to ENSO phase change, Geophys. Res. Lett., 46, 13920– 13927, doi:https://doi.org/10.1029/2019GL085415, 2019 (S.J. Warner and J.N. Moum). AGU Editor’s Highlight.