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Webinar: Vertical turbulent cooling of the mixed layer in the tropical Atlantic ITCZ and trade wind regions

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Thursday, 08 November 2018, 2:00

Thursday, November 8, 2018. 2:00PM. Webinar: Vertical turbulent cooling of the mixed layer in the tropical Atlantic ITCZ and trade wind regions. Greg Foltz, NOAA. Sponsored by NOAA. More information here.


The processes responsible for vertical turbulent cooling of sea surface temperature (SST) and its seasonal modulation are not well known and potentially contribute to coupled model biases. In this study, the seasonal cycles of vertical turbulent cooling at the base of the mixed layer are examined using observations from moorings at 4N, 23W and 15N, 38W, together with one-dimensional mixing models. The 15N, 38W mooring samples the trade wind region, which experiences a strong annual cycle of SST and weak seasonal variations of wind speed. The 4N, 23W mooring is located in a region with much weaker seasonal variability of SST and a stronger seasonal cycle of winds. At 4N, 23W the modeled turbulent cooling agrees with indirect estimates from the mooring heat budget residual. Both show a maximum in cooling of 25-45 W/m^2 during boreal winter and spring and a minimum of 0-15 W/m^2 during summer and fall. The maximum in winter and spring is found to be driven by strong high-frequency variations of current shear (period < 1 day) that are likely generated by tides and other remotely forced internal waves. In summer and fall the mixed layer and thermocline are deeper and current shear is much weaker, leading to reduced turbulent cooling of SST. At 15N, 38W the seasonal cycle of turbulent cooling is out of phase with that at 4N, 23W, with largest cooling during boreal fall. However, the mechanism appears to be similar: a thinner mixed layer in the fall leads to stronger current shear and mixing. These results suggest that most of the turbulent cooling at these locations is driven not by local wind or surface buoyancy forcing but by a complex mix of remotely-generated internal waves.

 

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