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

The causes of the seasonal cycle of vertical turbulent cooling at the base of the mixed layer are assessed using observations from moored buoys in the tropical Atlantic ITCZ (4N, 23W) and trade wind (15N, 38W) regions together with one-dimensional models. At 4N, 23W the modeled turbulent cooling agrees with indirect estimates from the mooring heat budget residual: both show maximum cooling of 30-40 W/m^2 during boreal winter and spring and a minimum of 0-10 W/m^2 during summer and fall. Stronger cooling in winter and spring is found to be associated with weaker mean winds, a thinner mixed layer, and stronger shear-induced mixing at the base of the mixed layer. The variations in shear and mixing are driven by phenomena on a wide range of timescales, including internal waves, near-inertial waves, and tropical instability waves. All appear to contribute significantly to turbulent mixing and cooling of the mixed layer. At 15N, 38W the seasonal cycle of turbulent cooling is out of phase compared to 4N, 23W, with largest cooling during boreal fall. However, the relationships between wind speed, mixed layer depth, and turbulent mixing are similar: weaker mean winds and a thinner mixed layer in the fall are associated with stronger current shear and mixing. These results suggest that a significant portion of the seasonal cycle of the turbulent cooling at these locations may not be driven directly by local wind or surface buoyancy forcing but indirectly through their modulation of mixed layer depth, upper-ocean stratification, and shear.