Vertical Turbulent Cooling of the Mixed Layer in the Atlantic ITCZ and Trade Wind Regions

Foltz, G. R., Hummels, R., Dengler, M., Perez, R. C., & de Araujo, M. (2020). Vertical turbulent cooling of the mixed layer in the Atlantic ITCZ and trade wind regions. Journal of Geophysical Research: Oceans, 125, e2019JC015529.


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 Intertropical Convergence Zone (ITCZ) (4N, 23W) and trade wind (15N, 38W) regions together with mixing parameterizations and a one-dimensional model. At 4N the parameterized turbulent cooling rates during 2017–2018 and 2019 agree with indirect estimates from the climatological mooring heat budget residual: both show mean cooling of 25–30Wm-2 during November–July, when winds are weakest and the mixed layer is thinnest, and 0–10Wm-2 during August–October. Mixing during November–July is driven by variability on multiple time scales, including subdiurnal, near-inertial, and intraseasonal. Shear associated with tropical instability waves (TIWs) is found to generate mixing and monthly mean cooling of 15–30Wm-2 during May–July in 2017 and 2019. At 15N the seasonal cycle of turbulent cooling is out of phase compared to 4N, with largest cooling of up to 60Wm-2 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 mixing and turbulent cooling of SST. These results emphasize the importance of seasonal modulations of mixed layer depth at both locations and shear from TIWs at 4N.

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