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 Intertropical Convergence Zone (ITCZ) (4°N, 23°W) and trade wind (15°N, 38°W) regions together with mixing parameterizations and a one‐dimensional model. At 4°N 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 25–30 W m −2 during November–July, when winds are weakest and the mixed layer is thinnest, and 0–10 W m −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 15–30 W m −2 during May–July in 2017 and 2019. At 15°N the seasonal cycle of turbulent cooling is out of phase compared to 4°N, with largest cooling of up to 60 W m −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 4°N. Plain Language Summary: In the tropics, vertical turbulent mixing transports heat from the surface waters to the deeper ocean, regulating sea surface temperature (SST). Turbulent mixing has been shown to cause pronounced cooling of SST in the eastern equatorial Pacific and Atlantic Oceans. This cooling is driven to a large extent by strong vertical shear of the zonal equatorial currents, which give rise to instability and mixing. In this study the seasonal cycles of turbulent cooling are assessed at two off‐equatorial locations in the tropical North Atlantic. Despite the different conditions at these two locations, it is found that turbulent cooling at both is strongest during the time of year when the mean winds are weakest and the mixed layer is thinnest. This is surprising, since it might be expected that stronger mean winds and associated deepening of the mixed layer would directly force much of the mixing and turbulent cooling at off‐equatorial locations. Instead, the shallow mean mixed layer, which is more responsive to surface forcing, sets the stage for stronger bursts of cooling. In addition, intraseasonal variations in shear and mixing from tropical instability waves are important at 4°N. These results will be useful for evaluating the mixing parameterizations used in numerical models. Key Points: The seasonal cycle of vertical turbulent cooling was assessed at 4°N, 23°W and 15°N, 38°WTurbulent cooling of sea surface temperature is strongest during the seasons when mean winds are weakest and the mixed layer is thinnestTropical instability waves contributed 15–30 W m −2 of monthly mean cooling at 4°N, 23°W during May–July 2017 and May–July 2019 [ABSTRACT FROM AUTHOR]