Direct Measurements of Upper Ocean Horizontal Velocity and Vertical Shear in the Tropical North Atlantic at 4°Ν, 23°W

The Tropical Atlantic Current Observations Study measured upper ocean horizontal velocity from a mooring at 4 degrees N, 23 degrees W, at discrete depths between 7 and 87 m, in order to observe the temporal and vertical structure of the currents. Between March 2017 and March 2018, mean zonal velocity and vertical shear were strongest between 32 and 37 m. Near-surface mean eastward currents during this period were weaker than the long-term mean but within the range of previously observed values given the high interannual variability. Interannual variability of the zonal velocity exceeded that of the meridional velocity, was primarily geostrophically driven, and was strongly influenced by the large-scale currents. Energetic tropical instability waves (TIWs) were observed in early summer and late fall of 2017. Meridional velocity fluctuations associated with the TIWs were far larger than those of zonal velocity and extended down to 87 m. These fluctuations propagated upward to the surface with vertical phase speeds between 12 and 15 m/day. Coherent velocity and vertical shear variations emerged in a TIW composite. However, the vertical shears observed during the TIW season were modest compared to the large shear measured in spring 2017 and winter 2018. As a result, TIWs may be less crucial for vertical turbulent cooling at 4 degrees N, 23 degrees W than they are near the equator. To observe the intense winter/spring vertical shear events, fine vertical (less than 10 m) and diurnal temporal sampling is needed. Plain Language Summary Changes in upper ocean currents influence temperature, salinity, and air-sea fluxes in the tropical North Atlantic, which in turn affect the weather, climate, and fisheries of the surrounding continents. The Tropical Atlantic Current Observations Study measured upper ocean currents and vertical shear of velocity for the first time at a moored buoy at 4 degrees N, 23 degrees W. These data yielded several new insights: The average zonal currents and vertical shear are strongest at a depth of 32 to 37 m below the surface. Both the average east-west and north-south currents are fairly weak relative to strong high-frequency fluctuations in the north-south currents associated with tropical instability waves (TIWs). However, vertical shears observed during the TIW season were modest compared to large shear measured in spring 2017 and winter 2018. This suggests that TIWs at 4 degrees N, 23 degrees W may be less crucial for vertical turbulent cooling and upper ocean temperature budgets than they are near the equator. Given the high year-to-year variability in zonal velocity at this location, partly driven by changes in the large-scale background currents, multiple years of fine vertical scale (less than 10-m spacing) and high-frequency (subdiurnal) velocity and shear data are needed to evaluate the robustness of these findings.