Direct measurements of upper ocean horizontal velocity and vertical shear in the tropical North Atlantic

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 Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) consists of eighteen moorings in the tropical Atlantic that have been used for climate research, numerical weather prediction, and ocean forecasting for over two decades. The Tropical Atlantic Current Observations Study (TACOS) measured upper ocean currents and vertical shear of velocity for the first time at the 4°N, 23°W PIRATA mooring from 2017 to 2019. Analysis of the first year of TACOS 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). 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°N, 23°W may be less crucial for vertical mixing and turbulent cooling at the base of the mixed layer than they are near the equator. Analysis of the second and third year of TACOS data confirms the significant interannual variability of zonal velocity in this region, with much stronger near-surface eastward flow in 2018 and 2019 than in 2017. Despite the Atlantic zonal mode being in a positive phase in 2017-2019, very energetic TIWs were also observed in 2019 with up to ±100 cm/sec velocity perturbations and elevated vertical shear. This observed vertical shear exists on fine vertical scales (less than 10 m spacing) at 4°N, 23°W, and a reduced array with coarser vertical sampling would underestimate the shear.