Diurnal to Subseasonal Air-Sea Interaction from a Mooring in the Atlantic ITCZ

The Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) is a network of 18 buoys spanning the tropical Atlantic Ocean. Since 1997, PIRATA has contributed to improved understanding of air-sea interaction and upper-ocean processes on multiple timescales. In this study, a unique collection of measurements from the 4˚N, 23˚W PIRATA mooring is used to explore interannual changes in the interplay between tropical instability waves (TIWs) and the diurnal cycles of the upper ocean and near-surface atmosphere during boreal summer. The measurements include two years (2017 and 2019) of enhanced upper-ocean velocity measurements from 11 current meters of the Tropical Atlantic Current Observations Study (TACOS). Time series of meridional velocity in the upper 80 m shows the passage of TIWs during both years, with northward and southward velocities reaching ~1 m/s during the 2019 TACOS deployment. The TIWs generate enhanced shear and reduced Richardson number near the base of the mixed layer, suggesting they may be an important source of vertical mixing and associated cooling of the mixed layer. A robust diurnal cycle of near-surface currents is also found during both years, with a maximum in the late afternoon and evening and a minimum in early morning. The diurnal peak in meridional velocity coincides with a diurnal minimum in mixed layer depth and maximum in stratification, suggesting that the velocity maximum results from vertical trapping of wind momentum in a thinner mixed layer. A numerical ocean model is used for comparisons with the hourly mooring data to assess properties of mixing, upper-level shear, and other important air-sea constituents.