Variations in Wave Slope and Momentum Flux From Wave-Current Interactions in the Tropical Trade Winds

Abstract

Observations from six Lagrangian Surface Wave Instrument Float with Tracking drifters in January-February 2020 in the northwestern tropical Atlantic during the Atlantic Tradewind Ocean-atmosphere Mesoscale Interaction Campaign are used to evaluate the influence of wave-current interactions on wave slope and momentum flux. At observed wind speeds of 4–12 ms−1, wave mean square slopes are positively correlated with wind speed. Wave-relative surface currents varied significantly, from opposing the wave direction at 0.24 ms−1 to following the waves at 0.47 ms−1. Wave slopes are 5%–10% higher when surface currents oppose the waves compared to when currents strongly follow the waves, consistent with a conservation of wave energy flux across gradients in currents. Assuming an equilibrium frequency range in the wave spectrum, wave slope is proportional to wind friction velocity and momentum flux. The observed variation in wave slope equates to a 10%–20% variation in momentum flux over the range of observed wind speeds (4–12 ms−1), with larger variations at higher winds. At wind speeds over 8 ms−1, momentum flux varies by at least 6% more than the variation expected from current-relative winds alone, and suggests that wave-current interactions can generate significant spatial and temporal variability in momentum fluxes in this region of prevailing trade winds. Results and data from this study motivate the continued development of fully coupled atmosphere-ocean-wave models.