Variations in wave slope and momentum flux from wave-current
interactions in the tropical trade winds
Observations from six Lagrangian Surface Wave Instrument Float with
Tracking (SWIFT) drifters in January-February 2020 in the northwestern
tropical Atlantic during the Atlantic Tradewind Ocean-atmosphere
Mesoscale Interaction Campaign (ATOMIC) are used to evaluate the
influence of wave-current interactions on wave slope and momentum flux.
At wind speeds of 4-12 m/s, wave mean square slopes are positively
correlated with wind speed. Wave-relative surface currents varied
significantly, from opposing the wave direction at 0.16 m/s to following
the waves at 0.57 m/s. For a given wind speed, wave slopes are up to
20% higher when surface currents oppose the waves compared to when
currents strongly follow the waves, consistent with a theoretical
Doppler shift between the absolute (fixed) and intrinsic (relative)
frequency. 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 up to a 40% variation
in momentum flux for a given wind speed. This is 30% greater 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.