Diurnal Cycles of Near-Surface Currents Across the Tropical Pacific

Abstract

Solar radiative heat and wind-forced momentum can be trapped at the surface and transmitted into the ocean interior via a daily descending shear layer that mixes heat and momentum downwards. Here, we characterize the extent of this mechanism across the tropical Pacific via seven mooring deployments spanning the eastern and western tropical Pacific and the Inter-Tropical and South Pacific Convergence Zones. We find a wide range of diurnal cycles in temperature and velocity, from very weak cycles trapped in the upper 10 m to strong cycles that reach as deep as 60 m. These deeper diurnal cycles appear where the strong background shear above the equatorial undercurrent in the eastern equatorial Pacific helps destabilize the fluid, setting up a persistent state of marginal instability. At the sites located in these marginally unstable regions, we find a linear relationship between wind speed and the depth and strength of the diurnal cycle in velocity.