FY 2021

Diurnal cycles of near-surface currents across the tropical Pacific

Masich, J., W.S. Kessler, M.F. Cronin, and K.R. Grissom

J. Geophys. Res., 126(4), e2020JC016982, doi: 10.1029/2020JC016982, View online (open access) (2021)

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.

Plain Language Summary. The daily cycle of daytime heating and nighttime cooling over the tropical ocean can inject heat and wind momentum from the atmosphere into the ocean. Daytime warm layers at the ocean surface can trap wind momentum, and the heat and momentum in these layers can be mixed downward each day. We explore this process at seven different locations across the tropical Pacific, finding that this near‐surface daily heat and momentum mixes far deeper into the ocean near the equator than away from it. We find that the deeper mixing occurs only where strong near‐surface currents make the fluid more “mixable,” so that the wind‐driven jet can mix downwards more easily. We also find that stronger wind is associated with a stronger, deeper jet in these regions.