Penetration of surface-forced diurnal cycles observed by enhanced TAO moorings across the tropical Pacific

As part of a Tropical Pacific Observing System (TPOS) 2020 pilot project, operational Tropical Ocean Atmosphere (TAO) moorings in nine locations spanning the tropical Pacific have been enhanced to observe the ocean near-surface boundary layer. Enhancements include approximately one-year deployments of upward-looking acoustic Doppler current profilers mounted at 45-65 meters depth; one or two current meters in the upper 25 meters; temperature and salinity sensors at five meter resolution in the mixed layer; and long wave and short wave radiometers for calculation of air-sea heat exchanges. Real-time telemetry of this data has allowed us to begin analysis of these observations while the instruments are still in the water.

We find a surprising diversity of diurnal cycles in upper ocean heat and velocity across the enhanced sites, ranging from cycles that penetrate as deep as 65 meters to those that are confined to the upper 10-15 meters. We explore possible controls on these penetration depths, including wind forcing, insolation, and background ocean dynamics like Kelvin and tropical instability waves. In general, we find that deeper penetration of surface-forced heat and velocity occurs in locations where there is strong insolation and strong subsurface background shear. This background shear creates a marginally unstable flow, so that additional shear associated with the near-surface afternoon warm layer can trigger deeply-penetrating diurnal mixing.