Rain and Sun Create Slippery Layers in Eastern Pacific Fresh Pool

An autonomous Lagrangian float equipped with a high-resolution acoustic Doppler current profiler observed the evolution of upper-ocean stratification and velocity in the Eastern Pacific Fresh Pool for over 100 days in August–November 2016. Although convective mixing homogenized the water column to 40 m depth almost every night, the combination of diurnal warming on clear days and rainfall on cloudy days routinely produced strong stratification in the upper 10 m. Whether due to thermal or freshwater effects, the initial strong stratification was mixed downward and incorporated in the bulk of the mixed layer within a few hours. Stratification cycling was associated with pronounced variability of ocean surface boundary layer turbulence and vertical shear of wind-driven (Ekman) currents. Decoupled from the bulk of the mixed layer by strong stratification, warm and fresh near-surface waters were rapidly accelerated by wind, producing the well-known “slippery layer” effect, and leading to a strong downwind near-surface distortion of the Ekman profile. A case study illustrates the ability of the new generation of Lagrangian floats to measure rapidly evolving temperature, salinity, and velocity, including turbulent and internal wave components. Quantitative interpretation of the results remains a challenge, which can be addressed with high-resolution numerical modeling, given sufficiently accurate air-sea fluxes.