Slippery Near-Surface Layer of the Ocean Arising Due to Daytime Solar Heating

Abstract

Measurements made in the Equatorial Atlantic during the 35th cruise of the R/V Akademic Vernadsky using a free-rising profiler and drifters revealed a near-surface slippery layer of the ocean arising due to daytime solar heating. The solar heating warms and stabilizes the surface layer of the ocean. This suppresses turbulent exchange and limits the penetration depth of the wind-induced turbulent mixing. The heated near-surface layer is then slipping over the underlying water practically without friction. At daytime warming of 1°C the resistance coefficient in the upper 5-m ocean, Cu = (U*/?Us)2 became smaller by a factor of 25?30 as compared with the case of neutral stratification. The effect of slipping results in forming a daytime near-surface current. At low wind speed the velocity of this current was observed to achieve 19 cm s?1. A simple one-dimensional integral model reproduces the main diurnal variation of the temperature and the current velocity in the near-surface layer of the ocean. For daytime the experimental data suggest the existence of a self-regulating state of the diurnal thermocline, which predicts linear temperature and velocity profiles and an equilibrium value of the bulk Richardson number. This provides simple relations coupling the temperature and velocity differences and the thickness of thermocline. An estimation of the upper velocity limit of the daytime near-surface current is equal to 29 cm s?1.