Enhanced turbulence associated with the diurnal jet in the ocean surface boundary layer

Under conditions of relatively low winds and high solar insolation, a strong diurnal warming signal is expected, which increases stratification near the surface. Although this has been shown to repress turbulent dissipation below the diurnal warming signal, little is known about what occurs in the shallow newly formed boundary layer. Presented are microstructure observations and high resolution ADCP measurements near the ocean surface in the subtropical Atlantic during several diurnal warning events. Microstructure observations are obtained with the Air-Sea Interaction Profiler (ASIP), an autonomous upwardly rising profiler designed to observe the ocean surface boundary layer (OSBL). The turbulent dissipation rate in the upper few metres is observed to increase during daytime restratification. This enhanced turbulence coincides with the presence of a diurnal jet, which forms during the day due to the wind stress input being restricted to the shallow reformed OSBL. The high shear layer is associated with the active mixing layer depth (XLD), which reforms in the early morning about 3 hours before a density based mixed layer depth (MLD), causing the generation of shear instabilities. The diurnal jet is simulated using a damped slab model with observed values of the wind stress input as well as the time varying OSBL depth. The model shows the diurnal jet to generate shear instabilities for relatively small wind speeds with these instabilities occurring within a couple of hours after OSBL reformation.