Sack Lunch Seminar (SLS)

During the daytime, under conditions of relatively low winds and high solar insolation, the nearsurface ocean warms. This stratified diurnal warm layer traps momentum from the wind near the surface, generating shear across the layer. Horizontal advection associated with this diurnal warm layer shear is likely responsible for observed nearsurface freshening in the upper meter of the ocean during the SPURSI field campaign.

This shear associated with the diurnal warm layer, a fractional depth of the deeper mixed
layer, horizontally displaces water near the surface during the daytime converting horizontal gradients into vertical ones. At night, convection vertically mixes the horizontally displaced water throughout the deeper mixed layer. This cycle of advection and then vertical mixing is a mechanism for effective submesoscale lateral diffusion on the scale of 110 km. We will discuss a simplified representation of the mechanism, and estimates of effective submesoscale horizontal diffusivity for the mixed layer. The calculated effective diffusivity associated with this mechanism is 1100 m2/s, depending on the advective timescale, and depth and speed of the diurnal warm layer current.

The importance of the daily cycle as a mechanism for smoothing lateral inhomogeneities within the mixed layer will be considered. Additionally, we examine how the advection associated with the diurnal warm layer may locally enhance or suppress the diurnal warming observed at a location. The frequency of occurrence of diurnal warm layers and global perspectives will be discussed.