Physics of Diurnal Warm Layers: Turbulence, Internal Waves, and Lateral Mixing

Alec Bogdanoff, Ph.D., 2017
Tom Farrar, Co-Advisor
Carol Anne Clayson, Co-Advisor

The daily heating of the ocean by the sun can create a stably stratified near-surface layer (Diurnal Warm Layer, DWL) when the winds are slight and solar insolation is strong.

During the Salinity Processes in the Upper ocean Regional Study (SPURS-I), strong shear across the DWL was coincident with enhanced turbulent kinetic energy (TKE) dissipation (𝜖, 𝜖 > 10−5 W/kg) observed from glider microstructure profiles. This thesis examines surface forcing and internal waves as possible source of the observed TKE dissipation. However, surface forcing alone does not appear to explain the observed enhanced TKE dissipation.

Internal waves are able to transfer energy from the deep ocean into the DWL through the unstratified remnant mixed layer, which is the intervening layer between the DWL and seasonal thermocline. The analysis demonstrates that internal waves can generate strong enough shear to cause a shear-induced instability, and are a plausible source of the observed enhanced TKE dissipation.

Additionally, vertically-varying horizontal transport across the upper ocean occurs because a diurnal current exists within the DWL, but not in the unstratified remnant mixed layer below. Coupled with nocturnal convection, this is a mechanism for submesoscale (1-10 km) lateral diffusion across the upper ocean.