Abstracts Category : Other

Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing

by Alec S Bogdanoff

The daily heating of the ocean by the sun can create a stably stratified near-surface layerwhen the winds are slight and solar insolation is strong. This type of shallow stable layeris called a Diurnal Warm Layer (DWL). This thesis examines the physics and dynamicsof DWLs from observations of the subtropical North Atlantic Ocean associated with theSalinity Processes in the Upper ocean Regional Study (SPURS-I).Momentum transferred from the atmosphere to the ocean through wind stress becomestrapped within the DWL, generating shear across the layer. During SPURS-I, strong diurnalshear across the DWL was coincident with enhanced turbulent kinetic energy (TKE) dissipation(, > 105 W/kg) observed from glider microstructure profiles of the near-surface.However, a scale analysis demonstrated that surface forcing, including diurnal shear, couldnot be the sole mechanism for the enhanced TKE dissipation.High-frequency internal waves ( ) were observed in the upper ocean during thedaytime within the DWL. Internal waves are able to transfer energy from the deep oceaninto the DWL through the unstratified remnant mixed layer, which is the intervening layerbetween the DWL and seasonal thermocline. As the strength of the stratification of theDWL increases, so does the shear caused by the tunneling internal waves. The analysisdemonstrates that internal waves can generate strong enough shear to cause a shear-inducedinstability, and are a plausible source of the observed enhanced TKE dissipation.Vertically-varying horizontal transport across the upper ocean occurs because a diurnalcurrent exists within the DWL, but not in the unstratified remnant mixed layer below.Therefore, when a DWL is present, the water within DWL is horizontally transported adifferent distance than the water below. Coupled with nocturnal convection that mixesthe DWL with the unstratified layer at night, this cycle is a mechanism for submesoscale(1-10 km) lateral diffusion across the upper ocean. Estimates of a horizontal diffusioncoefficient are similar in magnitude to current estimates of submesoscale diffusion based onobservations, and are likely an important source of horizontal diffusion in the upper ocean.