Abstracts Category : Other

Physics of diurnal warm layers : turbulence, internalwaves, and lateral mixing

by Alec Setnor Bogdanoff

The daily heating of the ocean by the sun can createa stably stratified near-surface layer when the winds are slightand solar insolation is strong. This type of shallow stable layeris called a Diurnal Warm Layer (DWL). This thesis examines thephysics and dynamics of DWLs from observations of the subtropicalNorth Atlantic Ocean associated with the Salinity Processes in theUpper ocean Regional Study (SPURS-I). Momentum transferred from theatmosphere to the ocean through wind stress becomes trapped withinthe DWL, generating shear across the layer. During SPURS-I, strongdiurnal shear across the DWL was coincident with enhanced turbulentkinetic energy (TKE) dissipation ([epsilon], [epsilon] > 10W/kg) observed from glider microstructure profiles of thenear-surface. However, a scale analysis demonstrated that surfaceforcing, including diurnal shear, could not be the sole mechanismfor the enhanced TKE dissipation. High-frequency internal waves([omega] >> f) were observed in the upper ocean during thedaytime within the DWL. Internal waves are able to transfer energyfrom the deep ocean into the DWL through the unstratified remnantmixed layer, which is the intervening layer between the DWL andseasonal thermocline. As the strength of the stratification of theDWL increases, so does the shear caused by the tunneling internalwaves. The analysis demonstrates that internal waves can generatestrong enough shear to cause a shear-induced instability, and are aplausible source of the observed enhanced TKE dissipation.Vertically-varying horizontal transport across the upper oceanoccurs because a diurnal current exists within the DWL, but not inthe unstratified remnant mixed layer below. Therefore, when a DWLis present, the water within DWL is horizontally transported adifferent distance than the water below. Coupled with nocturnalconvection that mixes the DWL with the unstratified layer at night,this cycle is a mechanism for submesoscale (1-10 km) lateraldiffusion across the upper ocean. Estimates of a horizontaldiffusion coefficient are similar in magnitude to current estimatesof submesoscale diffusion based on observations, and are likely animportant source of horizontal diffusion in the upperocean.Advisors/Committee Members: J. Thomas Farrar, Clayson (advisor).