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

• Main Author: Bogdanoff, Alec S.
• Format: Thesis
• Language: English
• Published: Massachusetts Institute of Technology and Woods Hole Oceanographic Institution 2017
• Subjects: SPURS: Salinity Processes in the Upper Ocean Regional Study; Ocean circulation; Ocean waves; Ocean currents; Diffusion; Knorr (Ship : 1970-) Cruise KN209; Endeavor (Ship: 1976-) Cruise EN522; North Atlantic
• Online Access: https://hdl.handle.net/1912/8524

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2017 The daily heating of the ocean by the sun can create a stably stratified near-surface layer when the winds are slight and solar insolation is strong. This type of shallow stable layer is called a Diurnal Warm Layer (DWL). This thesis examines the physics and dynamics of DWLs from observations of the subtropical North Atlantic Ocean associated with the Salinity Processes in the Upper ocean Regional Study (SPURS-I). Momentum transferred from the atmosphere to the ocean through wind stress becomes trapped within the DWL, generating shear across the layer. During SPURS-I, strong diurnal shear across the DWL was coincident with enhanced turbulent kinetic energy (TKE) dissipation (????, ???? > 10−5 W/kg) observed from glider microstructure profiles of the near-surface. However, a scale analysis demonstrated that surface forcing, including diurnal shear, could not be the sole mechanism for the enhanced TKE dissipation. High-frequency internal waves (???? ≫ ????) were observed in the upper ocean during the daytime within the DWL. 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. As the strength of the stratification of the DWL increases, so does the shear caused by the tunneling internal waves. 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. 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. Therefore, when a DWL is present, the water within DWL is horizontally transported a different distance than the water below. Coupled with ...