Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing
Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing
Date
2017-02
Authors
Bogdanoff, Alec S.
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DOI
10.1575/1912/8524
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Keywords
SPURS: Salinity Processes in the Upper Ocean Regional Study
Ocean circulation
Ocean waves
Ocean currents
Diffusion
Ocean circulation
Ocean waves
Ocean currents
Diffusion
Abstract
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
(𝜖, 𝜖 > 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 (𝜔 ≫ 𝑓) 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 nocturnal convection that mixes
the 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 diffusion
coefficient are similar in magnitude to current estimates of submesoscale diffusion based on
observations, and are likely an important source of horizontal diffusion in the upper ocean.
Description
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
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Citation
Bogdanoff, A. S. (2017). Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/8524
Cruise ID
EN522
Cruise DOI
https://doi.org/10.7284/900990
Vessel Name
Endeavor