Submesoscale turbulence in the upper ocean
Citable URI
https://hdl.handle.net/1912/7570DOI
10.1575/1912/7570Abstract
Submesoscale flows, current systems 1–100 km in horizontal extent, are increasingly
coming into focus as an important component of upper-ocean dynamics. A range of processes
have been proposed to energize submesoscale flows, but which process dominates
in reality must be determined observationally. We diagnose from observed flow statistics
that in the thermocline the dynamics in the submesoscale range transition from geostrophic
turbulence at large scales to inertia–gravity waves at small scales, with the transition
scale depending dramatically on geographic location. A similar transition is shown to
occur in the atmosphere, suggesting intriguing similarities between atmospheric and oceanic
dynamics.We furthermore diagnose from upper-ocean observations a seasonal cycle
in submesoscale turbulence: fronts and currents are more energetic in the deep wintertime
mixed layer than in the summertime seasonal thermocline. This seasonal cycle hints
at the importance of baroclinic mixed layer instabilities in energizing submesoscale turbulence
in winter. To better understand this energization, three aspects of the dynamics
of baroclinic mixed layer instabilities are investigated. First, we formulate a quasigeostrophic
model that describes the linear and nonlinear evolution of these instabilities.
The simple model reproduces the observed wintertime distribution of energy across scales
and depth, suggesting it captures the essence of how the submesoscale range is energized
in winter. Second, we investigate how baroclinic instabilities are affected by convection,
which is generated by atmospheric forcing and dominates the mixed layer dynamics at
small scales. It is found that baroclinic instabilities are remarkably resilient to the presence
of convection and develop even when rapid overturns keep the mixed layer unstratified.
Third, we discuss the restratification induced by baroclinic mixed layer instabilities.
We show that the rate of restratification depends on characteristics of the baroclinic eddies
themselves, a dependence not captured by a previously proposed parameterization.
These insights sharpen our understanding of submesoscale dynamics and can help focus
future inquiry into whether and how submesoscale flows influence the ocean’s role in
climate.
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
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Suggested Citation
Thesis: Callies, Joern, "Submesoscale turbulence in the upper ocean", 2016-02, DOI:10.1575/1912/7570, https://hdl.handle.net/1912/7570Related items
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