Circulation and Stirring by Ocean Turbulence
Balwada, Dhruv (author)
Speer, Kevin G. (Kevin George) (professor directing dissertation)
Landing, William M. (university representative)
Dewar, William K. (committee member)
Sura, Phillip (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Program in Geophysical Fluid Dynamics (degree granting department)
Ocean turbulence is responsible for stirring and spreading ocean tracers, and contributes to the mean circulation as eddy bolus fluxes. The influence of the eddies on the mean circulation becomes particularly important in regions where mean geostrophic flows are weak, such as the meridional flow across the Antarctic Circumpolar Current. However, high resolution observations of eddies and their influence on the circulation are generally lacking, particularly in the deep ocean that cannot be observed via satellites. The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was designed to observe the transport and stirring associated with the eddies in the Southern Ocean, using RAFOS floats and a passive tracer. In the first half of the thesis, the imprint of the eddies on the large scale circulation ([greater than] 100 km) is assumed to be diffusive, and the corresponding eddy diffusivities are quantified using the long term behavior of the RAFOS floats. The eddy diffusivities are found to be suppressed in the presence of mean flows. These eddy diffusivity estimates from DIMES, along with estimates from a couple of other diffusivity studies, are then used to quantify the eddy bolus fluxes in the Southern Ocean, which were found to vary in response to the bottom bathymetry. The second part of this work, addresses the flow at the length scales of the submesoscale and mesoscale ([less than] 100 km). Here, in addition to the DIMES RAFOS floats, we also used surface drifter observations from an experiment, Grand Lagrangian Deployment (GLAD), conducted in the Gulf of Mexico. The goal was to observe the kinematic stirring properties at these smaller scales, and also to characterize the dynamics of the turbulence that is active by investigating the energy spectrum. At the surface ocean in the Gulf of Mexico, we characterized the scale dependent energy distribution over 5 orders of length scales (10 m - 1000 km) using second order velocity structure functions. Divergent motions were found to be dominant, over non-divergent motions, at length scales smaller than 5km, where the Rossby number was greater than one and the third order velocity structure functions indicated the presence of a forward energy cascade. These methods were also used to explain subsurface turbulence in the Southern Ocean with DIMES RAFOS floats. The RAFOS floats showed that divergent flows are also present in the deep ocean at length scales smaller than 30 km, and become comparable in magnitude to the non-divergent flows near 5 km. The observed dispersion of the floats was used to address the question - is the mixing at small scales due mainly to large scale shear (non-local) or small scale eddies (local)? The associated stirring was found to be local at depth.
Eddy diffusivity, Lagrangian Observations, Meridional overturning circulation, Ocean Turbulence, Relative Dispersion, Structure Functions
November 10, 2016.
A Dissertation submitted to the Geophysical Fluid Dynamics Institute in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Kevin Speer, Professor Directing Dissertation; William Landing, University Representative; William Dewar, Committee Member; Phillip Sura, Committee Member.
Florida State University
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