A model of a Mediterranean salt lens in external shear
MetadataShow full item record
LocationEastern North Atlantic
A pair of simple models representing the interaction of a continuously stratified f-plane quasigeostrophic lens with a uniform external shear flow is examined. The study is motivated by the desire to understand the processes that affect Mediterranean Salt Lenses and other mesoscale lenses in the ocean. The first model represents the eddy as a pair of quasigeostrophic 'point potential vortices' in uniform external shear, where the two point vortices are imagined to represent the top and bottom of a baroclinic eddy. While highly idealized, the model succeeds in qualitatively reproducing many aspects of the behavior of more complex models. In the second model the eddy is represented by an isolated three dimensional patch characterized by quasigeostrophic potential vorticity linear in z, in a background flow with constant potential vorticity. The boundary of the lens may be deformed by interactions with a uniform background shear. A family of linearized analytical solutions representing such a vortex is discussed in Chapter 3. These solutions represent lens-like eddies with trapped fluid cores, which may propagate through the surrounding water when there is external vertical shear. The analysis predicts the possible forms of the boundary deformation in a specified external flow, and the precession rate of normal mode boundary perturbations in the absence of external flow. The translation speed of the lens with respect to the surrounding fluid is found to be a simple function of the external vertical shear and the core baroclinicity. A numerical algorithm which is a generalization of the contour dynamics technique to stratified quasigeostrophic flow is used to extend the linear results into the nonlinear regime. This numerical analysis allows a determination of the range of environmental conditions (e.g., the maximum shear and/ or core baroclinicity) in which coherent vortex solutions can be found, and allows the stability of the steadily translating solutions to be examined directly. It is found that the solutions are stable if neither the external shear nor the core baroclinicity is too large, and that the breakdown of the unstable solutions is characterized by the loss of an extrusion of core fluid to the surrounding waters. The translation speeds of the large amplitude numerical solutions are found to have the same functional dependence on the external vertical shear and the core baroclinicity that was found in the linear analysis, and it is demonstrated that the solutions translate at a rate which is equal to the background flow speed at the center of potential vorticity of the lens. As a test of the model results, new data from a recent SOFAR float experiment are presented and compared with the model predictions. The data show that the cores of two different Mediterranean Salt Lenses are tilted, presumably as a result of interactions with external flows. Both the sense of the tilt and its relation to the translation of the lens are in qualitative agreement with the model solutions.
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 April 1992
Suggested CitationThesis: Walsh, David, "A model of a Mediterranean salt lens in external shear", 1992-04, DOI:10.1575/1912/5488, https://hdl.handle.net/1912/5488
Showing items related by title, author, creator and subject.
Shyu, Esther (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2015-09)Males and females may differ in stage-specific survival, maturation, fertility, or mating availability. These demographic differences, in turn, affect population growth rates, equilibrium structure, and evolutionary ...
MEMS IMU navigation with model based dead-reckoning and one-way-travel-time acoustic range measurements for autonomous underwater vehicles Kepper, James (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-09)Recent advances in acoustic navigation methodologies are enabling the way for AUVs to extend their submerged mission time and maintain a bounded XY position error. Additionally, advances in inertial sensor technology ...
French, Katherine L. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2015-02)Tracing the evolution of Earth’s redox history is one of the great challenges of geobiology and geochemistry. The accumulation of photosynthetically derived oxygen transformed the redox state of Earth’s surface environments, ...