Mesoscale oceanic convergence and divergence: Quasi-geostrophic theory, analytical modeling and observations

Abstract

[eng] Mesoscale oceanic features, like fronts, meanders, eddies, gyres, are characterized by temporal and spatial scales that range from a few days to several months and from a few meters to 100 km. Vertical motions associated with mesoscale motions play a key role in the ocean circulation and ocean-atmosphere interaction; supplying nutrients, conveying heat, salinity and momentum fluxes. However, the measurement of these vertical velocities represents a challenge, since they are three to four orders of magnitude smaller than the horizontal velocities. Traditional observing systems lack accuracy and resolution to capture the small scale signal of these vertical fields. Vertical motions are related to the horizontal convergence and divergence as a consequence of the continuity equation. In this master’s thesis we study the convergence/divergence of a mesoscale eddy using two different approaches: an analytical model and Lagrangian observations. We implement and evaluate a code based on an analytical model of baroclinic instability to generate fields of convergence/divergence. The code is written in Python and is made freely available through github. The model consists in a two-layer region where the upper layer has a constant shear flow and the bottom layer has no motion. It assumes quasi-geostrophic equilibrium and recreates the baroclinic instabilities that can be found in a region such as the Algerian Basin. The Lagrangian observations used here were obtained by a set of drifters deployed at the south of Almeria (Western Mediterranean); which eventually got caught inside an eddy formed by a baroclinic instability of the Algerian Current. By studying the rate of change of the area of a parcel formed by a set of drifters, a calculation of the horizontal convergence/divergence is performed. In order to compare both methods, we simulate an eddy with the properties of an eddy located at the Algerian Basin such as the sampled by the drifters. To obtain its hydrographical characteristics we use data from a glider, an autonomous underwater vehicle that sampled the same eddy. In addition, to corroborate the origin of the eddy as an instability of the Algerian Current, we use satellite altimetry data to track its origin. Both results are analyzed and a discussion between the different methods and its value to the calculation of convergence/divergence zones is provided. Ultimately, we estimate vertical velocities from the model and compare with those obtained from the glider data set using the omega equation within the Quasi-geostrophic approximation.