Vertical mixing and vertical velocities in an anticyclonic eddy south of Gran Canaria Island. A first approach.

Abstract

During February of 2007 an anticyclonic eddy has been localized and cross-measured in two transects; from 6 to 7 and from 26 to 27 of February south of the Gran Canaria Island in the framework of RODA- II project. Hydrographic measurements such as CTD profiles and altimeter derived data from AVISO were used to determine the structure of such mesoscalar structure. In order to obtain a contextual vision of the diapycnal mixing of this eddy, potential density profiles were used to compute Thorpe scales through the displacements of each overturn. Moreover, ADCP measurements were used to establish the critical Richardson gradient number as a parameter to determinate the presence of mixing below the mixed layer in stable stratified fluid. The results show that, the highest level of vertical active mixing is concentrated on the surface layers, over the mixed layer, which can lead to a diapycnal transport between the upper mixed layer and the stratified layer below favoring then the turbulent vertical entrainment. Moreover, high Thorpe length scales are found at the periphery of the first transect which match with critical gradient Richardson numbers below the mixed layer and high vertical shear levels. The second transect has shown higher Thorpe scales at the core of the eddy and critical gradient Richardson numbers were found in some stations that not necessarily match to those enhancement of Thorpe scales. An additional meteorological-related quantity, the Monin-Obukhov length scale, was determined for each cast through surface buoyancy and heat fluxes as well as the friction velocity calculations, using empirical formulations and the bulk flux of TOGA/COARE for surface heat flux and wind stress. Across the eddy we find Monin-Obukhov scales less than zero which could indicate that the wind-generated turbulence can be overwhelmed by an stable stratification. On the other hand, absolutes depths of Monin-Obukhov were lower than mixed layer depths indicating that the wind-induced mixing could not be enough to explain the total turbulence over the mixed layer. In order to determine in a first approach the relevance of vertical mixing in the vertical velocities and the magnitude of this vertical velocities a regional ocean model solution is used which is part of the PUMP project. Although terms of vertical mixing are small compared to other components of vertical velocity, first results indicates a downwelling on the dense side of the periphery and upwelling on the light side at the core of the eddy when a simple vertical velocity scaling is used. This agrees with theoretical and observational studies of mesoscalar structures. In addition, further analysis related to the structure of the vertical velocities inside an anticyclonic eddy using realistic model solutions will also be addressed.