A Methodology for Assessing the Postconvection Mesoscale Dynamics in the Gulf of Lion from Composite Datasets

Abstract

In order to access the statistical properties of the mesoscale dynamics in the Western Mediterranean, and its associated transport and heat fluxes during the postconvection period, the authors have applied data combination methods for analyzing a wide range of in situ measurements collected during the Thetis 1 and Convhiv experiments. CTD and XBT profiles were merged with times series at a fixed or moving point and also with integral time series obtained from acoustic tomography data. Estimates of temperature and currents within a box of approximately one degree square, over a time period of 35 days during the postconvection period, were produced. During this winter, convection has been only partial, rarely penetrating deeper than 1200 m. The analysis concentrates on the upper 1000 m, where most changes occur. Geostrophy is used as a dynamical constraint relating the parameters. The time evolution is controlled by a Kalman filter using simple persistence. The contribution of the different datasets to the estimation indicates their complementarity in the time and space dimensions. Hydrography and Eulerian measurements provide a major contribution to the estimation of the baroclinic modes. Tomography data complement the estimate at all horizontal scales. Float data bring some information on the barotropic mode but the major contribution on this mode comes from the reciprocal tomography data, particularly at the largest scales. The period analyzed mostly covers the postconvection. Estimation of the kinetic energy indicates that the barotropic contribution represents 85% of the total energy. Horizontal advection transfers heat toward the central area at a mean rate of 50 W m?2 compensating for the heat losses through the surface. The mesoscale flow field observed is characterized by strongly barotropic coherent vortices with a size O(30?40 km). These barotropic eddies are present during all phases of convection.