OBSERVATION OF A DEEP CONVECTION REGIME WITH ACOUSTIC TOMOGRAPHY

In the winter of 1991-1992 a convection experiment was conducted in the western Mediterranean Gulf of Lions, combining a variety of observational techniques. An essential component was an acoustic tomography array, consisting of six moorings, designed to observe the time evolution of the large-scale processes believed to be relevant in a convection regime. Here two-dimensional slice inversions in three directions from the central mooring are used to estimate the volume of convected water and the mean convection depth and to observe the preconditioning and restratification processes before and after the convection. The near-surface layer is well sampled by the acoustics, which show cooling and subsequent entrainment of the warmer Levantine Intermediate Water (LIW) from below, in agreement with mixed-layer calculations. During the 2 months prior to the main convection event the total heat loss of the large-scale field is in approximate agreement with the surface heat fluxes, showing that little net warm-water advection takes place from outside the convection region. The implied confinement of water by the local circulation should be an important factor in setting the location and extent of the deep convection patch. The volume of water modified by convection in this winter is estimated to correspond to an area of order 60 km radius and 1500 m depth. It is argued that the homogenized area implies an annual mean deep water replenishment of 0.3 Sv. The restratification, by the return of less dense water in the surface and LIW layers, occurs first by rapid capping in the near-surface region. Thereafter, the southern and eastern parts of the region restratify in the deeper layers on a 40-day timescale, while a dense core of 50-60 km diameter remains in the northwest until the end of the experiment.