OBSERVATION OF A DEEP CONVECTION REGIME WITH ACOUSTIC TOMOGRAPHY

In the winter of 1991-1992 a convection experiment was conducted in th e western Mediterranean Gulf of Lions, combining a variety of observat ional techniques. An essential component was an acoustic tomography ar ray, consisting of six moorings, designed to observe the time evolutio n of the large-scale processes believed to be relevant in a convection regime. Here two-dimensional slice inversions in three directions fro m the central mooring are used to estimate the volume of convected wat er and the mean convection depth and to observe the preconditioning an d restratification processes before and after the convection. The near -surface layer is well sampled by the acoustics, which show cooling an d subsequent entrainment of the warmer Levantine Intermediate Water (L IW) from below, in agreement with mixed-layer calculations. During the 2 months prior to the main convection event the total heat loss of th e 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 locati on and extent of the deep convection patch. The volume of water modifi ed 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 homogeni zed area implies an annual mean deep water replenishment of 0.3 Sv. Th e restratification, by the return of less dense water in the surface a nd LIW layers, occurs first by rapid capping in the near-surface regio n. 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 experimen t.