LAGRANGIAN OBSERVATIONS OF MEDDY FORMATION DURING A MEDITERRANEAN UNDERCURRENT SEEDING EXPERIMENT

Mediterranean eddies (meddles) play an important role in maintaining t he temperature and salinity distributions in the North Atlantic, but r elatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field progr am, called A Mediterranean Undercurrent Seeding Experiment, has been c arried out to directly observe meddy formation and the spreading pathw ays of Mediterranean Water into the North Atlantic. Between May 1993 a nd March 1994, 49 RAFOS floats were deployed sequentially in the Medit erranean Undercurrent south of Portugal and cracked acoustically for u p to 11 months. The float deployments were accompanied by high-resolut ion XBT sections across the undercurrent, Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope . Meddy formation thus occurs where the continental slope turns sharpl y to the right (when facing in the downstream direction of the undercu rrent). After conditionally sampling the float dataset to identify flo ats that were well seeded in the undercurrent, the authors have estima ted a meddy formation rate of 15-20 meddles per year. The timescale fo r meddy formation at Cape St. Vincent was found to be 3-7 days, shorte r than previous estimates based on the volume of larger meddles. Meddl es were observed to form most frequently when the speed of the Mediter ranean Undercurrent was relatively fast. The meddy formation process a t Cape St. Vincent resembles the conceptual model of E. A. D'Asaro, wh ereby anticyclonically rotating eddies are formed by separation of a f rictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shea r zone of the undercurrent and that of the newly formed meddles shows that much of the anticyclonic relative vorticity in meddles can be acc ounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generatio n of submesoscale coherent vortices, by collapse and geostrophic adjus tment of a weakly stratified fluid injected into a stratified ocean, m ay not be the principle mechanism at work in the formation of meddles at Cape St. Vincent.