Hydrographic Structure of the Convection Regime in the Gulf of Lions: Winter 1987

Abstract

Deep winter convection in the northwestern Mediterranean (Gulf of Lions) and the subsequent formation of Mediterranean Deep Water were observed using advanced oceanographic instrumentation during a six-week long experiment in 1987. The severe 1987 European winter forced an intense outbreak of the Mistral, a cold, dry wind blowing down the Rhone valley, in mid-January. Surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began. However, several more Mistral events took place during the experiment. During several cruises into the area, CTD and absolute horizontal velocity profiles were measured in the mixed area over the Rhone fan as well as across the southern front of this region; in addition, continuous records of shipboard meteorological and oceanographic parameters (air temperature, surface salinity, etc.) were made. An early-February Mistral apparently did not produce enough surface cooling to reinitiate convection. In contrast, further convection during an intense, mid-February Mistral was observed in both hydrographic and current-meter data. The 0.02°C cooling observed in CTD data at 1950 m, as well as in data from moored temperature sensors at other depths by the end of this storm, is consistent with what would be expected if the estimated surface heat loss were mixed over the 2200 m depth of the water column. Analysis of CTD data shows that the presence of unstably stratified surface layers was correlated with periods of strong surface cooling. An inverse relation appears to exist between the thickness and density excess of these layers. Comparison with surface cooling rates suggests that these layers could be formed in periods as short as one hour. Finally, comparison of our results to historical data suggests that deep water formed in the northwestern Mediterranean has become progressively warmer and saltier over the past several decades.