Deep water formation in the western Mediterranean

Abstract

Wintertime deep convection was studied in the Gulf of Lions (northwestern Mediterranean) using the chlorofiuoromethane (CFM) distributions (components F11 and F12), from December 1991 and February–March 1992. Convection occurred on small spatial and temporal scales and it was not complete: only convection to 1900 m depth was observed. The vertical mixing of the water column above the convection depth turned out to be of major influence regarding the characteristic of newly formed deep water and regarding the boundary conditions for CFMs in modeling deep water formation. Air‐sea gas exchange and entrainment were found to be negligible. These findings are important in estimating deep water formation rates for all water masses renewed by convection (e.g., Greenland Sea Deep Water, Labrador Sea Water) and in evaluating the spreading of deep water masses along the western boundary current. A box model is presented which uses CFM and tritium distributions to estimate formation of Western Mediterranean Deep Water below 1000 m. The model starts with zero tracer concentrations in 1945, the integration step is 1 year, and the deep water formation rate is assumed to be constant from 1945 to 1992. The best fit between CFM model concentrations and measurements were obtained at a mean deep water renewal time of 5–7 years, equivalent to a conversion of 0.46 Sv Modified Atlantic Water (0‐ to 150‐m depth) and 0.76 Sv Levantine Intermediate Water (150‐ to 400‐m depth) into deep water. However, the tritium measurements are only partly matched by the model, by assuming that convection stopped in the years 1964–1968.