Air-sea Interactions: the winter convection

 

Strong northern winds blowing over the Mediterranean Sea are involved in deep ocean convection events in keys areas (Figure 1a). In particular, the winds above the Alps and Pyrenees, the Mistral and Tramontane, help the generation of a cyclonic circulation, called gyre, centred at 42°N-5°E in the Gulf of Lions. In autumn, high heat losses, mainly due to evaporation linked to wind enhancement, are present over the sea and break the stratification of the mixed layer in the surface layer. The energy injected allows a homogeneous layer in the first 100 to 200 meters depth. Moreover, the cyclonic circulation in the gyre is enhanced by the winds, causing an isolation of the water gyre compared to the adjacent currents. Then the mixing is less intense before a second violent mixing phase occuring during the winter. This phase is maximum when the cumulated heat losses in the gyre are reached. At this time, during a period that usualy ranges from two weeks and one month and a half, either by intermittance or during one strong event, the deep convection reaches the bottom sea in the north-western Mediterranean (Figure 1b). Dense waters formed, called Westerrn Mediterranean Deep Water (WMDW) contribute to the thermohaline circulation of the Mediterranean Sea, the slow branch of the water cycle.


The FGO team studies the thermohaline circulation of the Mediterranean Sea in present climate. We are interested in understanding the air-sea interaction processes involved in the winter convection in order to reproduce coherently the water mass characteristics and their spreading into the basin. In particular, the FGO team has highlighted the key role of the wind position over sea in the Gulf of Lions.

 

Figure 1 : Model results (a) Map of the mixed layer depth maximum reached during the period 1998 à 2006 (b) Vertical north-south section of salinity at 4.7°E showing the winter convection et 42°N with a strong vertical mixing reaching the sea bottom near 2000 m depth at the beginning of March 2005.