Dense water formation in the Gulf of Lion shelf: Impact of atmospheric interannual variability and climate change
Herrmann, M. ; Estournel, C. ; Déqué, Michel ; Marsaleix, P. ; Sevault, Florence ; Somot, Samuel
Dense water formed over the continental shelf and cascading down the slope is responsible for shelf-slope exchanges in many parts of the world ocean, and transports large amounts of sediment and organic matter into the deep ocean. Here we perform numerical modeling experiments to investigate the impact of atmospheric interannual variability and climate change on dense water formation over the Gulf of Lions shelf, in the Northwestern Mediterranean Sea. Results obtained for a 140 years eddy-permitting simulation (1960-2100) performed over the whole Mediterranean Sea under IPCC A2 scenario forcings are used to force a regional eddy-resolving model of the Northwestern Mediterranean Sea.<br>For the years selected in the present period, the quantity of dense water formed over and exported from the shelf is well correlated with atmospheric conditions, and dense water cascading is in agreement with available observations. During years colder than the average, most of the dense water formed over the shelf sinks into the deep ocean by cascading. During warmer years, dense water is mainly consumed by mixing with lighter surrounding water, and only a small quantity escapes the shelf, flowing along the coast without sinking.<br>For the years selected in the future period, dense water formation over the shelf is strongly reduced, due to the stronger stratification of the water column. Most of the dense water formed is consumed over the shelf by mixing. A very small part escapes the shelf, flowing mainly in the surface layer: cascading practically disappears.<br>The extrapolation of the results obtained for the selected years to the whole present and future periods suggests that volumes of dense water annually formed on the shelf, exported and cascading from the shelf are reduced by, respectively, 50%, 90% and 90% between the 20th century and the end of the 21st century. Uncertainties regarding our results are evaluated: the uncertainty due to the choice of the atmospheric forcing model is the most important, however, a decrease of cascading of at least 60% for the end of the 21st century compared to the present climate is obtained for every atmospheric model examined.</p>
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