Violent storms within the Sea: dense water formation episodes in the NW Mediterranean

摘要

Heat and water exchanges in the ocean occur almost exclusively at thesurface. As water compressibility is very low, stratification of the fluidis expected and horizontal motion is predominant in the sea interior. Amongthe few processes that may introduce a vertical component in the watermotion are those that increase surface water density by freezing, cooling orevaporation. Those processes triggering convective motion are enhanced bycold surface air, dry wind and low solar radiation. Therefore, convectivecells are more likely to occur when the temperature of the air at the seasurface is lower than sea surface temperature. Conversely, rain, riverrunoff, solar heating, calm and condensation at surface enhancestratification. Convective motion at sea has several scales ranging from fewmeters at the upper ocean, causing the surface mixed layer, to the entirewater column, in what is known as deep convection. Only few places in theworld ocean are suitable for deep convection, and only under particularweather conditions. In this paper, a brief review of the response to theseparticular conditions in the NW Mediterranean is presented in what is knownas dense water formation. The violent sinking and spreading of water parcelsthat reach the deep sea floor in few hours is described. These are"hidden" mediterranean storms, occurring under the sea surface, "on theother side of the mirror".