APPLICATION OF THE STOMMEL MODEL TO SHALLOW MEDITERRANEAN ESTUARIES AND THEIR CHARACTERIZATION

During summer, the basins of shallow estuaries in mediterranean climat es are usually vertically mixed, hypersaline, and sometimes inverse (i n the sense of their densities being elevated above ocean values). The se estuaries are commonly controlled by heat and salt exchange with th e ocean. Because of the compensating effect of estuarine heating, the magnitude of the density inversion (due to hypersalinity) depends on t he net surface buoyancy flux. The surface heat and evaporative fluxes are functions of the estuarine temperature and so the estuarine state (and its associated surface buoyancy flux) depends on the strength of the ocean exchange processes. A simple Stommel model of a summer medit erranean estuary is presented here in which this exchange is controlle d by density-driven processes. The Stommel model has two possible stab le stationary states and it is shown that the estuary can be character ized by four types of steady state. They include the familiar classica l and inverse estuaries plus a solution, called ''quasi-neutral'', in which the estuary is close to neutrality but just marginally inverse; that is, the net buoyancy flux is small and negative. This quasi-neutr al solution resembles;the steady state in Tomales Bay (California) in late summer and shows the characteristic high level of thermal compens ation of the density inversion from hypersalinity. It also has the obs erved very long flushing time which is responsible for some of the env ironmental fragility of this class of estuary. This appears to be the first detailed confirmation of the applicability of the Stommel model to estuaries. The fourth type of estuarine state arises from the simul taneous existence of both stable states. It is here called ''intermitt ent'' and contains both the classical and a quasi-neutral Stommel solu tion. The type of estuarine state achieved by a narrow basin in autumn is highly dependent on its length and depth. This produces extreme Se nsitivity to changes in basin geometry from human influences.