Analysis of non-linear behaviour of shallow drought estuaries using the Stommel model

Shallow estuarine basins have small ocean exchange rates during periods of drought (when they are vertically mixed). This exchange is primarily due to convective processes. These are forced by a density difference between the basin and ocean produced by the surface buoyancy flux. This flux is depend ent on the estuarine temperature, which is itself affected by the ocean exc hange of heat. This creates a strong interaction between the buoyancy flux, and the density difference, and may be represented by a Stommel (H. Stomme l, Tellus. 13 (2) (1961) 224-230) non-linear estuarine model. It involves a meteorological constant Gamma, and a parameter epsilon, which measures the strength of the (buoyancy forced) convective exchange processes. The model has two stable states, or attractors, one of which corresponds to a classi cal, and the other to an inverse estuary. The analysis considers the range of (epsilon, Gamma(2)) under which both of these steady states exist and in vestigates their comparative attractions. On seasonal time scales, the estu ary may be in a transitory state (described here by manifold theory) in whi ch the estuary may be 'positive' or 'negative' according to the sign of the buoyancy flux. For typical meteorological conditions, four categories of t ransitory estuary emerge which are dependent on the magnitude of epsilon. ( C) 1999 Elsevier Science Inc. AU rights reserved.