Two-layer Exchange in an Estuary Basin, with Special Reference to the Baltic Sea

Abstract

Stationary and transient states of a two-layer fjord-type estuary are discussed analytically. The forcing functions are the outer salinity S0, the fresh-water supply qf, and a meteorologically forced barotropic transport qm. Forced nonlinear, time-dependent cases have been studied numerically. Some associated laboratory experiments are described. The main results obtained are as follows: (i) A single steady state exists; this is approached in an exponential-like way. (ii) The total mixing through the interface must vary with depth (decrease for increasing interface depth) to allow a stable steady state. (iii) The static stability increases with increasing fresh-water supply, up to a critical value where the two-layer model breaks down. (iv) An added oscillatory component in S0 increases and in qf decreases the esturay salinity and the static stability. The effect of an oscillatory qm may go in either direction. (v) The statistical steady state is sensitive to certain high-order statistical features of the forcing functions. It is suggested that changes in such statistical features, rather than changes in mean forcing conditions, may explain observed physical-chemical secular variations in the Baltic, in particular the drop of oxygen concentrations of the deep water.