Three-layered circulation in reverse estuaries

Driven by an upper-layer density deficit and lower-layer density surplus from an adjacent sea, circulation in an estuarine embayment may be three-layered, with top and bottom inflows separated by a mid-layer outflow. The characteristics of this type of circulation as functions of density forcing, background stratification and lateral depth variation in an embayment are examined using a three-dimensional primitive-equation model with a free surface. Among all the details, the dominance of bottom intrusion over the surface one stands out and presents a departure from early models that have missed the importance of bottom intrusion in regulating the three-layered layered circulation. A strong or deeper upper density deficit from the adjacent sea, bottom stress and narrowing estuary width at depths all produce transients discouraging the development of the bottom intrusion. In time, the bottom inflow invariably gains strength and thickness and squeezes up the core of the mid-layer outflow to above the mid-depth. A heuristic argument follows to explain this counter-intuitive result.