Dynamics of Rotating Shallow Gravity Currents Passing through a Channel. Part II: Analysis

The physics of frictional control for channelized rotating gravity currents are analyzed using an extensive dataset including hydrographic, current, and microstructure measurements from the western Baltic Sea. Rotational effects in these gravity currents, characterized by Ekman numbers of the order of one and subcritical Froude numbers, induce a complex transverse circulation that strongly affects the internal dynamics. The key component of this circulation is a geostrophically balanced transverse jet in the interface that modifies the entrainment process by (i) laterally draining the interface and (ii) providing additional interfacial shear comparable to the down-channel shear. The recirculation of mixed interfacial fluid into the interior distorts the internal density structure of the gravity current, and creates a thermal wind shear in the interior that is comparable to the observed shear. Using a theoretical model, this effect is shown to be responsible for the three-layer structure of the transverse velocity with the near-bottom velocity and stress directed opposite to the Ekman transport. The analysis confirms the key assumption in available models for frictional control in rotating gravity currents: the transverse Ekman transport is balanced by the geostrophic transport due to the down-channel tilt of the interface.