The Alternative Density Structures of Cold/Saltwater Pools on a Sloping Bottom: The Role of Friction

Observed density sections through dense-water pools or lenses on sloping topography typically have an asymmetric structure. One side of the dense lens usually is bounded by isopycnals that slope steeply down to the seabed while, on the other side, the slope of isopycnals is more gentle. A common situation is for the steepest-sloping isopycnals to be on the upslope side of a lens (which is termed the “head-up” state), but the reverse is occasionally true (which is called the “head-down” state). Here a 1½-layer reduced-gravity model, which resolves the bottom boundary layer to provide physical insight into the three-dimensional evolution of these alternative forms, is used. It is found that the head-up state arises when the thickness of the central core of a lens exceeds about two Ekman depth scales, and the head-down state arises when the converse is true. The speed of along- and cross-slope motion of the central, thick core of a dense lens is also investigated, and the results from an ensemble of runs with the three-dimensional reduced-gravity model are found to accord suprisingly well with some approximations derived from bulk dynamics. From a practical point of view, the results concerning the shape of isopycnals bounding dense lenses on slopes can provide valuable information from which to infer important aspects of the underlying dynamics.