A SIMPLE THEORY FOR THE FATE OF BUOYANT COASTAL DISCHARGES

A simple theory that predicts the vertical structure and offshore spre ading of a localized buoyant inflow onto a continental shelf is formul ated. The theory is based on two competing mechanisms that move the bu oyant fluid offshore: 1) the radial spread of the lighter water over t he ambient water, being dent cted by the Coriolis force and producing an anticyclonic cyclostrophic plume, and 2) offshore transport of buoy ant water in the frictional bottom boundary layer that moves the entir e plume offshore while maintaining contact with the bottom. The surfac e expression of the cyclostrophic plume moves offshore a distance y(s) = 2(3g'h(o) + upsilon(r)(2))/(2g'h(o) + upsilon(r)(2))(1/2)f, where g ' is reduced gravity based on the inflow density anomaly, h(o) is the inflow depth, upsilon,is the inflow velocity, and f is the Coriolis pa rameter. The plume remains attached to the bottom to a depth given by h(b) = (2L upsilon(i)h(o)f/g')(1/2), where L is the inflow width. Both scales are based solely on parameters of the buoyant inflow at its so urce. There are three possible scenarios. 1) If the predicted h(b) is shallower than the inflow depth, then the bottom boundary layer does n ot transport buoyancy offshore, and a purely surface-advected plume fo rms, which extends offshore a minimum of more than four Rossby radii. 2) If the h(b) isobath is farther offshore than y(s), then transport i n the bottom boundary layer dominates and a purely bottom-advected plu me forms, which is trapped along the h(b) isobath. 3) If the h(b) isob ath is deeper than the inflow depth but shoreward of y(s), then an int ermediate plume forms in which the plume detaches from the bottom at h (b) and spreads offshore at the surface to y(s). The theory is tested using a primitive equation numerical model. All three plume types are reproduced with scales that agree well with the theory. The theory is compared to a number of observational examples. In all cases, the pred iction of plume type is correct, and the length scales are consistent with the theory.