Penetration of buoyant coastal discharge onto the continental shelf: A numerical model experiment

Abstract

Plumes of buoyant water produced by inflow from rivers and estuaries are common on the continental shelf. Typically they turn anticyclonically to flow alongshelf as buoyancy-driven coastal currents. During this passage, mixing with ambient shelf water gradually erodes the plume buoyancy so that its alongshelf penetration is finite. This paper addresses the extent of this penetration and how it is determined by fundamental dimensionless flow parameters. A three-dimensional numerical model is applied to an idealized flow regime. Ambient shelf conditions include tidal motion, but neither wind stress nor ambient alongshelf current. The alongshelf extent of penetration is evaluated after the plume reaches a stationary condition downshelf. A total of 66 model experiments are conducted, including variations in buoyant source and ambient shelf properties. Five dimensionless parameters determine the alongshelf and across-shelf penetration, the latter the coastal current width. The most critical of these is T, the source volume transport scaled by the associated source geostrophic transport. For fixed shelf bottom slope and no tides, similarity forms are found for both alongshelf and across-shelf penetration for a wide range of T. Increased shelf bottom slope and increased shelf tidal amplitude shorten the alongshelf penetration. The vertical turbulent closure scheme itself contributes one of the five model parameters, the background eddy viscosity or diffusivity. This background viscosity is added to the viscosity determined from the MellorYamada level 2.5 closure scheme to give N, the vertical viscosity used by the model. Where the local Richardson number exceeds about 0.2, as in most buoyant plumes, the Mellor-Yamada scheme "switches off," forcing N to default to v. Plume penetration properties thus are found to depend significantly on v. At present one must choose v arbitrarily, thus introducing uncertainty into the model results. © 1999 American Meteorological Society.