The wind-driven response of the Zambezi River plume along the Sofala Bank: A numerical model study

Abstract

The Zambezi River plume constitutes a sizable feature in the horizontal distribution of both surface salinity and suspended sediments, and it spreads both downstream and upstream, influencing substantially the coastal ecosystems. Here we present the results of several numerical experiments conducted using the Regional Oceanic Modeling System, to investigate the plume’s response to impulsive wind forcing. The model uses realistic geometry and bathymetry, as well as constant discharges emanated from three point sources. Different wind directions and magnitudes were explored, including a daily oscillating wind field. In the absence of wind forcing, a modest discharge generates a plume that propagates northeastwards trapped to the coast. A constant wind field can alter the plume shape to become either a “coastal current” or a “large bulge”, depending on the predominant wind direction. The “coastal current” characterized by a northeastward (downstream) spreading of the plume reaching up to 180 km was directly related to a downwelling favorable winds. While the “large bulge” characterized by an increased upstream penetration is related to upwelling-favorable winds. Diurnal breezes were effective in transforming the trajectories of surface water particles into ellipsoids, as well as promoting additional mixing of the plume and ambient waters. The realistic wind field can transport plume waters to nearly 120 km downstream, 75 km upstream, and 65 km seaward.