Modelling the Zambezi River plume using the regional oceanic modelling system

Abstract

The Zambezi River constitutes an important source of freshwater to the Sofala Bank in the western Mozambique Channel, where it is believed that the out ow from the river infuences the secondary production of coastal waters. Observational data is analyzed alongside simulation outputs to describe the main features of the Zambezi River plume, and its response to ambient factors. These plumes spread over both the downstream and upstream coasts, with the downstream part remaining attached to the shoreline and con#ned to the region onshore from the 50 m isobath. The plume naturally penetrates downstream beyond the City of Quelimane, but only under a strong buoyancy input or an ambient current oriented equatorward do the plume waters spread further north (beyond the mouth of the Licungo River). The upstream portion of the plume consists of a tongue of brackish water leaving the mouth and spreading poleward and o#shore, beyond the 50 m isobath. Plume waters occupy the entire water column in the region immediately seaward from the delta, to a distance of about 30 km from the coast, where they become limited to the top 10 m . In the absence of wind, tides, and ambient current, a sizable unsteady plume forms under a constant-slope bathymetry and straight shoreline, consisting of a bulge that grows continually as it re-circulates anti-cyclonically (counter-clockwise) in front of the mouth, and a coastal current that ows equatorward along the coast. Under realistic geometry and bathymetry, the multiple mouth Zambezi River gen- erates a surface-advected plume with diminished upstream penetration. The plume response to constant impulsive winds is characterized by a change of the shape into either a coastal current that moves equatorward along the coast or a large bulge that spreads seaward and poleward, depending on whether the wind eld is equa - torward or poleward. Asymmetric sea-breezes and tides deepen the plume, hence changing the plume structure from surface-advected to bottom-advected. Strong poleward ambient current forces the plume water in the far-eld region to spread southward beyond 20ºS Sat times when the water in the near-#eld region is trans-ported downstream