Numerical Investigation of the Influence of Wind and Tides on Salt Mixing and Cross-Shore Transport in River Plumes

Abstract

River plumes play an essential role in the transport of terrestrially derived materials (like nutrients, sediments, pollutants, etc.) into the coastal ocean. Quantifying the cross-shore transport in river plumes can help to better understand the contribution of river-borne substances to marine biogeochemical cycles and to parameterize these processes in global ocean models which are usually too coarse to resolve individual rivers. It is known that besides external factors (like runoff, latitude, wind, and tides) also internal estuarine processes like salt mixing affect the exchange flow between an estuary and the coastal ocean. A theoretical framework to separate the plume and the estuary mixing in isohaline coordinates is presented. An idealized coastal ocean model setup resolving the whole plume-estuary continuum is used to validate the theoretical relation and to study the link between the estuarine pre-conditioning and the cross-shore export of river water under different forcing scenarios. It is found that the most effective cross-shore transport of river water happens under moderately upwelling favorable wind conditions and weak tidal forcing. This scenario is characterized by relatively weak estuarine mixing, strong stratification, and little interaction between the surface and bottom boundary layers such that a thin layer of buoyant river water can extend far into the ocean. We conclude that reduced estuarine mixing is indicative of an enhanced accumulation of fresh water near the shore, but is not directly related to the cross-shore transport in river plumes.