Impact of exchange flow on shelf-plume interactions

Estuarine exchange flow draws in water from the shelf which can impact coastal ecology (with nutrient content or low aragonite saturation) or, in the case of fjords, glacial melt (through thermal content). Channel-bound shelf currents arise to meet the demand of exchange flow within the estuarine channel. These channel-bound currents interact with buoyant plumes. Here we test whether channel-bound currents are driven by forcing on the shelf or within the channel. To investigate the interactions between estuarine inflow and outflow on the shelf, we analyze a model built using Regional Ocean Modeling Systems that includes an estuarine channel with exchange flow that debouches onto a sloping, stratified shelf. To isolate the salient dynamics, the only forcing imposed is a single frequency tide and there are no ambient currents on the shelf. Channel-bound currents arise from the direction of coastal trapped wave propagation, with half of the transport occurring directly beneath the plume and the remaining half occurring offshore of the plume. The shelf-plume interface is steepened by counter flow beneath the plume. Similarly, the sea surface offshore of the plume is tilted to allow offshore channel-bound flow. Despite these geostrophic adjustments, ageostrophic forcing is ultimately necessary to bring shelf water across isobaths into the channel. Ekman transport is insufficient because cross-isobath flow happens in the interior. This suggests that exchange flow within the estuarine channel requires more shelf water than is provided by geostrophic flow in and around the buoyant plume, and this deficit diffuses from the estuary across the shelf as a channel-directed pressure gradient force.