Secondary circulation driven by artificial downwelling ventilates stratified coastal waters

Artificial downwelling (AD), typically implemented by pumping surface water downward, has been proposed to alleviate hypoxia in stratified coastal waters. However, its hydrodynamic effects in open-water settings remain unclear. This study uses a coupled near- and far-field hydrodynamic model to investigate AD operation under stratified cross-flow conditions. The model reproduces observed dye patterns from a fjord-based field experiment and reveals that, under cross-flow, AD induces a secondary circulation: narrow upwelling flanked by broader downwelling zones. This circulation extends several hundred meters downstream and transport water at rate up to 90 times greater than the pumping rate, effectively ventilating bottom waters across a large area. The circulation strength scales with the total jet-and-plume entrainment flux and its boundary mimics “sonic barrier” depending on the Froude number, indicating that ventilation efficiency can be tailored by adjusting engineering parameters such as pipe flow rate and jet orientation. Additional simulations show that sourcing water from below the pycnocline, rather than from the surface, enhances near-bottom oxygenation while minimizing stratification disruption and preventing hypoxic water upwelling. These findings identify secondary circulation as a key mechanism driving AD ventilation and provide a mechanistic basis for optimizing oxygenation strategies in stratified coastal environments.