Extreme bed shear stress during coastal downwelling

The wind-driven circulation of coastal oceans has been studied for many decades. Using a 2.5-dimensional hydrodynamic model, this work unravels new aspects inherent with this circulation. In agreement with previous studies, downwelling-favorable coastal winds create an overturning cross-shelf circulation that operates to mix nearshore water. On timescales of days, this circulation tends to eliminate itself causing a “shutdown” of the cross-shelf circulation. For the first time, here, the author demonstrates that this shutdown is accompanied by creation of a zone of extremely high bed shear stresses (> 0.35 Pa) that operates to “plow” the seabed over an offshore distance of ~ 10–20 km. The author postulates that the associated sediment erosion episodes and their likely ammonification of the water column are key in the understanding of the biogeochemistry shaping coastal marine ecosystems.