On the Creation of Extreme Bed Shear Stresses in Coastal Oceans (#186)
Using a 2.5-dimensional hydrodynamic coastal ocean model, here I show that downwelling-favourable synoptic winds create an oceanic circulation that includes a zone of extremely large bed shear stresses >0.4 Pa. The chain of events yielding such extreme bed shear stresses is outlined as follows. (i) The wind forcing creates a swift coast-parallel downwelling jet. (ii) The across-shelf circulation on the landward side of this jet operates to weaken the density stratification and triggers the Kelvin-Helmholtz instability mechanism. (iii) The associated water-column stirring also affects the momentum and leads to a shut-down of the across-shelf circulation, which is well documented. (iv) Importantly – and this is a new finding – this instability also operates to mix the enhanced momentum of the downwelling jet into close vicinity of the seafloor where it induces a zone of extreme bed shear stresses. During the process, the zone of extreme shear stresses develops near the coast and gradually propagates offshore over a distance of up to 20 km thereby “ploughing” the seabed. I postulate that the associated ammonification of the water column plays an important role in the functioning of coastal ecosystems. These findings are discussed in a manuscript (Kämpf, 2018) submitted to Ocean Dynamics.