The Influence of Stokes Drift on Baroclinic Instabilities in the Ocean Mixed Layer


Surface gravity waves can change the stability of the ocean mixed layer by altering the near surface shear. A wave-averaged current, called Stokes drift, induces a Stokes Coriolis force and a Stokes shear force that change the horizontal and vertical momentum balances respectively. In geostrophically balanced flows, such as the submesoscale density fronts occurring in the ocean mixed layer, the Stokes Coriolis force reduces the burden on the Eulerian Coriolis force to balance the pressure gradient force. This effectively induces an Eulerian flow opposing the Stokes drift, while maintaining the Lagrangian (Eulerian plus Stokes) flow. The criteria for geostrophic instabilities is unchanged with the appropriate reinterpretation of the Lagrangian shear, however the size, growth rate, and vertical structure of these instabilities is slightly changed. The flow remains symmetrically unstable when the potential vorticity (PV; the dot product of Eulerian vorticity with the buoyancy gradient) is negative. Since the Stokes drift induces an opposing Eulerian flow, it alters the PV, thereby favoring or suppressing symmetric instabilities (SI) depending on the alignment between the Stokes drift and the front. Since SI reduce the shear and increase the stratification of a front, this is a mechanism by which wave forcing may induce restratification rather than mixing as is commonly thought.

Jan 13, 2015 3:30 PM — 4:30 PM
Bechtel Collaboratory, Discovery Learning Center
Engineering Center, University of Colorado at Boulder, Boulder, CO 80309

University of Colorado Boulder