A Case Study of Inertial Oscillations and Diurnal Dynamics Offshore of Mobile Bay

Book, J. W., Parra, S. M., Dykstra, S., Dzwonkowski, B., Howden, S. D., Warner, S. J., Moum, J. N., Pan, C., Cambazoglu, M. K., Fitzpatrick, P. J., & Lau, Y. H. (2019). A Case Study of Inertial Oscillations and Diurnal Dynamics Offshore of Mobile Bay. 2019 Gulf of Mexico Oil Spill and Ecosystem Conference. New Orleans, LA.

The northern Gulf shelf has a natural resonance at diurnal frequencies because the period where the circular acceleration of currents can perfectly balance Coriolis acceleration at 30°N is close to 24 hours. Complicating this normal ocean resonance in this region are land-sea breezes forced by solar radiation and diurnal tides that also act in 24 hour cycles. These dynamics are investigated using six sets of moorings that were deployed as part of the CONsortium for oil exposure pathways in COastal River-Dominated Ecosystems (CONCORDE) spring experiment to observe the spatial structure of the riverine driven outflow of Mobile Bay. Three acoustic Doppler current meter moorings were deployed to form a fan across the Mobile Bay outflow path and three additional mooring sets were deployed in a line array extending offshore from the fan array. Together these six short-term mooring sets complemented the long-term observations made by one Dauphin Island Sea Lab mooring located nearby. A major inertial oscillation event was observed from April 3-14 by all seven moorings. The inertial currents were strongest further offshore and had a subsurface peak of opposite phase of the surface currents as is typical for such events. More surprising was that inertial-like currents with sub-surface patterns were observed as far inshore as the fan mooring array at the 10 m isobath. Also, the semi-minor to semi-major aspect ratios of the currents were typically much less than one everywhere and were close to 0.75 for the strongest inertial currents. This indicates that frictional forces, wind stress, and/or coastal convergences/divergences are playing important roles in breaking the Coriolis and circular acceleration balance for these events. The implications are that significant vertical energy and mass fluxes are occurring during these events and these could have important biological and oil dispersal consequences that have not yet been adequately studied.