Effects of tropical cyclones on circulation and momentum balance in a subtropical estuary and inlet

Estuaries along the coast of U.S. are subjected to extreme events such as tropical cyclones, nor'easters, and heavy rainfall. These extreme events can bring very drastic and sudden changes to the estuarine environment and habitat. This study focuses on the hydrodynamics of the subtropical Northern Coastal Basin (NCB) estuary during regular (non-storm) time periods as well as tropical cyclones. Hydrodynamics (water level, currents and salinity) of the system is simulated with a three-dimensional model, CH3D (Curvilinear-grid Hydrodynamics in 3D), during four major hurricanes (Charley, Frances, Ivan, Jeanne) in 2004 and two tropical storms (Fay and Hanna) in 2008. After successful calibration and verification of using the observed water level, salinity and current (2008) data throughout the estuary, the model is used to examine the residual circulation, momentum balance, and stratification in order to understand the effects of tropical cyclones on the estuarine hydrodynamics. Model results during a typical tidal cycle show formation of four residual eddies around the headlands of the St. Augustine Inlet due to the nonlinear advection. The dynamic structure of the residual eddies strongly influences the cross-sectional residual flow inside the inlet. Analysis of the residual momentum balance simulated by the model reveals that during a typical tidal cycle with weak external forcing, the balance is between the barotropic pressure gradient and the nonlinear advection. Prior to the landfall of storms, strong northeasterly winds disrupted the ocean side eddies and created a uniform along-shore southerly current which forced complete inflow through the inlet due to Ekman transport, but eddies inside the inlet persisted. After the landfall, the wind direction changed to easterly followed by southeasterly winds. The change in wind direction caused the along-shore current to completely reverse and become northerly on the ocean side, while the eddies inside the inlet disrupted and the flow through inlet became complete outflow. The duration of the changes were dependent on the duration of the storm that affected the region, ranging from 3-4 days for Charley and Fay, and up to 12 days for Frances. The momentum balance within the system also changed significantly during storms. Wind stress generated by strong winds was mostly balanced by a combination of barotropic pressure gradient and bottom friction. Baroclinic pressure gradient became important coinciding with increased precipitation or river discharge events in the system and it was more significant in 2004 compared to the 2008 simulations due to the higher precipitation and river discharge (3 times higher). Salinity inside the estuary was significantly affected by the increased precipitation and river discharge especially during the storms. When the residual flow rate at the inlets was directed towards the ocean, the size of low salinity area was at its maximum. Although the salinity within the system was highly affected by precipitation and river discharge, the stratification within the system was generally very weak due to the shallow and dynamic nature of the system except the regions closer to the rivers and creeks. The model results show two potential regions with a very slow flushing rate.