Wind Stress Dynamics in Chesapeake Bay: exploring variability through observation & numerical modeling

Wind may play an important role in estuarine dynamics, yet the specific nature of wind forced responses in estuaries remains relatively understudied. While significant attention has been paid to tidal dynamics, recent studies have contributed to a growing recognition that the wind can be of first order importance for wave generation, vertical mixing, and circulation in estuaries.

As part of an ongoing collaborative investigation of wind-driven estuarine physics in Chesapeake Bay, we investigate the spatiotemporal variability of wind stress through a combination of direct observations and numerical modeling. Data were collected in the spring of 2012 and autumn of 2013 using an extensive field array and observations conducted aboard the R/V Hugh R. Sharp. Wave measurements were collected using acoustic wave and current profilers (AWACs) and surface meteorological measurements were made from instrumented surface buoys. Direct measurements of air-sea momentum and sensible heat fluxes were provided by a sonic anemometer mounted atop a stationary tower deployed in the mid-Bay.

Basin-scale variability was examined by optimally interpolating the 10 meter neutral wind field. Following Large and Pond (1981), direct wind, atmospheric, and water temperature observations were used to adjust over-water wind observations from multiple sources to uniform 10 meter neutral conditions. Over-land stations were corrected using a standard log profile. A universal kriging scheme was applied to corrected observations using an algorithmically-fit exponential variogram model. Results suggest that significant fine scale structure, including topographic influences, exists in the wind field over Chesapeake Bay.

Variable wind forcing can influence the development of wind stress directly and indirectly through the generation of surface waves. A third-generation wave model, Simulating WAves Nearshore (SWAN), was used to investigate the spatiotemporal variability of the surface wave field generated by variable atmospheric conditions. The model solves the spectral action density equation using a curvilinear grid and accounts for tidal elevation and bottom friction. The kriged wind field was used to force wind-wave generation and wave measurements from NDBC buoy 44014 were used as the oceanic boundary condition. We used modeled wave parameters to estimate wind stress with a wave age relationship, where wave age is defined as the wave phase speed divided by the shear velocity. Results suggest that complex interdependencies between wind, waves, and currents may lead to spatially variable wind stress development within the estuary.