Influence of Wind on Stratification and Mixing in Mobile Bay, Alabama, a
Wide Microtidal Estuary
Abstract
Extensive research has shown that wind has a strong influence on
estuarine circulation and salt transport. However, the response to wind
forcing in estuarine systems presents challenges due in part to the
complexities of realistic forcing conditions, system states, and
geomorphologies. To further advance the understanding of estuarine
responses to wind forcing, a comprehensive analysis of stratification
and mixing during a typical southeast wind event was conducted in Mobile
Bay, a microtidal, wide, shallow, and river-dominated estuary in the
northern Gulf of Mexico. An analysis of the vertical salinity variance
and its associated budget terms shows that the system generally becomes
less stratified and fully mixed across much of the system; however,
there was significant spatial heterogeneity in physical processes
driving the evolution of the water column stratification over the course
of the event. Surprisingly, in some regions of the bay, dissipation of
salinity variance was secondary to contributions from straining and
advection. Furthermore, local wind stress and remote wind driven Ekman
transport affected stratification responses and their relative impacts
varied spatially across the estuary. Direct turbulent mixing from local
wind stress and straining dominated the stratification responses away
from the main tidal inlet where estuarine-shelf exchange (i.e., current
velocity structure and advection of salinity) from Ekman transport
controlled the vertical mixing. This detailed case study highlights the
complexity of wind influences in a system like Mobile Bay, a
representative typical of the northern Gulf of Mexico and other coastal
region.