Surface Wave Effects on the Translation of Wind Stress across the Air–Sea Interface in a Fetch-Limited, Coastal Embayment

Abstract

The role of surface gravity waves in structuring the air-sea momentum flux is examined in the middle reaches of Chesapeake Bay. Observed wave spectra showed that wave direction in Chesapeake Bay is strongly correlated with basin geometry. Waves preferentially developed in the direction of maximum fetch, suggesting that dominant wave frequencies may be commonly and persistently misaligned with local wind forcing. Direct observations from an ultrasonic anemometer and vertical array of ADVs show that the magnitude and direction of stress changed across the air-sea interface, suggesting that a stress divergence occurred at or near the water surface. Using a numerical wave model in combination with direct flux measurements, the air-sea momentum flux was partitioned between the surface wave field and the mean flow. Results indicate that the surface wave field can store or release a significant fraction of the total momentum flux depending on the direction of the wind. When wind blew across dominant fetch axes, the generation of short gravity waves stored as much as 40% of the total wind stress. Accounting for the storage of momentum in the surface wave field closed the air-sea momentum budget. Agreement between the direction of Lagrangian shear and the direction of the stress vector in the mixed surface layer suggests that the observed directional difference was due to the combined effect of breaking waves producing downward sweeps of momentum in the direction of wave propagation and the straining of that vorticity field in a manner similar to Langmuir turbulence.

Effects of Locally Generated Wind Waves on the Momentum Budget and Subtidal Exchange in a Coastal Plain Estuary

“…For instance, a changing wind direction can increase the fetch (Donelan et al, 1985) and bring local wind seas closer to saturation. In estuaries such as Chesapeake Bay, the misalignment between wind and waves due to the basin geometry is translated into a wind stress divergence at the air-sea surface (Fisher et al, 2017).…”

Wind-Wave Effects on Estuarine Turbulence: A Comparison of Observations and Second-Moment Closure Predictions

“…At the center of an extensive field experiment conducted in the fall of 2013, a turbulence tower was deployed on the western shoal of Chesapeake Bay (38827 0 39 00 N, 76824 0 44 00 W) in a 14-m-deep region of slowly varying bathymetry. The tower was held vertically rigid using four guy wires, which were secured to the top of the tower and anchored to 450-kg railcar wheels (Scully et al 2015;Fisher et al 2017). The tower was deployed on 18 September 2013 and recovered on 29 October 2013.…”

“…The tower was deployed on 18 September 2013 and recovered on 29 October 2013. A schematic of the tower and map of the deployment site are shown in Scully et al (2015) and Fisher et al (2017).…”

“…Atmospheric measurements of wind stress were calculated by integrating velocity cospectra for frequencies less than 2 Hz in 30-min blocks (Rieder et al 1994). The sensitivity of vertical flux measurements to variations in vertical velocity prompted a tilt correction using the planar fit method (Wilczak et al 2001) on daily subranges of the anemometer data as described in Fisher et al (2015). The anemometer was deployed on 25 September 2013 and recovered on 28 October 2013.…”

“…During an experiment conducted in the middle reaches of Chesapeake Bay in the fall of 2013, breaking waves dominated the transfer of momentum and energy in the oceanic surface layer (Scully et al 2016), and coherent turbulent structures consistent with Langmuir turbulence were documented (Scully et al 2015). Building on analyses presented in a series of manuscripts describing windforced responses observed during this experiment (Scully et al 2015(Scully et al , 2016Fisher et al 2017), this paper examines the effects of wind waves on vertical profiles of estuarine turbulence and compares observations to the predictions of second-moment turbulence closure schemes.…”