Characterization and Modulation of Langmuir Circulation in Chesapeake Bay

Malcolm E. Scully Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Alexander W. Fisher University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland

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Steven E. Suttles University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland

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Lawrence P. Sanford University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland

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William C. Boicourt University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland

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Abstract

Measurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness.

Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.

Corresponding author address: Malcolm E. Scully, Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, MS 10, Woods Hole, MA 02543. E-mail: mscully@whoi.edu

Abstract

Measurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness.

Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.

Corresponding author address: Malcolm E. Scully, Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, MS 10, Woods Hole, MA 02543. E-mail: mscully@whoi.edu
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