Wave-generated Turbulence in the Coastal Ocean During Passage of a Tropical Cyclone

Ann Gargett, Institute of Ocean Sciences, Sidney, BC, Canada and Dana K Savidge, Skidaway Inst Oceanography, Savannah, GA, United States
In September 2007, tropical storm (TS) Barry passed northwards over US Navy Tower R2 on the Georgia continental shelf. Tower instruments quantified wind and buoyancy flux forcing of the ocean, while a single VADCP, bottom-mounted nearby, provided continuous measurements of directional surface wave spectra (hence of directional Stokes functions of depth and time) as well as mean, tidal, and turbulent velocities throughout the water column. Full-depth turbulent velocity and backscatter structures observed during TS Barry have the characteristics of Langmuir supercells (LS), present when a surface Langmuir circulation (LC) growth rate growth rate g* exceeds a critical value. However despite g* values that remain above critical, full-depth velocity/backscatter structures disappear completely for a time between the two wind maxima associated with the TS, as the wind veers rapidly clockwise with eye passage to the west of R2. The most likely explanation for this hiatus is the observed absence of wave breaking during the period of wind veering, decreasing surface supply of quasi-organized vertical vorticity that is more “effective” in generating LC than the randomly oriented vorticity associated with a (wind) stress-driven boundary layer . This result has significant implications for LES modelling of LC in both shallow and deep water.