No abstract

Measurements just beneath the ocean surface demonstrate that the primary mechanism by which energy from breaking waves is transmitted into the water column is through the work done by the covariance of turbulent pressure and velocity fluctuations. The convergence in the vertical transport of turbulent kinetic energy (TKE) balances the dissipation rate of TKE at first order and is nearly an order of magnitude greater than the sum of the integrated Eulerian and Stokes shear production. The measured TKE transport is consistent with a simple conceptual model that assumes roughly half of the surface flux of TKE by wave breaking is transmitted to depths greater than the significant wave height. During conditions when breaking waves are inferred, the direction of momentum flux is more aligned with the direction of wave propagation than with the wind direction. Both the energy and momentum fluxes occur at frequencies much lower than the wave band, consistent with the time scales associated with wave breaking. The largest instantaneous values of momentum flux are associated with strong downward vertical velocity perturbations, in contrast to the pressure work, which is associated with strong drops in pressure and upward vertical velocity perturbations.

No abstract

The spatiotemporal variability of wind stress dynamics in Chesapeake Bay has been investigated using a combination of observations and numerical modeling. Direct measurements of momentum and surface heat fluxes were collected using an ultrasonic anemometer deployed on a fixed tower in the middle reaches of Chesapeake Bay in the spring of 2012 along with collocated wave measurements. These measurements were compared to bulk estimates of wind stress using wave-dependent formulations of the Charnock parameter (alpha). Results indicate that a constant alpha value of 0.018 reasonably represents observed stress values, but estimates can be improved by the inclusion of surface wave information in the parameterization of alpha. Using a wave age formulation of alpha in combination with an optimally interpolated 10-m neutral wind field, a third-generation numerical wave model, Simulating Waves Nearshore (SWAN), was employed to investigate the spatiotemporal variability of wind stress across the estuary. Alpha values were found to be wind speed dependent and displayed spatial distributions that ranged between open-ocean values and strongly fetch-limited values. Model results suggest that variable wind stress dynamics stemming from a combination of variable surface winds and fetch-limited wave growth may result in the 10-m neutral drag coefficient varying by a factor of 2 across the estuary. Up to 20% of these changes can be directly attributed to the effects of variable waves.