Shear stress model for the aqueous boundary layer near the air‐sea interface

Abstract

Two sets of data on currents in the near‐surface boundary layer are reanalyzed to more accurately determine shear stress and characteristics of the mean current profile (friction velocity and roughness length). The results are used to examine the mechanisms responsible for momentum transfer through the aqueous boundary layer (the top several meters of water). The relationship between friction velocity and roughness length found here is consistent with the prevailing concepts on wave‐generated mixing. This result conflicts with a commonly used parameterization of momentum transfer in the aqueous boundary layer, where molecular viscosity is treated as the dominant transfer mechanism. The shear‐related stress and current profiles are shown to follow Charnock's [1955] relationship, which indicates that the shear‐related mixing is due to gravity waves. Charnock's constant for water is found to be within an order of magnitude of 850 (105 times greater than that for the atmospheric boundary layer). Shear‐related stress in the aqueous boundary layer is compared to the wind stress: for conditions of near local wind‐wave equilibrium, the aqueous shear stress is ∼20% of the atmospheric stress, indicating that the majority of the vertical momentum flux is transferred through other mechanisms. These limited data sets suggest that this ratio increases (approaching 100%) for decaying wave fields.