Wind stress and wind wave observations in the presence of swell

At any given time, much of the global ocean's wave energy is defined bythe swell fields, generated by past storm events and now propagating elsewhere. The role of swell upon the air-sea momentum transfer that drives the ocean's circulation is not well determined or parameterized. Aircraft and in situ measurements of the atmospheric boundary layer and underlying ocean waves during the ONR Shoaling Waves Experiment (SHOWEX) are used to examine this issue. Friction velocity, wave height spectra,and wave slope variance estimates for both m and cm-scale wavelets are examined during events where the near-surface wind direction was either aligned or opposed to the swell propagation direction. Preliminary results indicate that the largest observed effect is the substantial decrease in the surface drag coefficient and wave slope variance underthe aligned condition. While such a smooth flow case is often associated with and attributed to a stable boundary layer, results here suggest that wave-ABL coupling is also a controlling factor. Implications for the open ocean drag parameterization are addressed.