Wave breaking efficiency of bubble-related air-sea interaction processes Paul A. Hwang, Ivan B. Savelyev, and Magdalena D. Anguelova Naval Research Laboratory, 4555 Overlook Ave SW, Washington DC 20375 USA E-mail: paul.hwang@nrl.navy.mil Abstract. Simultaneous measurements of sea spray aerosol (SSA), wind, wave and microwave brightness temperature are obtained in the open ocean onboard the Floating Instrument Platform (FLIP). These data are analysed to clarify the ocean surface processes important to SSA production. Parameters are formulated to represent surface processes with characteristic length scales spanning over a broad range. The investigation also reveals distinct differences of the SSA properties in rising winds and falling winds, with higher efficiency of breaking production in low or falling winds. Also, in closely related measurements of whitecap coverage, higher whitecap fraction as a function of wind speed are found in falling winds than in rising winds or in older seas than in younger seas. In the research of length and velocity scales of breaking waves, it has been pointed out that the length scale of wave breaking is shorter in mixed seas than in wind seas. For example, source function analysis of short surface waves shows that the characteristic length scale of the dissipation function shifts toward higher wavenumber (shorter wavelength) in mixed sea compared to that in wind seas. Similarly, based on feature tracking or Doppler analysis of microwave radar sea spikes, which are closely associated with breaking waves, the average breaking wave velocity is smaller with swell presence compare to that in pure wind seas. Applying the result of breaking length and velocity scale analyses to the SSA and whitecap observations described earlier, it is suggestive that larger air cavities resulting from the longer breakers are entrained in rising high winds. The larger air cavities escape due to buoyancy before they can be fully broken down into small bubbles for the subsequent SSA production or whitecap manifestation. In contrast, in falling winds (with mixed seas more likely), the shorter breaker entrains smaller and more numerous air cavities that stay underwater longer for more efficient bubble breakup by turbulence. For low winds, the breaking scale is small and with high efficiency for SSA or whitecap generation; the trend of rising or falling wind is less important. 1. Introduction Breaking of ocean surface waves plays an important role in many air-sea interaction processes. For example, it is a main source of turbulence on the upper ocean layer; breaking waves entrain bubbles and enhance air-sea gas exchange; returning to the ocean surface, the entrained bubbles appear as whitecaps enhancing the surface albedo and modifying heat transfer; and the bursting of bubbles is one of the main generation sources of sea spray aerosols, which represent an important member of the atmospheric aerosol population. In this paper, we report results of a field experiment addressing the effect of wave breaking on the sea spray aerosol (SSA) production. Simultaneous measurements of SSA, wind, wave and microwave