Defining the Enhancement of Air-Water Interfacial Oxygen Exchange Rate due to Wind-Forced Microscale Waves

Abstract

Over the last few years, compelling evidence has emerged that the exchange of low-solubility gases across air-water interfaces is strongly enhanced by microscale breaking (e.g. Jähne and Haußecker [12], Zappa et al. [28]). Jähne and Haußecker [12] observe that low-solubility gas flux rates are enhanced by up to a factor of 5 in the presence of small scale waves. Investigations using surface infrared imagery [10, 22, 27, 28] have demonstrated a strong correlation between total flux and a proportional area of surface with a high infra-red radiation emission associated with the passage of microscale breaking waves. The mechanisms causing this significant enhancement in exchange rate remain unclear. Zappa et al. [28] proposed that thinning of the aqueous diffusion sublayer by subsurface turbulence in the vicinity of the high infra-red emission region was primarily responsible for this enhancement. Alternate to this is a relationship between the air-water surface exchange rate and the passage rate of wind-forced microscale breaking waves proposed by Peirson and Banner [21]. They have suggested that subduction of the aqueous diffusion sublayer by the microscale wave spilling regions coupled with a weak surface divergence on the upwind faces of the waves primarily determines the microscale-breaking associated flux rate. We have completed a sequence of precise oxygen re-aeration measurements with the specific objective of testing the findings of Peirson and Banner [21]. Specifically, we have compared the flux rates of wind-forced, flat water surfaces in the absence of waves with those in the presence of wind-forced, steep, unbroken waves and wind-forced, microscale breaking waves. With the introduction of steep, unbroken micro-scale waves the surface exchange rate is enhanced by a factor of approximately 2.5. The transition from incipient breaking of the waves to the microscale breaking state induces a significant increase in the associated wind stress [1]. The observed rapid increase in flux rate is approximately proportional to the increase in the wind stress. For the microscale-breaking state, the observed flux rates show good agreement with the predictions of Peirson and Banner [21].