The warm oceanic surface layer: Implications for CO2 fluxes and surface gas measurements

The ocean and air‐sea interface are important for the exchange of heat, momentum, water vapour and carbon‐dioxide. The details of the exchange mechanisms, which are often coupled and complex, have to be understood in order to assess the future role of the global oceans in climate change. Recently, much attention has focused on the ‘thermal skin effect’, cooling or warming of the uppermost millimeters of the sea surface, and its global implications for enhancing CO2 uptake [Robertson and Watson, 1992; Van Scoy et al., 1995]. Routinely, air‐sea flux estimates derived from oceanic pCO2 measurements are corrected for the usually cooler thermal skin by applying a correction factor [Sarmiento and Sundquist, 1992; Wong et al., 1995]. Here we describe how near surface warming of the upper few meters of the ocean by solar radiation, routinely observed but not fully appreciated in this context, can significantly affect the net daily exchange of CO2 over and above the skin effect, and can even lead to a reversal of the direction of the air‐sea flux calculated from pCO2 measurements and wind speed alone. The warming and cooling cycle produces a net asymmetry between CO2 invasion and evasion, having the effect of decreasing CO2 invasion and increasing CO2 evasion.