Biases in the air-sea flux of CO₂ resulting from ocean surface temperature gradients

The difference in the fugacities of CO₂ across the diffusive sublayer at the ocean surface is the driving force behind the air-sea flux of CO₂. Bulk seawater fugacity is normally measured several meters below the surface, while the fugacity at the water surface, assumed to be in equilibrium with the atmosphere, is measured several meters above the surface. Implied in these measurements is that the fugacity values are the same as those across the diffusive boundary layer. However, temperature gradients exist at the interface due to molecular transfer processes, resulting in a cool surface temperature, known as the skin effect. A warm layer from solar radiation can also result in a heterogeneous temperature profile within the upper few meters of the ocean. Here we describe measurements carried out during a 14-day study in the equatorial Pacific Ocean (GasEx-2001) aimed at estimating the gradients of CO₂ near the surface and resulting flux anomalies. The fugacity measurements were corrected for temperature effects using data from the ship's thermosalinograph, a high-resolution profiler (SkinDeEP), an infrared radiometer (CIRIMS), and several point measurements at different depths on various platforms. Results from SkinDeEP show that the largest cool skin and warm layer biases occur at low winds, with maximum biases of -4% and +4%, respectively. Time series ship data show an average CO₂ flux cool skin retardation of about 2%. Ship and drifter data show significant CO₂ flux enhancement due to the warm layer, with maximums occurring in the afternoon. Temperature measurements were compared to predictions based on available cool skin parameterizations to predict the skin-bulk temperature difference, along with a warm layer model.