The influence of applying skin temperature corrections to gas exchange models on air-sea oxygen flux estimates

Abstract

The skin of the ocean is often slightly cooler than the surface mixed layer due to net surface heat loss (cool skin effect), and sometimes slightly warmer in areas with extreme solar radiation (warm layer effect). In previous work (Yang et al., 2022), with the skin temperature correction term (ΔT) derived from the fifth generation European Center for Medium-Range Weather Forecasts Reanalysis (ERA5) and oxygen (O₂) data from three Argo profiling floats, we showed that skin temperature correction is critical for air-sea O₂ flux calculation. In this work, we applied the same method to the World Ocean Atlas 2018 (WOA2018) dataset with two widely used air-sea gas exchange models (an empirically derived quadratic bulk flux model W14, and a mechanistic bubble-mediated model E19), to evaluate the influence of skin temperature correction on large-scale air-sea O₂ flux estimate. To avoid the influence of sea ice on air-sea gas exchange (and possibly on the ERA5 reanalysis), we limited our analysis between 50°S and 50°N. The result revealed that for both W14 and E19 models the skin temperature correction lowered annual sea-to-air O₂ flux between 50°S and 50°N by 25 % for the E19 model and by 22 % for the W14 model. Larger ΔT (further from zero), higher temperature, higher wind speed, and larger O₂ concentration difference across the air-sea interface led to larger difference in O₂ fluxes calculated with and without the skin temperature correction. With the E19 model, using the ERA5-based ΔT for areas between 50°S and 50°N and a fixed ΔT of −0.17 K for high latitude areas (50°N-90°N and 50°S-90°S), we made an estimate of O₂-based global air-to-sea carbon flux of 3.84 Pg C yr⁻¹ (using O₂ to C ratio of 1.45 from Hedges et al., 2002), which was comparable to other latest estimates.