Thermal skin effect and the air‐sea flux of carbon dioxide: A seasonal high‐resolution estimate

Understanding the role the oceans play in sequestering anthropogenic CO 2 is crucial to understanding global climate change. Correct parameterization of air‐sea flux of CO 2 is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO 2 solubility (Robertson and Watson, 1992). We re‐evaluate the effect of the cool thermal skin and present a high‐resolution seasonal estimate of its effect on the air‐sea flux of CO 2 . We compare air‐sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO 2 (Tans et al., 1990), we estimate that excluding the southern ocean, the surface skin correction increases the air‐sea flux of carbon by 0.48 Gt yr ‐1 . This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Gt yr −1 . Conservatively, adding a 10% contribution from the southern ocean, we estimate a mean global increase in CO 2 flux due to the skin effect of 0.39 Gt C yr −1 . This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO 2 flux anomaly of only 0.17 Gt C yr −1 which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson and Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large‐scale flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (‐0.75°C) are found during the northern hemisphere winter in the regions of the Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient during the summer months.