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

摘要

Understanding the role the oceans play in sequestering anthropogenic CO2is crucial to understanding global climate change. Correct parameterization of air‐sea flux of CO2is 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 CO2solubility (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 CO2. 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 CO2(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 CO2flux 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 CO2flux anomaly of only 0.17 Gt C yr−1which 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