Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 1 2005 10.5194/acpd-5-325-2005 http://www.atmos-chem-phys-discuss.net/5/325/2005/ http://www.atmos-chem-phys-discuss.net/5/325/2005/acpd-5-325-2005.html http://www.atmos-chem-phys-discuss.net/5/325/2005/acpd-5-325-2005.pdf 325 346 2005-01-28 Systematic errors in global air-sea CO₂ flux caused by temporal averaging of sea-level pressure H. Kettle C. J. Merchant School of GeoSciences, University of Edinburgh, West Mains Rd, Edinburgh EH9 3JZ, UK Long-term temporal averaging of meteorological data, such as wind speed and air pressure, can cause large errors in air-sea carbon flux estimates. Other researchers have already shown that time averaging of wind speed data creates large errors in flux due to the non-linear dependence of the gas transfer velocity on wind speed (Bates and Merlivat, 2001). However, in general, wind speed is negatively correlated with air pressure, and a given fractional change in the pressure of dry air produces an equivalent fractional change in the atmospheric partial pressure of carbon dioxide (<i>p</i>CO_2air). Thus low pressure systems cause a drop in <i>p</i>CO_2air, which together with the associated high winds, promotes outgassing/reduces uptake of CO₂ from the ocean. Here we quantify the errors in global carbon flux estimates caused by using monthly or climatological pressure data to calculate <i>p</i>CO_2air (and thus ignoring the covariance of wind and pressure) over the period 1990&ndash;1999, using two common parameterisations for gas transfer velocity (Wanninkhof, 1992 (W92) and Wanninkhof and McGillis, 1999 (WM99)). Results show that on average, compared with estimates made using 6 hourly pressure data, the global oceanic sink is systematically overestimated by 7% (W92) and 10% (WM99) when monthly mean pressure is used, and 9% (W92) and 12% (WM99) when climatological pressure is used.