Systematic errors in global air-sea CO₂ flux caused by temporal averaging of sea-level pressure

H. Kettle (1) , C. J. Merchant (1)

1 School of Geosciences [Edinburgh]

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 (pCO_2air). Thus low pressure systems cause a drop in pCO_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 pCO_2air (and thus ignoring the covariance of wind and pressure) over the period 1990?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.

Domaines: Océan, Atmosphère

H. Kettle, C. J. Merchant. Systematic errors in global air-sea CO₂ flux caused by temporal averaging of sea-level pressure. Atmospheric Chemistry and Physics Discussions, 2005, 5 (1), pp.325-346. ⟨hal-00300925⟩