Global average of air-sea CO2 transfer velocity from QuikSCAT scatterometer wind speeds
Résumé
The absolute calibration of the relationship between air-sea CO2 transfer velocity, k, and wind speed, U, has been a topic of debate for some time, because k global average, $\langle$k$\rangle$, as deduced from Geochemical Ocean Sections Study oceanic 14C inventory has differed from that deduced from experimental k-U relationships. Recently, new oceanic 14C inventories and inversions have lead to a lower $\langle$k$\rangle$. In addition, new measurements performed at sea in high-wind speed conditions have led to new k-U relationship. Meanwhile, quality and sampling of satellite wind speeds has greatly improved. The QuikSCAT scatterometer has provided high-quality wind speeds for more than 7 years. This allows us to estimate the global distributions of k computed using k-U relationships and temperature-dependent Schmidt numbers from 1999 to 2006. Given the difficulty of measuring in situ wind speed very accurately, we performed a sensitivity study of the $\langle$k$\rangle$ uncertainty which results from QuikSCAT U uncertainties. New QuikSCAT-buoy U comparisons in the northern Atlantic Ocean and in the Southern Ocean confirm the excellent precision of QuikSCAT U (RMS difference of about 1 m s-1), but it is possible that QuikSCAT overestimates wind speeds by 5%, leading to a possible overestimation of k derived with quadratic relationships by 10%. The $\langle$k$\rangle$ values obtained with two recent experimental k-U relationships are very close, between 15.9 and 17.9 cm h-1, and within the error bar of k average deduced from the new oceanic 14C inventory.
Origine : Fichiers éditeurs autorisés sur une archive ouverte