Global average of air‐sea CO2 transfer velocity from QuikSCAT scatterometer wind speeds

Abstract

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, 〈k〉, 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 〈k〉. 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 〈k〉 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 〈k〉 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.