Excess radiocarbon constraints on air-sea gas exchange and the uptake of CO2 by the oceans

We re-assess the constraints that estimates of the global ocean excess radiocarbon inventory (I-E) place on air-sea gas exchange. We find that the gas exchange scaling parameter a(q) cannot be constrained by I-E alone. Non-negligible biases in different global wind speed data sets require a careful adaptation of aq to the wind field chosen. Furthermore, aq depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed- and inventory-normalized gas exchange parameter a(q)(N) which takes into account these biases and which is easily adaptable to any new estimate of I-E. Our study yields an average estimate of a(q) of 0.32 +/- 0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wanninkhof ( 1992). We calculate a global annual average piston velocity for CO2 of 16.7 +/- 2.9 cm/hr and a gross CO2 flux between atmosphere and ocean of 73 +/- 10 PgC/yr, significantly lower than results from previous studies.