Assessing the seasonality of the oceanic sink for CO2 in the northern hemisphere

Abstract

Seasonal CO2 fluxes are estimated from quarterly maps of ΔpCO2 (difference between the oceanic and atmospheric partial pressure of CO2) and associated error maps. ΔpCO2 maps were interpolated from pCO2 measurements in the North Atlantic and the North Pacific Oceans using an objective mapping technique. Negative values correspond to an uptake of CO2 by the ocean. The CO2 flux for the North Atlantic Ocean, between 10°N and 80°N, ranges from −0.69 GtC/yr, for the first quarter (January‐March), to −0.19 GtC/yr for the third quarter (July‐September) using the gas exchange coefficient of Tans et al. [1990], satellite wind speeds, and a correction for the skin effect. On annual average, the North Atlantic ocean (north of 10°N) is a sink of CO2 ranging from −0.23 ± 0.08 GtC/yr (gas exchange coefficient of Liss and Merlivat [1986] with Esbensen and Kushnir [1981] wind field) to −0.48 ±0.17 GtC/yr (gas exchange coefficient of Tans et al. with satellite wind field). The CO2 flux for the North Pacific, between 15°N and 65°N, ranges from −0.66 GtC/yr from April to June to zero from July to September. For the Atlantic, the errors are generally small, that is, less than 0.19 GtC/yr, but for the Pacific considerably larger uncertainties are generated due to the less extensive data coverage. The northern hemisphere ocean (north of 10°N) is a net sink of CO2 to the atmosphere which is stronger in spring (April‐June), due to the biological activity, with an estimate of −1.23 ± 0.40 GtC/yr averaged over this period. The annual mean northern hemisphere ocean flux is −0.86 ± 0.61 GtC/yr.