Influences of the NAO on the North Atlantic CO_2 Fluxes in Winter and Summer on the Interannual Scale

The differences in the influences of the North Atlantic Oscillation (NAO) on the air–sea CO₂ fluxes (fCO₂) in the North Atlantic (NA) between different seasons and between different regions are rarely fully investigated. We used observation-based data of fCO₂, surface-ocean CO₂ partial pressure (pCO_2sea), wind speed and sea surface temperature (SST) to analyze the relationship between the NAO and fCO₂ of the subtropical and subpolar NA in winter and summer on the interannual time scale. Based on power spectrum estimation, there are significant interannual signs with a 2–6 year cycle in the NAO indexes and area-averaged fCO₂ anomalies in winter and summer from 1980 to 2015. Regression analysis with the 2–6 year filtered data shows that on the interannual scale the response of the fCO₂ anomalies to the NAO has an obvious meridional wave-train-like pattern in winter, but a zonal distribution in summer. This seasonal difference is because in winter the fCO₂ anomalies are mainly controlled by the NAO-driven wind speed anomalies, which have a meridional distribution pattern, while in summer they are dominated by the NAO-driven SST anomalies, which show distinct zonal difference in the subtropical NA. In addition, in the same season, there are different factors controlling the variation of pCO_2sea in different regions. In summer, SST is important to the interannual variation of pCO_2sea in the subtropical NA, while some biogeochemical variables probably control the pCO_2sea variation in the subpolar NA.