Collapse of the tropical and subtropical North Atlantic CO2 sink in boreal spring of 2010

International audience; Following the 2009 Pacific El Niño, a warm event developed in the tropical and subtropical North Atlantic during boreal spring of 2010 promoted a significant increase in the CO2 fugacity of surface waters. This, together with the relaxation of the prevailing wind fields, resulted in the reversal of the atmospheric CO2 absorption capacity of the tropical and subtropical North Atlantic. In the region 0–30°N, 62–10°W, this climatic event led to the reversal of the climatological CO2 sink of −29.3 Tg C to a source of CO2 to the atmosphere of 1.6 Tg C from February to May. The highest impact of this event is verified in the region of the North Equatorial Current, where the climatological CO2 uptake of −22.4 Tg for that period ceased during 2010 (1.2 Tg C). This estimate is higher than current assessments of the multidecadal variability of the sea-air CO2 exchange for the entire North Atlantic (20 Tg year−1), and highlights the potential impact of the increasing occurrence of extreme climate events over the oceanic CO2 sink and atmospheric CO2 composition. Anthropogenic CO2 emission to the atmosphere is widely considered the main cause of current climate change. Since the industrial revolution, the oceans have absorbed about 40–50% of all the anthropogenic CO2 emissions 1,2 , thus mitigating its effects over the Earth climate system. Nevertheless, studies have suggested that the oceanic C sink may be decreasing for the last 50 years 3,4. Whether these changes are caused from anthropogenic climate change or internal climate variability is still uncertain 4–6 , but they could significantly impact future atmospheric CO2 levels. The North Atlantic north of 18°N is one of the oceanic regions of strongest CO2 uptake (420 ± 110 Tg C y−1) representing 30% of the global oceanic CO2 sink 7 , and an estimated interannual and multidecadal CO2 uptake variability of 20 Tg C yr −1 7–9. The area of the North Atlantic with CO2 uptake that is most sensitive to climate forcing (changes in sea surface temperature (SST) and wind speed) is the subtropical North Atlantic 10. There, the sea-air CO2 exchange is mainly controlled by sea surface temperature (SST) changes due to its permanent oligotrophic conditions outside upwelling areas, thus presenting the strongest seasonal variability in the sea-air CO2 exchange of this ocean 10. Recent warming identified in the region, partially linked to anthropogenic forcing, is already reducing its CO2 uptake 11. Large-scale climate modes such as the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the El Niño-Southern Oscillation (ENSO) can mitigate or exacerbate anthropogenic-driven SST increase in the North Atlantic and its effects over the sea-air CO2 exchange 12. In 2009, a strong El Niño event occurred in the Pacific. ENSO events are known to promote positive SST anomalies in the northern tropical Atlantic through a teleconnection driven through the troposphere with a time lag of a few months 13. In boreal spring of 2010, this event coincided with a strong positive AMO resulting in a strong positive SST anomaly associated with negative wind speed anomalies in the tropical Atlantic 14,15 (Fig. 1). Its impact in the equatorial Atlantic sea-air CO2 exchange has been explored by Lefèvre et al. 14 , who showed that during this period the Intertropical Convergence Zone (ITCZ) was shifted northward compared to its climatological position, associated with a significant reduction of rainfall. The CO2 undersaturation promoted by the intense rainfall associated with the ITCZ was thus significantly reduced and, consequently, CO2 outgassing in this area increased. As their study mainly focused on the Western equatorial Atlantic, the impact of this climatic event in the sea-air CO2 exchange in the North Atlantic is currently unknown.