Disentangling the impact of Atlantic Niño on sea-air CO flux.

Atlantic Niño is a major tropical interannual climate variability mode of the sea surface temperature (SST) that occurs during boreal summer and shares many similarities with the tropical Pacific El Niño. Although the tropical Atlantic is an important source of CO to the atmosphere, the impact of Atlantic Niño on the sea-air CO exchange is not well understood. Here we show that the Atlantic Niño enhances (weakens) CO outgassing in the central (western) tropical Atlantic. In the western basin, freshwater-induced changes in surface salinity, which considerably modulate the surface ocean CO partial pressure (pCO), are the primary driver for the observed CO flux variations. In contrast, pCO anomalies in the central basin are dominated by the SST-driven solubility change. This multi-variable mechanism for pCO anomaly differs remarkably from the Pacific where the response is predominantly controlled by upwelling-induced dissolved inorganic carbon anomalies. The contrasting behavior is characterized by the high CO buffering capacity in the Atlantic, where the subsurface water mass contains higher alkalinity than in the Pacific.