Disentangling the impact of Atlantic Niño on sea-air CO2 flux

Abstract The Atlantic Niño is a major tropical interannual climate variability mode of the sea surface temperature (SST) occurring during boreal summer that shares many similarities with the tropical Pacific El Niño. Although the tropical Atlantic is an important source of CO2 to the atmosphere, the impact of Atlantic Niño on the sea-air CO2 exchange is not well understood. We reveal that the Atlantic Niño enhances (weakens) CO2 outgassing in the central (western) tropical Atlantic. In the western basin, freshwater-induced changes in surface salinity, which considerably modulate the surface ocean CO2 partial pressure (pCO2), is the primary driver for the observed CO2 flux variations. Contrastingly, pCO2 anomaly in the central basin is dominated by the SST-driven solubility change. Here, we reveal that this multi-variable mechanism for pCO2 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 CO2 buffering capacity in the Atlantic, where the subsurface water mass contains higher alkalinity than in the Pacific. The different mechanisms driving the internannual variability in sea-air CO2 exchange in the tropical Atlantic and Pacific suggests that they may respond differently to climate change.