Disentangling the impact of Atlantic Ni(n)over-tildeo on sea-air CO2 flux

Atlantic Nino 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 Nino. Although the tropical Atlantic is an important source of CO2 to the atmosphere, the impact of Atlantic Nino on the sea-air CO2 exchange is not well understood. Here we show that the Atlantic Nino 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 (pCO(2)), are the primary driver for the observed CO2 flux variations. In contrast, pCO(2) anomalies in the central basin are dominated by the SST-driven solubility change. This multi-variable mechanism for pCO(2) 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 response of sea-air CO2 flux to the Atlantic Nino variability shows a dipole pattern in the equatorial Atlantic characterized by a freshwater-induced anomaly in the western basin, and a sea surface temperature-induced anomaly in the central basin.