ENSO-driven CO2 efflux variability and the role of the upwelling region on the carbon exchange in the Northern Humboldt Current System

The role played by El Niño-Southern Oscillation (ENSO) on modulating the oceanic carbon outflux in the Northern Humboldt Current System (NHCS) is assessed with the use of outputs from a coupled physical-biogeochemical hindcast which permits to evaluate the interannual variability of the surface seawater partial pressure of carbon dioxide, hereinafter pCO2sw, and reconstruct associated air-sea CO2 fluxes from 1998 through 2015 at monthly timescales. In addition, surface dissolved inorganic carbon (DIC) concentrations were inferred from a recent neural network methodology prescribed with the same outputs of the biogeochemistry hindcast. Results show a large spatiotemporal variability of the CO2 exchange to nine El Niño and La Niña episodes throughout the period of analysis. Within the coastal and equatorial upwelling region, that variability results from combined ENSO-driven ΔpCO2 (pCO2sw - pCO2air) and wind speed variations, although the latter in a minor proportion during ENSO peaks. Specifically, it is found that the weak CO2 source behavior during an average El Niño episode is mainly caused by a weakening of ΔpCO2 which is partially compensated by more efficient CO2 exchange at the air-sea interface due to the strengthening of the upwelling-favorable winds. Conversely, the relatively strong CO2 source behavior during an average La Niña episode results from more efficient upwelling which brings colder but CO2-rich waters to the surface, therefore, increasing pCO2sw and the associated CO2 efflux. Moreover, it is estimated that during an average La Niña episode, 3.3 million metric tons of carbon is additionally emitted contributing to the atmospheric CO2 accumulation. In contrast, during an average El Niño episode, the total amount of carbon retained is about 24.7 million metric tons of carbon that normally would have been lost to the atmosphere as CO2, rendering the NHCS a key region for the global atmospheric carbon budget.