The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. Interannual variability of CO2 fluxes to the atmosphere in the central and eastern equatorial Pacific are primarily affected by changes in the rate of upwelling of CO2-enriched waters from the Equatorial Undercurrent and advection of CO2-depleted waters from the west at the surface along the equator. Both processes are affected strongly by El Niño.

In the above figures, DfCO2 and CO2 fluxes from the ocean to atmosphere are compared for El Niño and non-El Niño conditions. The top panel shows the DfCO2 and CO2 flux in the equatorial Pacific from December 1995 to June of 1996, when non-El Niño conditions prevailed. The bottom panel shows DfCO2 and CO2 fluxes from October 1997 to April 1998 during a period of a fully developed El Niño. The CO2 fluxes during the El Niño period are reduced by more than 100% compared with the non-El Niño condition. During 1998, this difference was sufficient to cause a measurable reduction in the growth rate of CO2 in the atmosphere as measured at NOAA Global network of atmospheric monitoring sites, including Moana Loa.