The equatorial ocean plays an important role in the global carbon cycle. Upwelling in the oceanic equatorial belt annually supplies approximately 0.6 - 1.5 Pg C as CO₂ to the atmosphere with most of the release occurring in the equatorial Pacific. During non-El Niņo years, waters enriched in CO₂ extend from the coastal waters west of South America to approximately 160°E. The large area affected by the upwelling process makes this region the largest oceanic source of CO₂ to the atmosphere. Physical processes and biological productivity controls the strength of this source from year to year. During the 1990s, the NOAA Global Carbon Cycle program supported research directed towards a quantitative understanding of the carbon budget in the equatorial Pacific. The data were gathered onboard the NOAA ships Baldridge, Discoverer, Brown and Ka'imimoana from 1992 through 2001. Over this period, surface water pCO₂ data indicate significant interannual variations. The largest decreases in fluxes were associated with the 1991-94 and 1997-98 El Niņo events. The lower sea-air CO₂ fluxes during these two El Niņo periods were the result of the combined effects of interconnected large scale and locally-forced physical processes: 1) development of a low-salinity surface cap as part of the formation of the warm pool in the western and central equatorial Pacific, 2) deepening of the thermocline by propagating Kelvin waves in the eastern Pacific, and 3) the weakening of the winds in the eastern half of the basin. These processes serve to reduce pCO₂ values in the central and eastern equatorial Pacific towards near-equilibrium values at the height of the warm phase of ENSO. In the western equatorial Pacific there is a small but significant increase in seawater pCO₂ during strong El Niņo events (i.e., 1982-83 and 1997-98) and little or no change during weak El Niņo events (1991-94). The net impact of these interannual variations is a lower-than-normal CO₂ flux to the atmosphere from the equatorial Pacific during El Niņo. The annual average fluxes indicate that during strong El Niņos the release to the atmosphere is 0.2 - 0.4 Pg C yr^-1 compared to 0.8 - 1.0 Pg C yr^-1 during non-El Niņo years.

Our observational data and physical-biogeochemical model results reveal decadal variations over the past 40 years with a decrease in the equatorward interior flow and a reduction of 20% in the equatorial upwelling transport since the 1976-77 climate shift. The decreased volume transports causes a rise of sea surface temperature in the equatorial upwelling zone by about 0.7 °C since mid 1970s. Slowdown of the meridional overturning and decrease of the equatorial upwelling transport have significant impacts on marine ecosystem and carbon flux. The modeled primary production and phytoplankton biomass decrease by 10% over the past 40 years mainly due to reduction of upward nutrient flux. Both our data-based estimates and physical-biogeochemical model results document that the equatorial Pacific sea-to-air CO₂ flux decreased by 20% after the 1976-77 climate shift.