The surface water pCO2 data from the past two decades indicate significant seasonal to interannual variability, and the size of the equatorial Pacific CO2 source is strongly influenced by El Ni¿o and La Ni¿a events. A basin-scale ocean circulation-biogeochemistry model is developed to study the carbon cycle in the equatorial Pacific for the period of 1980--2003. The model produces strong spatial and temporal variability in sea minus air pCO2 (50--170 ¿atm) and sea-to-air CO2 flux (1--5 mol C m-2 yr-1). The magnitude, spatial pattern, and seasonal to interannual variability in the model fields are in general agreement with the observations. Our analyses have demonstrated that dissolved inorganic carbon (DIC) plays a dominant role in determining the interannual variability of the sea surface pCO2 in the equatorial Pacific. However, sea surface temperature (SST) also has significant influence on the spatial and temporal variability of the sea surface pCO2 in particular during warm periods. At seasonal timescales, the sea surface pCO2 is relatively high both in boreal spring and fall, but low in boreal summer in the eastern equatorial Pacific. While the high sea surface pCO2 in boreal fall is associated with the seasonal upwelling of carbon-rich water, the high sea surface pCO2 in boreal spring results mainly from the seasonal warming (e.g., high SST). At interannual timescales, the sea surface pCO2 is largely associated with the El Ni¿o--Southern Oscillation (ENSO) phenomenon, showing high values during cold ENSO phase but low ones during warm ENSO phase. The overall spatial and temporal variations of the sea surface pCO2 are dominated by physical processes (e.g., seasonal upwelling and the ENSO cycle). However, biological uptake also plays an important role in modulating the variability of the sea surface pCO2, and determining the strength of the equatorial Pacific CO2 source. On an annual basis, the integrated DIC over the top 50 m of the equatorial Pacific is approximately balanced between the supply due to physical processes (1.47 Pg C) and removal due to the biological activity (0.87 Pg C) and outgassing (0.6 Pg C).