The controlling mechanism of the seasonal cycle of surface partial pressure of CO2 (pCO2) in the central/eastern equatorial Pacific is studied using a coupled physical-biogeochemical model. The model results indicate that the individual components of physical transport of total CO2 (TCO2) in the Ni¿o3 area are out of phase with each other such that the net TCO2 transport is an order smaller. This net transport is further nearly balanced by the biological drawdown and air-sea CO2 flux combined, resulting in a weak seasonality of surface TCO2. As a consequence, the seasonal cycle of sea surface temperature (SST) controls the surface pCO2 cycle with a maximum in boreal spring-summer and a minimum in boreal fall-winter. In turn, the seasonality of air-sea CO2 flux is largely controlled by SST and surface winds via the piston velocity of gas exchange. Perturbations to the subtle balance of TCO2 transport by dynamical processes on other timescales such as El Ni¿o, La Ni¿a and tropical instability waves, could induce significant change to the surface pCO2, in addition to the change due to SST variability.