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Paleoclimate evidence suggests that tropical volcanic eruptions could increase the likelihood of El Niño occurrence. Previous numerical model studies with zonally uniform volcanic aerosol forcing suggested the roles of land cooling‐induced monsoon suppression and ocean cooling‐induced air‐sea interaction in triggering an El Niño following the eruption. Here, we perform targeted sensitivity experiments by confining aerosol forcing over land or ocean only in the fully coupled Community Earth System Model to assess relative roles of these land and ocean cooling. Our results indicate that volcanic aerosol over land, especially over the large landmass of tropical Africa, is more effective in exciting an El Niño than over ocean. The suppressed African monsoon excites Kelvin‐wave westerly wind anomalies over the tropical central Pacific, triggering an El Niño through the Bjerknes feedback. Under the uniform ocean aerosol forcing, the Indian Ocean's fast dipole response induces a slow western Pacific El Niño‐like cooling in the second year. Plain Language Summary: Paleoclimate analyses suggest that tropical volcanic eruptions can increase the likelihood of El Niño. Scientists performed numerical model experiments with zonally uniform aerosol forcing to understand this phenomenon. Two triggering mechanisms have been proposed. One emphasizes the role of land cooling that could suppress monsoon, inducing westerly wind anomalies and triggering an El Niño. The other stresses the role of ocean cooling‐induced atmosphere‐ocean interaction (ocean dynamical thermostat mechanism) in generating an El Niño. Previous works could not answer which one is more important in exciting an El Niño since these two mechanisms happen simultaneously in their simulations. We perform a suite of sensitivity experiments to address this question by setting volcanic aerosol over land and ocean separately. Our simulations suggest that the fast land cooling‐induced land‐sea thermal contrast and tropical monsoon suppression, especially the tropical African monsoon suppression, are more critical in exciting an El Niño than the overall ocean cooling. The latter can only excite a western Pacific El Niño‐like cooling. Key Points: Pinatubo aerosol forcing is set over land and ocean respectively to investigate roles of land and ocean cooling in exciting an El NiñoThe volcano‐induced land cooling, especially tropical African cooling, plays an essential role in exciting an El Niño following eruptionsThe oceanic volcanic forcing could induce a fast dipole response in the Indian Ocean, exciting a slow western Pacific El Niño‐like cooling [ABSTRACT FROM AUTHOR]