This study proposes a mechanism that explains the marked shift in the correlation between the El Ni¿o/Southern Oscillation (ENSO) and the isotopic composition (δ18Oc) of a Porites coral from the Chagos Archipelago (71¿E/5¿S). Only after the mid-1970s a strong ENSO signal emerges in the δ18Oc during the analyzed period 1950--1994. In the 1970s, the increasing sea surface temperature (SST) shifted the mean SST closer to the deep convection threshold at about 28.5¿C. ENSO-related SST variability largely controls the deep convection and precipitation in the central equatorial Indian Ocean (CEIO) when the SST is at this critical level. The anomalies in the precipitation induce changes in the isotopic composition of the surface ocean waters. The precipitation signal amplifies the SST signal in the coral δ18Oc and raises the correlation to ENSO. The presented results have important implications for the reconstruction of ENSO indices from corals within the Indian Ocean.