We analysed the effects of local-(sea-surface temperature [SST] and windspeed) and large-(multivariate El Niño index) scale weather conditions and timing of breeding on reproductive parameters of tropical roseate terns Sterna dougallii on Aride Island, Seychelles, using up to 17 years of data. The size of the breeding population and initiation of breeding were negatively and positively correlated, respectively, with both SST and the multivariate El Niño index for the laying season (MayJune). It is the first time that an El Niño index obtained for the Pacific Ocean is shown to be correlated with reproductive parameters of seabirds in the Indian Ocean. Hatching success decreased significantly with later initiation of breeding. Virtually no chicks fledged when breeding started in June (40% of the years monitored). We suggest that oceanographic conditions over a relatively large scale have an influence on tern arrival date to the breeding grounds and that SST around the breeding colony influences the number of birds that attempt to breed. Despite the influence of factors such as predatory fish on food availability, this influence appears to be overridden by the importance of weather events and oceanographic conditions, which are likely to determine marine productivity. This study suggests that ecosystem-level phenomena appear to be important in shaping the population dynamics of tropical roseate terns.