It has been suggested, based on results from climate-model simulations, that because of global warming, the sea-surface temperature (SST) will likely increase, which will lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed TC intensity over the western North Pacific (WNP) for the past 40 years. It is found that the average TC intensity over a season shows no significant change with that of SST over the WNP but increases with the SST over the equatorial eastern Pacific Ocean. The mean annual TC intensity is generally higher (weaker) during an El Niņo (a La Niņa) year. Such an interannual variation of the annual TC intensity is mainly constrained by the large-scale atmospheric factors closely related to El Niņo/Southern Oscillation phenomenon. Large-scale dynamic and thermodynamic factors such as low-level relative vorticity, vertical wind shear and moist static energy are found to be more important than the local SST in controlling the annual TC intensity. Such results are shown to be physically consistent with one another and with those from previous studies on the interannual variations of TC activity. The results point to the danger of drawing conclusions on the effect of global warming on TC intensity based on current climate-model simulations that are not designed for such predictions.