Abstract. The zonal displacement of the western Pacific warm pool (WPWP) and the meridional displacement of the eastern Pacific warm pool (EPWP) and their responses to wind anomalies over the tropical Pacific were investigated. Joint effect of the WPWP and EPWP on ENSO was examined based on a joint effect index, which is a combination of the standardized anomaly time series of the eastern edge of the WPWP and the southern edge of the EPWP. Results show that both WPWP and EPWP are major providers of warm water in the eastern equatorial Pacific. The anomalous eastward extension of the WPWP and abnormal southward extension of the EPWP can supply a large amount of warm water into Nino3 region of the north equator, result in dramatic local SST increase, and trigger El Niño. To the contrary, as scope of the WPWP retreats westward and the EPWP retreats northward, a La Niña will outburst. One cannot separate apart the roles played by the WPWP and EPWP on ENSO, and the joint effect of both warm pools must be considered. A joint index of 1.6 means a new El Niño event is likely to happen.