The tropical maximum sea surface temperature (SST) occurs in the tropical western Pacific warm-pool (Fig. 1 ). The warm-pool is the region where tropical deep convection is concentrated and the latent heat release reaches a broad maximum (Spencer 1993). The latent heat release powers the Walker and Hadley circulation in the atmosphere which in turn drive the currents in the upper ocean (Sun and Liu 1996, Djikstra and Neelin 1995, Trenberth and Solomon 1994, Webster and Lukas 1992, Philander 1990, Held and Hou 1980). The atmospheric Hadley circulation and its counterpart in the ocean-the meridional branch of the wind-driven circulation-extends the influence of the warm-pool to the extratropical region (Sun 1999, Hou 1998, Lu et al. 1998). Therefore, understanding what determines the SST of the tropical Pacific warm-pool is a central issue in the study of global climate dynamics.