The anomalous, low-level anticyclone near the Philippines and suppressed convection over the western Pacific associated with the mature warm phase of El Ni¿o--Southern Oscillation (ENSO) have been suggested as important elements in the interaction between ENSO and the East Asian monsoon. We examined the causes of these anomalies in the circulation and convection using a newly developed, linear baroclinic model that includes interactive moist processes of the cumulus convection and surface heat fluxes. A conventional version of the linear model forced by prescribed heating indicates that the diabatic cooling due to suppressed convection over the maritime continent generates the Philippine Sea anticyclone. From a series of the moist linear model experiments, we found that the modest warming of the Indian Ocean, in addition to the strong warming in the central--eastern Pacific and weak cooling in the western Pacific, is significant to suppress the convection over the maritime continent. Observed data also show a coincidence of the development between the Philippine Sea anticyclone, Indian Ocean warming, and the ascending motion over the Indian Ocean, supporting the model results. The above results indicate that the atmosphere-ocean system in the Indian Ocean may be one of important factors to improve predictability of the East Asian climate during ENSO.