Satellite data show that the total amount of deep convection was nearly constant from year to year over the warm pool region of the tropical Indian and Pacific oceans, but the spatial distribution varied. In the 1986--1987 warm El Ni¿o/Southern Oscillation event, the maximum of deep convective activity was over the high sea surface temperature (SST) of the west central Pacific, with a local minimum over the eastern Indian Ocean. In the 1988--1989 cold ENSO event the maximum convective activity switched to the eastern Indian Ocean, with a minimum over the west central Pacific. The SST changed very little over the eastern Indian Ocean from year to year, and the centers of convective activity were always collocated with the four-month mean low-level westerlies in each year. The latitude of the cloudy region of the intraseasonal oscillation (ISO) varied interannually. The interannual differences in deep convective activity over the warm pool were accounted for almost exclusively by the occurrence or nonoccurrence of extremely large, long-lasting, deep mesoscale cloud systems (cloud tops <208 K with horizontal dimensions ~300--700 km). Such cloud systems occurred more frequently than normal over the western Pacific during the warm ENSO event and over the eastern Indian Ocean during the cold event. They occurred more frequently over the tropical eastern Indian and western Pacific oceans than over the maritime continent, thus accounting for the seesaw pattern of the observed cold cloudiness between the two oceanic regions on both interannual and intraseasonal timescales. ¿ 1997 American Geophysical Union