Some potential forcing mechanisms of the year-to-year variability of the tropical convection and its intraseasonal (25–70-day) variability

Three potential forcing mechanisms for the convective signal of the Madden–Julian Oscillation (MJO) are investigated. Outgoing longwave radiation (OLR) is used to indicate convection, and data from 1974 to 1994 are subjected to a 25–70-day bandpass filter to represent the MJO. The focus of the paper is on interannual variability and the three mechanisms diagnosed are: tropical sea-surface temperature (SST); the phase of El Niño–Southern Oscillation (ENSO); and atmospheric precipitable water (W). The respective roles of these potential forcings are also considered for the year-to-year variability of the seasonal anomalies of mean convection. Observed seasonal anomalies of both mean convection and MJO activity over the equatorial Pacific from 160°E to the coast of South America are highly correlated to collocated anomalies of SST and the phase of ENSO. During El Niño winters the eastward-propagating convective MJO events penetrate farther than normal into the central Pacific before curving poleward into the Southern Hemisphere. Over the maritime continent, moist convection is significantly suppressed during El Niño. A corresponding signal in the MJO activity is confined to the Philippine Sea and North Australia during boreal winter, and to the Bay of Bengal in autumn. In contrast, there is no uniform MJO response over the eastern Indian Ocean when the latest five ENSO warm events (1976–1977, 1982–1983, 1986–1987, 1991–1992 and 1993–1994) are considered. On a seasonal time-scale, W over the Indian and Pacific Oceans is closely related to convection anomalies, as well as to the variance of the high-frequency convective perturbations. In contrast, convective activity of the MJO shows no relation to variations in the amount of W. It is concluded that the interannual variability of the convective MJO signal over the Indian Ocean and western Pacific Warm Pool is largely determined by the frequency and duration of ‘weak-signal’ episodes rather than by modulations of its amplitude.