Interannual Variation of the Madden–Julian Oscillation during Austral Summer

Interannual variability of the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the Tropics, is investigated during the extended austral summer season November–March, which is when the MJO is most prominent. Indexes of the level of MJO activity are developed using outgoing longwave radiation and zonal wind analyses at 850 mb for 1974–98. Based on these indexes, interannual variations in the level of MJO activity are found to be primarily associated with changes in the number of discrete MJO events each year and with changes in the intensity of intraseasonal convection across the Indian and western Pacific Oceans, where the MJO is normally prominent. An eastward shift of MJO activity east of the date line does occur during El Niño events. However, the overall level of MJO activity is found to be uncorrelated with El Niño, except during exceptional warm events when MJO activity is diminished. The level of MJO activity is shown to be weakly related to sea surface temperature anomalies in the equatorial Indian and western Pacific Oceans, but the weak correlations imply that much of the year-to-year variability of the MJO is internally generated, independent of any slowly varying boundary forcing. Such year-to-year variations of the intensity of the MJO are, however, associated with changes in the distribution of seasonal mean convection across the tropical Indian and western Pacific Oceans. This interannual variation of convection unrelated to SST variability may thus act as a limit to seasonal predictions that rely heavily on equatorial Pacific SST anomalies.