Seasonality in the Madden-Julian oscillation

This study, using observational data and global reanalysis products, synthesizes existing knowledge on the seasonality in the Madden-Julian oscillation (MJO) and discusses its possible dependence on the mean background state. The seasonality in the MJO is documented in terms of its components in low-level zonal wind and precipitation. A single peak season (boreal winter) exists near the equator in the Indian and western Pacific Oceans. For the broad tropical region, however, the seasonality in the MJO is featured by a latitudinal migration across the equator between two peak seasons. The primary peak season is in boreal winter (December-March), during which MJO signals are mainly confined to the Indian and western Pacific Ocean and reach their maxima in the South Pacific convergence zone. The secondary peak season is boreal summer (June-September), during which the strongest MJO occurs north of the equator from the Bay of Bengal to the South China Sea and another, separated, region of strong MJO signals is located in the eastern Pacific warm pool off the Central American coast. The seasonal cross-equatorial migration is the strongest in the western Pacific in both wind and precipitation and much weaker in the Indian Ocean in precipitation. Strong seasonal activities of the MJO appear to prefer mean westerlies or weak zonal winds at the surface and low level (850 hPa) and mean lower-level moisture convergence in the western Pacific. The relationship between the MJO and its mean background state is much less clear in the Indian Ocean. Implications of these observations are discussed.