Intraseasonal (30–60 day) variability in the global tropics: principal modes and their evolution

The evolution of the 30–60 day intraseasonal (Madden–Julian) oscillation (MJO), during borealwinter and summer, is studied by means of extended empirical orthogonal function analyses.For both seasons, the patterns describe an eastward traveling large-scale oscillatory regime witha period of approximately 45 days. Several atmospheric variables display a zonal wavenumberone pattern approximately symmetric about the equator. The seasonal variation of the MJOis investigated, especially in terms of the intraseasonal variability of the tropical convection.We find that the latitudinal location of the largest outgoing longwave radiation anomalies isdirectly related to the seasonal meridional shifts of the tropical convection in the Indo-Australianand Central American–South American regions. In the eastern hemisphere, the MJO affectsthe monsoons over India during boreal summer and over Australia during austral summer. Inthe western hemisphere the MJO has the greatest impact on convection over Northeast Brazilduring austral summer and over Central America during boreal summer. The MJO-relatedevolution of 925-hPa specific humidity patterns (not previously documented) show remarkableseasonal dependence. The boreal summer patterns contain a large-scale component quite similarto that shown in the sea level pressure patterns. The austral summer patterns show strongregional variability that might indicate the interactions of the MJO with deep tropical convectionin certain areas, such as along the South Pacific Convergence Zone and over northernSouth America.