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 boreal winter 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 with a period of approximately 45 days. Several atmospheric variables display a zonal wavenumber one 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 is directly related to the seasonal meridional shifts of the tropical convection in the Indo-Australian and Central American–South American regions. In the eastern hemisphere, the MJO affects the 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 Brazil during austral summer and over Central America during boreal summer. The MJO-related evolution of 925-hPa specific humidity patterns (not previously documented) show remarkable seasonal dependence. The boreal summer patterns contain a large-scale component quite similar to that shown in the sea level pressure patterns. The austral summer patterns show strong regional variability that might indicate the interactions of the MJO with deep tropical convection in certain areas, such as along the South Pacific Convergence Zone and over northernSouth America.