The global scale structure of the MJO

The global scale structure of the MJO is analyzed using the globally gridded ERA-Interim dataset (1979-2011). The study is separated into two parts:;Part I of this study concentrates on how the tropical wind field evolves during a composite event. The wind field over the tropics (30° S - 30° N) is partitioned into three components using free-space Green's functions: (1) a non-divergent component (2) an irrotational component and (3) a background, or environmental flow that is interpreted as the influence on the tropical flow due to vorticity and divergence elements outside of the tropical region. The flow partitions are studied through a composite analysis based on the Wheeler-Hendon MJO index. Anomalies in the background flow are strongest in the western and central Pacific, possess an equivalent barotropic structure, and show an eastward propagation. By contrast, the irrotational and non-divergent winds possess a first mode baroclinic structure. An oscillation in the zonally averaged background flow with the MJO phases is observed but contributes little to tropical angular momentum when compared to the non-divergent flow.;Part II of this study uses the two leading principal components of the unfiltered daily 150 hPa minus 850 hPa global velocity potential in the ERA Interim data are used as time-varying indices of the Madden Julian Oscillation (MJO). The MJO vertical structure is decomposed into baroclinic and barotropic modes by performing a maximum covariance analysis on vertical profiles of zonal wind (u) and geopotential height (Z) in the equatorial belt. The baroclinic mode, which is present year round with much the same structure, is dominated by an equatorially-trapped Kelvin wave flanked by Rossby waves that propagate eastward with the same phase speed, circumnavigating the globe in a full MJO cycle. The weaker barotropic mode consists of equatorially-symmetric Rossby waves only. The primary centers of the Rossby waves lie in the 25°-30° latitude belt, beyond the range in which the baroclinic / barotropic decomposition is useful. While the u anomalies in the Rossby waves are passing through the climatological-mean jet exit region in the central Pacific, the waves extract kinetic energy from the background flow, giving rise to strong Z anomalies that project upon the Pacific / North American (PNA) pattern. This occurs in the phase of the MJO cycle when enhanced / suppressed convection is passing over the Maritime Continent.