Propagation and the Vertical Structure of the Madden–Julian Oscillation

The Madden-Julian oscillation (MJO) dominates tropical variability on time scales of 30-70 days. During the boreal winter-spring it is manifested as an eastward propagating disturbance, with a strong convective signature over the Eastern Hemisphere. The space-time structure of the MJO is described using the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, Advanced Very High Resolution Radiometer outgoing longwave radiation, observed sea surface temperature, and the Climate Prediction Center Merged Analysis of Precipitation. Empirical orthogonal function analysis is used to identify the convective signature of the MJO, and regression is used to identify key relationships with the convection. Compared to analyzing successive years of data, the selection of years of strong MJO activity results in a more robust lead-lag structure and an increase in explained variance. The MJO exhibits a rich vertical structure, with low-level moisture convergence being well defined when the convective anomalies are strong, and there is evidence that free-tropospheric processes also play a role in the MJO life cycle. The westward vertical tilt is most apparent over the western Pacific. Over the Indian Ocean the system is more vertically stacked, principally because of the strong subsidence of the inactive phase of the MJO, which lies to the east of the convection. As the Kelvin wave decouples from the convection near the date line, a sea level low pressure surge, previously discussed by A. J. Matthews, transits the eastern Pacific and Atlantic Oceans. Here the link of the zonal wind stress and low-level divergence to the pressure surge is explored. The pressure gradient gives rise to westerlies that propagate rapidly to the east, and it may play role in the development of the MJO convection in the western Indian Ocean, which occurs in an easterly basic state, and conditions not consistent with the low-level moisture convergence paradigm.