Investigating how background state affects the propagation of the MJO

The Madden-Julian Oscillation (MJO) is a cycle of anomalous tropical wind and precipitation that starts in the Indian Ocean and propagates eastward into the Pacific Ocean. Variability is dominated by 30-60 day timescales. The processes that cause the MJO to propagate eastward are not well understood. Maloney et al. (2010) argued that zonal moisture advection is important to the eastward propagation of the MJO, and that advection of anomalous humidity by the background low-level zonal wind is integral to this propagation. Here, NCEP reanalysis winds are used to show that MJO propagation does not speed up when background zonal winds in the warm pool increase. In fact, propagation speed and background low-level zonal winds are significantly negatively correlated. Further, it is shown here that while the strength of intraseasonal zonal moisture advection increases with MJO amplitude, MJO amplitude and zonal moisture advection strength show no significant relationship to MJO propagation speed. Other factors such as surface evaporation anomalies may help mediate MJO propagation speed. Understanding MJO propagation could advance the MJO and tropical cyclone forecasts on weekly timescales.