Why are the Slowest MJO Events Weaker than Those in its Normal Range?

The Madden Julian oscillation (MJO) is a disturbance of zonal wind and precipitation that propagates eastward in the equatorial region. The presence of the 40-50 day spectral peak implies that slow events, or those that occur at lower frequencies than the spectral peak, tend to be weaker than events at the spectral peak. If events on the low frequency side of the spectral peak continued to grow in amplitude with reduced phase speed, the spectrum would just be red. This work presents a wavelet regression analysis of slow and fast MJO signals during northern winter to assess how MJO moisture and wind patterns could explain why slow MJO events achieve lower amplitude. Results show that slow MJO events favor a ridge anomaly over Europe, which drives cool dry air toward Africa and Arabia as the active convection emerges over the Indian Ocean. Dry air from this source, together with slower event evolution, diminishes MJO convection, concentrating it in the Rossby gyres off the equator.