A triggering factor of the eastward propagation of the Madden–Julian oscillation observed in late October during CINDY2011

A triggering factor of the eastward propagation of the Madden–Julian oscillation (MJO) is proposed from a case study of the first MJO generated in late October during CINDY2011. The proposed scenario is that the eastward propagation of the MJO is triggered by an extratropical cyclone in the Southern Hemisphere. The ridge and trough pair meridionally extending between 30°S~15°N accompanied by the extratropical cyclone was found to be completely synchronized with the eastward propagating MJO convection. The ascending areas of the cold front extending from the extratropical cyclone partially combined with those of the MJO and the large-scale ridge and trough below 500 hPa were formed across the midlatitudes in the Southern Hemisphere and the tropics. The convection center of the MJO shifted eastward as a result of the westerly winds in the tropics, expanding eastward by the zonal pressure gradient force between the ridge and trough.