The Rossby Wave Train Related to the Madden–Julian Oscillation and Its Impacts over North America

Tropical diabatic heating can modulate the circulation not only in the tropics but also in the extratropics. Many previous studies have shown that tropical diabatic heating source could excite stationary Rossby waves that propagate into the extratropics and significantly modulate the midlatitude circulation. Therefore, the Madden-Julian Oscillation (MJO), which is characterized by large-scale eastward propagating tropical convection, can have large impacts on the extratropical circulation. Many previous studies found that MJO can influence the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), the Pacific-North American pattern (PNA), midlatitude temperature, precipitation and storm track activity.

We investigate the MJO impact on North America during boreal winter by examining lag composites of upper level stream function anomalies relative to individual MJO phases. We also make use of the wave activity flux, which is parallel to the local three-dimensional group velocity in a zonal varying basic flow, to illustrate the propagation of the MJO-induced Rossby wave trains. The results show clear signatures of Rossby wave trains that propagate from the tropics, across the Pacific into North America in lag composites relative to MJO phases 3 and 8. It is likely that these Rossby wave trains have originated from the MJO-related dipole structure of tropical convection anomalies in the Indian Ocean and western Pacific.

Further analyses show that the anomalous cyclonic and anticyclonic upper level anomalies, which is associated with the MJO induced Rossby wave trains, can significantly modulate the surface temperature over eastern US. These anomalous upper level flow can also shift the westerly jet on intraseasonal time scales. In addition, the north-south shift of the jet can lead to shifts of storm track activity, since a stronger jet stream generally results in more storminess. As winter precipitation are mostly brought by extratropical cyclones, the MJO induced Rossby wave trains are also found to give rise to significant impacts on precipitation over North America.