The dynamics of the extratropical response to Madden–Julian Oscillation convection

The Rossby wave source (RWS) and the corresponding extratropical wave response to tropical convection associated with different phases of the Madden–Julian Oscillation (MJO) is investigated with the dynamical core of a climate model. The initial flow is specified to correspond to the boreal winter climatological flow and an imposed tropical heating that is derived from the observed precipitation for all eight MJO phases. One key question addressed here is why does the extratropical Rossby wave train depart the subtropics at a longitude well to the east of the RWS. For all eight MJO phases, it is found that the extratropical response over the North Pacific and North America is almost entirely due to the MJO convection over the western tropical Pacific. The RWS is excited within the first 24 h after the model heating is turned on. For MJO phases 1–3 and 8, the RWS leads to the development of a cyclonic anomaly over southeast Asia via advection of the climatological absolute vorticity by the anomalous divergent wind in the subtropics and by horizontal convergence in the Tropics. MJO phases 4–7 show opposite features. The resulting anomaly is then advected eastward by the climatological zonal wind toward the central Pacific, after which dispersion into the extratropics and the excitation of a Pacific/North American teleconnection pattern takes place.