Impacts of the Madden–Julian Oscillation on wintertime Australian minimum temperatures and Southern Hemisphere circulation

As a potential source of multiweek predictability, we investigate impacts of the Madden–Julian Oscillation (MJO) on temperature extremes during Austral winter using observational data analysis. We find a significant MJO influence on weekly mean minimum temperatures across much of northern and eastern Australia, with lower than normal minimum temperatures tending to occur during MJO phases 6 and 7. The likelihood of extreme weekly mean minimum temperatures also increases by at least of factor of 2 during these phases. In contrast, negligible impacts on maximum temperatures are observed. The proximate cause of the lower than normal minimum temperatures in these MJO phases is the anomalous equatorward advection of cool/dry continental air and enhanced night-time radiative cooling due to the drier conditions. The lack of any impact on maximum temperatures presumably stems from compensating day time warming from enhanced incoming shortwave radiation. The circulation anomalies over Australia during MJO phases 5–7 are shown to be a combination of the direct baroclinic response to the anomalous tropical convection driven by the MJO and the Rossby wave train that propagates from the tropics to the extratropics that is primarily confined to the Australian sector. The confinement of the extratropical Rossby wave train to the Australian sector results from a combination of localization of the Rossby wave source to the north of Australia and localization of wave propagation stemming from the refractive characteristics of the mean state zonal wind, which does not support tropical-extratropical wave paths to the east of Australia. An extratropical wave source due to feedback from transient eddies acts as an effective source of the extratropical response to the south of Australia and for the wave train that impinges on South America.