Tropical-extratropical interactions associated with East Asian cold air outbreaks. Part II: Intraseasonal variation

Intraseasonal variations of East Asian cold air outbreaks (CAOs) in relation to the tropical atmosphere during 34 winters (DJF) are investigated. This study is a continuation of Part I, which discussed the interannual variability of East Asian CAOs. Two types of quantitative East Asian CAOs, western and eastern CAOs, are examined. Their variations are identified by the zonal integration of equatorward flux of cold air mass (CAM) below 280 K at 45°N over 90°-135°E and 135°E-180°. A day-lagged regression analysis reveals that peaks of intraseasonal western and eastern CAO events are preconditioned by large-scale tropical convection anomalies resembling particular phases of the Madden-Julian oscillation (MJO). Western CAO events tend to occur when the convective phase of the MJO crosses over the Maritime Continent. In contrast, eastern CAO events are triggered by the MJO over the western Pacific. Observations of MJO-related atmospheric anomalies indicate the important roles of poleward Rossby wave trains in affecting extratropical East Asian CAOs. The barotropic Rossby waves develop negative geopotential height anomalies in midlatitude East Asia, which then induce a low-level equatorward cold airflow. Several experiments in an atmospheric model using prescribed MJO-like heating anomalies demonstrate that the Maritime Continent MJO and the western Pacific MJO clearly affect the equatorward CAM flux over the western and eastern CAO regions, respectively. Compared with the western CAO, the eastern CAO shows a more robust response to the MJO because of stronger wave activity during the western Pacific MJO.