Extremely Weak Cold-Air Mass Flux and Extratropical Direct Meridional Circulation Linked to the Record-Warm Winter 2019/2020 over East Asia

抄録

We examine the relationship between the record-warm winter (DJF) 2019/2020 over East Asia and the extremely weak hemispheric circulation anomaly. During this period, the polar cold-air mass (PCAM) flux over East Asia was the weakest on record since the DJF 1958/1959 due to the weak Siberian High. The zonal averaged surface temperature over the Northern Hemisphere mid-latitudes in DJF 2019/2020 was the highest since DJF 1958/1959 and was linked to the weakest PCAM flux at the mid-latitudes. The zonal mean field during this period was characterized by weak stationary waves, weak wave activity as diagnosed by Eliassen-Palm flux, and, to balance with this, record-weak extratropical direct meridional circulation (EDC). The weak EDC corresponded to weaker-than-normal meridional heat exchange and was consistent with warm anomalies in the Northern Hemisphere mid-latitudes, since the lower branch of EDC corresponds to zonally averaged cold air outflow. In addition, the statistical relationship also indicates the EDC intensity is negatively correlated with the surface temperature anomaly over East Asia.