Combined effects of Arctic tropospheric warming and La Niña events on enhanced Eurasian cold anomalies

Abstract

This study investigates the combined effects of Arctic tropospheric warming and La Niña events on climate variations in Eurasia during boreal winter by utilizing the reanalysis data and the output of CMIP6 models. Results show that the “Warm Arctic-Cold Eurasia” pattern emerges with enhanced intensity under the combined effects compared with the individual effect of Arctic tropospheric warming or La Niña events. The significantly intensified Eurasian cold anomaly is observed in the north of Lake Baikal during the combined years. However, this cold anomaly shrinks substantially in warm Arctic years while shifting southwestward with a weaker intensity in La Niña years. Additionally, the winter atmospheric variability under the combined effects is much stronger than that under the individual effect of these two factors. Further investigation reveals that the La Niña event is probably crucial to dominating the basic distribution of winter temperature variability and the associated tropospheric circulation anomalies. Recent Arctic tropospheric warm anomalies are also essential, possibly amplifying the impact of the La Niña event on Eurasia through the weakened stratospheric polar vortex. These two factors jointly contribute to the enhanced “+−+” wave train that displays over mid-high latitudes under the combined effects. This “+−+” wave train exhibits alternative anticyclonic, cyclonic, and anticyclonic circulation anomalies over the Ural Mountains, Lake Baikal, and North Pacific, respectively. It guides wave energy from the Barents-Kara Seas (BKS) along the Ural Mountains to Lake Baikal and promotes the development of the Ural blocking anomaly that favors the intensified extreme cold temperature in Eurasia. The results of the CMIP6 models largely replicate some major features of the combined effects in the observations.