A potential driver of Eurasian winter cooling in CESM large ensemble

Despite the ongoing global warming, Eurasian winter surface air temperature (SAT) has been decreasing in recent decades. This study investigates the nature of Eurasian winter cooling and its reproductivity in the Community Earth System Model Large Ensemble simulation (CESM-LE). It is found that Eurasian winter cooling and the related atmospheric circulation change are not captured by the model ensemble mean. When 40 ensemble members are divided into two groups, ensembles with Eurasian cooling tend to show a positive sea surface temperature (SST) trend over the western Pacific warm pool, whereas the other group has the opposite SST trend. The causal relationship between tropical SST warming and Eurasian winter cooling is further tested by conducting a series of linear baroclinic model experiments. These experiments reveal that the warm pool warming and the resultant convection can effectively excite the Rossby wave train that resembles atmospheric circulation change shown in the Eurasian cooling ensembles. Specifically, a cyclonic circulation forms over the Aleutian region through the teleconnection and it is followed by an anticyclonic circulation over Siberia resulting from mass redistribution. This result indicates that Eurasian winter cooling in CESM-LE is possibly determined by the internal variability of tropical SST. It also suggests that the recent Eurasian winter cooling has been likely influenced by tropical climate variability.