Arctic Sea Ice Loss Modulates the Surface Impact of Autumn Stratospheric Polar Vortex Stretching Events.

The cold Eurasia has been proposed to be closely linked to the weakening of the stratospheric polar vortex (SPV), however, how the Arctic sea ice modulates the surface impacts of the weak SPV is unclear. This study explores the critical modulating role of reduced Arctic sea ice in the surface cooling response to SPV stretching events in autumn. Here, through ERA5 reanalysis and Whole Atmosphere Community Climate Model simulations, we show that Eurasian cold events are more likely (45%) to occur in days 30–50 after the onset of SPV stretching events under lower Barents‐Kara Seas (BKS) sea ice conditions, in contrast to under heavy BKS sea ice conditions when robust surface cooling is absent. The stratospheric and tropospheric pathways explain 46.8% and 53.2% of the total variance of Siberian coldness, respectively. The downward extension of anomalous stratospheric wave‐2 ridge to the troposphere intensifies the Arctic‐North European high, favoring the subsequent colder Siberia. Plain Language Summary: Stratospheric polar vortex (SPV) stretching events are characterized by a zonally asymmetric pattern of lower stratospheric variability with positive height anomalies over northern Eurasia and negative anomalies over Canada. In this study, we mainly focus on the SPV stretching events that occurred from October to November, and this is because the surface cooling over high‐latitude Eurasia following these events is associated with the stratospheric pathway. Here, the autumn events can be categorized into lower (LoSIC/BKSIC) and higher (HiSIC/Ctrl) sea ice groups based on observations and simulations. We further demonstrate that lower BKS sea ice conditions are favorable for intensified upward propagation of waves into the stratosphere in the Euro‐Siberian sector prior to the onset of SPV stretching events, and subsequently affect the geometry of the SPV (SPV stretching pattern). The stratospheric wave‐2 ridge anomalies migrate downward to the mid‐troposphere 30–40 days following SPV stretching onset, leading to the enhancement of the Arctic‐North European high and attendant colder Eurasia. These results emphasize the critical modulating role of Arctic sea ice in the Eurasian cooling response to SPV stretching events. Key Points: Barents‐Kara Seas (BKS) sea ice loss modulates the impacts of autumn SPV stretching events on subsequent Siberian coolingReduced BKS sea ice enhances the upward propagation of waves into the stratosphere over the Euro‐Siberian sectorDescent of the anomalous stratospheric wave‐2 ridge bridges the SPV stretching events and Siberian cooling under low sea ice conditions [ABSTRACT FROM AUTHOR]