Ural Blocking Driving Extreme Arctic Sea Ice Loss, Cold Eurasia, and Stratospheric Vortex Weakening in Autumn and Early Winter 2016–2017

Abstract

This study investigates the dynamics that led to the repeated cold surges over midlatitude Eurasia, exceptionally warm conditions and sea ice loss over the Arctic, and the unseasonable weakening of the stratospheric polar vortex in autumn and early winter 2016–2017. We use ERA‐Interim reanalysis data and COBE sea ice and sea surface temperature observational data to trace the dynamical pathways that caused these extreme phenomena. Following abnormally low sea ice conditions in early autumn over the Pacific sector of the Arctic basin, blocking anticyclones became dominant over Eurasia throughout autumn. Ural blocking (UB) activity was four times above climatological levels and organized in several successive events. UB episodes played a key role in the unprecedented sea ice loss observed in late autumn 2016 over the Barents‐Kara Seas and the weakening of the stratospheric vortex. Each blocking induced circulation anomalies that resulted in cold air advection to its south and warm advection to its north. The near‐surface warming anomalies over the Arctic and cooling anomalies over midlatitude Eurasia varied in phase with the life cycles of UB episodes. The sea ice cover minimum over the Barents‐Kara Seas in 2016 was not observed in late summer but rather in mid‐November and December shortly after the two strongest UB episodes. Each UB episode drove intense upward flux of wave activity that resulted in unseasonable weakening of the stratospheric vortex in November. The surface impact of this weakening can be linked to the migration of blocking activity and cold spells toward Europe in early winter 2017.

Eurasian Cooling Linked to the Vertical Distribution of Arctic Warming

“…The longer-lasting and larger magnitude of the central Eurasian winter cold anomalies could be related to a stronger weakening of westerly winds at midlatitude (second curves in Figures 4a and S4) due to deep warming over the Arctic which can lead to more persistence in weather patterns (Cohen et al, 2014;Francis & Vavrus, 2012) and in the UB (Luo et al, 2016). Though the influence of the UB on Arctic warming has also been suggested by some studies (Luo et al, 2016), we note that there are many different definitions of the Ural sector, which in some cases extend west to 10°W (Peings, 2019) or east to 100°E (Tyrlis et al, 2019). We do see dominant high-pressure anomalies over the Ural sector before and after the peaking of Arctic warming (in both shallow and deep warming cases) ( Figure S5).…”

Recent fall Eurasian cooling linked to North Pacific sea surface temperatures and a strengthening Siberian high

“…For example, there is a connection between snow cover and the atmosphere. Air temperature forcing causes variability in snow cover extent in the Northern Hemisphere 54 , 55 , while changes in snow cover can affect the phase and amplitude of atmospheric circulations (e.g., AO/NAO/SH) 56 – 59 . However, there are still many challenges in understanding snow-atmosphere coupling 55 .…”

Role of Ural blocking in Arctic sea ice loss and its connection with Arctic warming in winter

“…Chen et al (2018) suggested that the cases of quasi-stationary UB events, a main factor for sea ice reduction via increased downward LW, have increased in the recent 2 decades. Tyrlis et al (2019) analyzed the impact of UB on the sea ice loss in the BKS in 2016-2017, showing that anomalous 2 m air temperature is related to warm and moist advection during UB events. Peings (2019) showed that imposing the UB pattern in November yields the WACE pattern with sea ice anomalies in the BKS.…”