Relationship of Ural Blocking Zonal Width with Cold Air Intrusion in China under Varying Barents–Kara Sea Ice Conditions

The zonal width of the Ural Blocking (UB) is a potentially crucial characteristic related to cold events in Eurasia. While most studies focus on the intensity, duration, frequency, and mobility of blocking events, the importance of UB zonal width has often been overlooked. Particularly amid declining Arctic sea ice concentration (SIC), the changes in UB zonal width and its association with extreme cold events in winter in China remain unclear and require further investigation. This study defines the UB zonal width as the longitude difference between the easternmost and westernmost points of the 120-gpm contour line in the 500-hPa geopotential height anomaly field. Based on the NCEP/NCAR reanalysis data and SIC observations in 1970-2020, the UB width and cold air events under varied SIC conditions are examined. It is found that as SIC decreases in the Barents-Kara Sea (BKS), the UB extends zonally, coinciding with the transition of the North Atlantic Oscillation (NAO) from positive to negative. Additionally, Linear Baroclinic Model (LBM) simulations indicate that as the BKS warms, the zonal width of the UB broadens. Concurrently, warming sea surface temperatures (SST) in the BKS lead to decreased 500-hPa zonal wind (U500) and meridional potential vorticity gradient ( PV_y ) over mid-high latitudes in Eurasia. This results in weakened energy dispersion and enhanced nonlinearity in the Ural region, causing the UB to extend zonally. Consequently, the frequency of winter cold events of varying intensities in China increases, including high-to-low latitude (H-L) and middle-to-low latitude (M-L) cold events. For H-L cold events, both intensity and frequency are higher in low SIC years when the UB widens. Conversely, for M-L events, there is no significant difference in the relationship of UB and cold air intrusion between high and low SIC years. Overall, when BKS SIC is low, the UB widens zonally, facilitating the southward movement of cold air and affecting southern China.