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. However, most studies, while focusing on the intensity, duration, frequency, and mobility of blocking events, have overlooked the UB zonal width. Particularly in the context of Arctic sea ice concentration (SIC) decline, the changes in UB zonal width and its association with extreme cold events in winter in China remain unclear. In this study, we define the UB zonal width as 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, accompanied by the positive phase of the North Atlantic Oscillation (NAO+). Concurrently, with the warming of the BKS sea surface temperature (SST), both the 500-hPa zonal wind (U500) and the meridional potential vorticity gradient (PVy) decrease over mid-high latitudes in Eurasia. This results in weakened energy dispersion and strengthened nonlinearity in the Ural region, causing the UB to extend zonally. Under these conditions, 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 zonally. Conversely, for M-L events, there is no clear difference in the relationship of UB width and cold air intrusion between high and low SIC years. Overall, the UB widens zonally when BKS SIC is low, facilitating the southward movement of cold air and affecting southern parts of China.