Impact of Ural blocking on Winter warm Arctic-cold Eurasian anomalies. Part II:The link to the North Atlantic oscillation

In Part I of this study, the Ural blocking (UB)-induced amplification role of winter warm Arctic–cold Eurasian (WACE) anomalies has been examined. It was found that the long-lived UB together with the positive North Atlantic Oscillation (NAO⁺) significantly contributes to the amplification of the WACE pattern. The present study examines how the UB variability affects quasi-biweekly WACE (QB-WACE) anomalies and depends on the NAO⁺ and North Atlantic conditions by classifying the UB based on a case study of a cold event that occurred over southern China in January 2008. A composite analysis during 1979–2013 shows that the QB-WACE anomalies associated with the UB that often occur with the NAO⁺ are strong and influenced by the North Atlantic jet (NAJ) and zonal wind strengths over Eurasia. For NAO⁺-related UB, the QB-WACE anomaly depends strongly on the location of UB, and the UB anomalies lag the NAO⁺ by approximately 4–7 days.

The strength of the NAJ determines whether the combined NAO⁺ and UB anomalies exhibit a negative East Atlantic/West Russia (EA/WR⁻) pattern, while the region of weak zonal winds over Eurasia and the zonal extent of the NAJ dominate the location of UB. For southward-, eastward-, and westward-displaced UBs associated with a strong NAJ, the NAO⁺ favors the UB with a southward-displaced QB-WACE anomaly through wave train propagation like an EA/WR⁻ pattern. Eastward- and southward-displaced UB anomalies induce similarly displaced cold anomalies with intrusion into southern China. However, for a northward-displaced UB, this happens without pronounced EA/WR⁻ patterns because of a weak NAJ and is accompanied by a northward-displaced QB-WACE anomaly.