Analysis of Longstanding Summer Blocking Highs over the Ural Mountains and Their Relationship with Tropical Thermal Anomalies

Severe floods occurred in the Yangtze River valley in 1998 and 2010. The atmospheric circulation patterns responsible for the disastrous events share a common feature: frequent occurrence and persistent presence of the blocking high over the Ural Mountains. To understand the formation and longstanding presence of the Ural blocking high, a comparative analysis was conducted by utilizing the daily data from the NCEP/NCAR reanalysis. The results suggest that (1) a significant positive persistent anomaly event occurred over the Ural in the summer of 2010, which was the second longest (37 days) since 1948 with the peak of the anomaly located slightly west of the Ural, whereas the 1998 event was shorter (22 days). (2) Significant abnormal planetary wave activities were seen in both cases. The planetary wave components with wavenumbers 2 and 4 played a dominant role in the 2010 case, but those with wavenumbers 1 and 2 were more important in the 1998 case. (3) Significant abnormal convections occurred over the tropics, which may have modulated the planetary wave activity and were responsible for the difference between the cases. (4) An intensified convection pattern emerged in the east tropical Indian Ocean region west of Sumatra in both cases. It might have significantly contributed to the geopotential height anomaly chain originating from the tropical Indian-Pacific Ocean and propagating toward the middle and high latitudinal Pacific; thus, the anomalous planetary wave activities excited a wave-train similar to the well-known East-Asian Pacific pattern.