The maintenance of the blocking over the Ural Mountains during the second Meiyu period in the summer of 1998

The 1998 summer-time floods at the Yangtze River basin of China, the severe st in last 50 years or so, directly resulted from the abnormal extension of Meiyu (rainy season), which was related to a weak East Asian summer monsoo n and persistent anomalies of extratropical circulation. The long persisten ce of blocking over the Ural Mountains is a conspicuous feature. The physic al processes responsible for the prolonged maintenance of this key system a re investigated in terms of internal forcing (transient eddy upon basic flo w) and external forcing (tropical heating forcing) via diagnosis and numeri cal experiments in the paper. Using the adjoint method, the location and structure of optimal perturbatio ns favorable for the development and maintenance of Ural blocking are ident ified, which shows an apparent coincidence with the observed storm track at the eastern Atlantic to Europe sector. The diagnosis of E-vector and the r esponse of baroclinic stationary wave to transient forcing both suggest fur ther that the enhanced transient eddy activity favors the occurrence and ma intenance of positive anomalies. The upper-level jet and heat sources (sinks) during that period are calcula ted, and the results indicate that the anomaly of upper jet and tropical he ating is evident. The ensemble forecasting experiments by a GCM, IAP T42L9 show that the anomalous heating over the tropics, especially over the centr al-western Pacific and Atlantic, favors the formation of positive anomalies of height at the Ural region. Finally, a self-sustain mechanism of positiv e anomalies through two-way interaction between planetary stationary wave a nd transient eddy under the stimulation of anomalous tropical heating is pr oposed.