Abstract
Using precipitation data from rain gauge stations over China, the authors examine the long-term variation of the durations of persistent rainfall over eastern China for the past 40 years. The variation in the regional rainfall was related to a change in the global-mean surface temperature from the relatively cold period of the 1960s–70s to the relatively warm period of the 1980s–90s. Compared to the cold period, the persistent rainfall in the warm period began earlier and ended later over southern China, lengthening the rainy season by 23 days, but it began later and ended earlier over northern China, shortening the rainy season by 14 days. This change in the durations of persistent rainfall contributed to the pattern of the long-term change in rainfall: southern floods and northern droughts. The earlier beginning of the rainy season over southern China was associated with a more westward subtropical high over the western North Pacific and a stronger low-level low near the eastern Tibetan Plateau during spring. On the other hand, the later ending of the rainy season over southern China and the shorter rainy season over northern China were related to a more westward subtropical high over the western Pacific and a weaker trough near the eastern Tibetan Plateau during summer.
The snow cover over the Tibetan Plateau exhibited a positive trend in winter and spring, which increased the local soil moisture content and cooled the overlying atmosphere during spring and summer. The sea surface temperature over the tropical Indian Ocean and the western North Pacific also displayed a positive trend. The cooling over land and the warming over oceans reduced the thermal contrast between East Asia and the adjacent oceans. Moreover, the low-level low pressure system over East Asia weakened during summer. Under such circumstances, the East Asian summer monsoon circulation weakened, with anomalous northerly winds over eastern China. Correspondingly, the mei-yu front stagnated over the Yangtze River valley, and the associated pattern of vertical motions increased the rainfall over the valley and decreased the rainfall over northern China.
Relationship between western North Pacific typhoon activity and Tibetan Plateau winter and spring snow cover
