青藏高原热源异常对1999年东亚夏季风异常活动的影响

Influence of Anomalous Heat Sources over the Tibetan Plateau on the Anomalous Activities of the 1999 East Asian Summer Monsoon

以1999年青藏高原的热源异常为出发点,讨论了其对东亚夏季风异常活动的影响,并从陆气相互作用的角度分析了该年热源异常的原因。结果表明,1999年青藏高原大气热源建立的时间明显偏晚,春夏季热源强度异常偏弱。这使得向高原的低层流入气流明显偏弱,垂直上升运动减弱,向高原的辐合减少,季风经圈环流变弱,高原南侧、东南侧的西南夏季风减弱,引起了夏季风的爆发偏晚及在中国东部北进的偏弱。而进一步对热源异常成因的分析表明,陆面因子的异常变化所引起的感热加热偏弱是热源偏弱的主要因子。高原积雪的减幅在春夏季变小,地表温度的增加变慢,地表温度偏低,引起了感热加热在春夏季的偏弱,进而导致了热源异常。

In the context of the anomalous heat sources over the Tibetan Plateau in 1999, we discuss its influence on the anomalous activities of the East Asian summer monsoon and analyze the formation reason of anomalous heat sources. The results show, that the establishment of heat sources over the Plateau was very late and the intensity of heat sources was very weak in spring and summer. This made the low-level inflow to the Plateau become weak and the ascending motion be depressed. And the convergence around the Plateau was smaller and the monsoonal meridional cells became weaker than normal. The southwesterlies on the southern and southeastern flanks of the Plateau were depressed, which caused the late onset and weak northward advance of the summer monsoon in the eastern China during 1999. The further exploration to the formation cause of the anomalous heat sources indicate that the abnormal weakening of sensible heat flux, which can cause anomalous evolution of land surface characteristics, is the main cause of the anomalous heal sources. The decrease amplitude of the Plateau's snow cover is smaller than normal in spring and summer, which made the increase of land surface temperature be slow and the temperature lower. As a result, the sensible heat flux was weaker than long-term mean in spring and summer, which caused the anomalous heat sources.