青藏高原的热力和动力作用对亚洲季风区环流的影响

利用NCEP/NCAR再分析资料,研究了青藏高原热状况的季节变化、动力和热力作用对周围环流,特别是对亚洲热带季风环流的影响。高原对西风带的机械作用在冬季最强,春季次之。冬季的机械作用形成以高原为主,南侧气旋性、北侧反气旋性的"偶极子"偏差环流,它比传统认识的爬坡、绕流的影响范围大得多,遍及东亚的高、低纬度。随着西风带的北移和高原总加热在4月由负变正,南侧气旋性偏差环流增强并逐渐北移,6月形成气旋盘踞整个高原的夏季型。在高原南侧,高原冬季偶极型、夏季加热的作用导致孟加拉湾地区常年存在印缅槽,使得印度半岛的感热加热始终强于中南半岛,而中南半岛上空的潜热加热大于印度半岛。印缅槽的演变存在明显的半年周期,证明2月初和8月初的较强低压槽分别对应冬季高原最强的动力强迫和夏季高原最强的热力强迫。对低纬经向风场的分析还表明,季风爆发前高原的热力作用尤为重要,是导致江南春雨的形成,亚洲季风最早在孟加拉湾东部爆发,最后在印度半岛爆发的原因。

Influence of the Mechanical and Thermal Forcing of Tibetan Plateau on the Circulation of the Asian Summer Monsoon Area

The NCEP/NCAR reanalysis data were used to study the seasonal variation of thermal condition over Tibetan Plateau(TP),and its mechanical and thermal forcing on the atmospheric circulation over the surrounding regions,particularly for the tropical Asian monsoon circulation.Results show that the TP mechanical effects on the circulation,is the strongest in winter,get reduced in spring.The mechanical forcing result in a winter pattern of lower atmosphere deviation circulation,characterized by the cyclonic/anticyclonic circulations over the south/north of the TP,which is called the "TP Dipole Mode".Following northward shift of the westerly and transition of total heating of TP from negative to positive in April,the cyclonic deflection circulation in the southern TP becomes stronger and moves northward gradually,and becomes a typical summer cyclonic circulation patterns over the whole TP in June.To the southern TP,the TP mechanical and thermal forcing result in Indian-Burma trough all the time over the Bay of Bengal, and thus,lead to a stronger sensible heating on the Indian Peninsula than that over the Indochina Peninsula,and a stronger latent heating on the Indochina Peninsula than that on the Indochina Peninsula.The variation of Indian-Burma trough exhibits a semi-annual periodic feature.The maximum strength of Indian-Burma trough in the early February and August correspond to the strongest of the winter mechanical and summer thermal forcing,respectively.The analysis of the tropical meridian wind shows that the TP thermal forcing before the monsoon onset is very important,particularly for the persistent rainfall in early spring in South China,and occurrence of the earliest Asian monsoon onset in the eastern of the Bay of the Bengal and the latest in the Indian Peninsula.