Abstract
The influence of a large-scale orography on climate depends not only on its mechanical forcing and thermal forcing which it exerts on the atmosphere, but also on the background atmospheric circulation. Empirical Orthogonal Function Analysis demonstrates the existence of two leading heating modes over the Tibetan Plateau (TP) in winter. However, numerical modeling indicates that these heating modes are the consequences relevant to dominant atmospheric circulations, in particular the Northern Atlantic and the North Pacific Oscillations. The prevailing effect of the mechanical forcing of the TP in wintertime generates a dipole type of circulation, in which the anticyclonic gyre in middle and high latitudes contributes to the formation of warm air in inland area to the west, and the cold air in seashore area to the east. Whereas the cyclonic gyre in low latitudes contributes to the formation of prolonged dry season over middle and south Asia and moist climate over southeastern Asia. The dipole circulation also generates a unique persistent rainfall in early spring over South China.
The thermal forcing of the Tibetan Plateau on the lower tropospheric circulation looks like a Sensible Heat driven Air Pump (SHAP). It is the surface sensible heating on the sloping sides of the Plateau that the SHAP can effectively influence the Asian monsoon circulation. In spring the SHAP contributes to the seasonal abrupt change of the Asian circulation, and anchors the earliest Asian summer monsoon onset at the eastern Bay of Bengal (BOB) by modulating the air-sea interaction over the BOB in producing a unique short-life but strong “spring BOB warm pool” and in generating the Asian summer monsoon onset vortex. It is demonstrated that the Asian summer monsoon is basically thermally-controlled by the land-sea contrast across South Asia and by the thermal status of the large-scale mountain ranges, i.e. the Iran Plateau and the TP. In summer, the pumping of the TP-SHAP, together with the thermal forcing over Iran Plateau, produces bimodality in the South Asian High activity in the upper troposphere, which is closely related with the climate anomaly patterns over South and East Asia.
Station observation and reanalysis data both indicate that the TP thermal forcing was weakened from 1980's to the end of the twentieth century, and accompanied with the a climate trend of “wetter in south and drier in north” in eastern China. Numerical experiments demonstrate that such a climate trend can be attributed substantially to the weakening trend of the TP thermal forcing. This result thus provides a supplemental means for understanding and verifying the regional climate changes over East Asia.
About the speaker
Prof Guoxiong Wu received his PhD from Imperial College of Science and Technology of the University of London in 1983. He has been Professor of the Institute of Atmospheric Physics, Chinese Academy of Sciences since 1985. He has served many key executive positions in Chinese and international academic communities, which include Director of National Key Laboratory of Atmospheric Sciences and Geophysical Fluid Dynamics, President of International Association of Meteorology and Atmospheric Sciences, Officer of WMO/ICO/ICSU Joint Science Committee and World Climate Research Program, and Member of the Executive Board of the International Council for Science.
Prof Wu is a world renowned scientist in the field of monsoon and climate dynamics studies. His main research interests are weather dynamics, climate dynamics and atmospheric general circulation. He developed the extended Ertel PV-theory to the theory of "slantwise vorticity development". By using this theory, he revealed the mechanism on the formation and development of torrential rain. He also extended quasi-geostrophic non-acceleration theorem to primitive equation non-acceleration theorem. He is closely involved in studies on the effects of the Tibetan Plateau on atmospheric general circulation, weather and climate, on the formation and variation of the subtropical anticyclones, and on the dynamics of the Asian monsoon. He has published more than 200 papers on domestic and international academic journals.
Prof Wu was elected as an Academician of the Chinese Academy of Sciences in 1997 and Honorary Fellow of the Royal Meteorological Society in 2012.
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