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Impact of deposition of light absorbing aerosols on Himalayas-Tibetan-Plateau snowcover and influence on the Asian summer monsoon: A possible mechanism for the Blanford hypothesis
Speaker Name
Dr. William K.M. LAU
Speaker Detail

Earth System Science Interdisciplinary Centre (ESSIC), The University of Maryland

B5-210, 5/F, Yeung Kin Man Academic Building , City University of Hong Kong, Kowloon Tong, Kowloon, Hong Kong

Over a century ago, Blanford (1884) discovered a significant relationship between snowcover in the Himalayas and the Indian monsoon strength, referred to in the modern era as the Blanford Hypothesis (BH).   Despite a large number of studies on the BH, fundamental mechanisms linking the physics of snowcover and snowmelt to monsoon dynamics remain poorly known.   In this study, we investigated the impact of snow darkening by deposition of light-absorbing aerosols (LAAs) on snow cover over the Himalayas–Tibetan Plateau (HTP) and subsequent influence on the Asian summer monsoon. We used the NASA Goddard Earth Observing System Model Version 5 (GEOS-5) that possesses improved aerosol physical package, including detailed radiative transfer by LAAs, i.e., dust, black carbon and organic carbon, as well as emission, transport, and deposition processes in the atmosphere and in snow.

Results show that during April–May–June, the deposition of LAAs on snow leads to a reduction in surface albedo, initiating a sequence of radiative and dynamical feedback processes, starting with increased net surface solar radiation, rapid snowmelt in the HTP and warming of the surface and upper troposphere, followed by enhanced low-level southwesterlies and increased dust loading over the Himalayas–Indo-Gangetic Plain. The warming is accelerated by increased dust aerosol heating, and subsequently amplified by latent heating from enhanced precipitation over the Himalayan foothills and northern India, via the elevated heat pump (EHP) effect during June–July–August. The reduced snow cover in the HTP anchors the enhanced heating over the Tibetan Plateau and its southern slopes, in conjunction with an enhancement of the Tibetan Anticyclone, and the development of an anomalous Rossby wave train over East Asia, leading to a northward displacement of the subtropical westerly jet, and the Mei-Yu rain belt, with reduction in rainfall in southern China.   The impacts by pre-monsoonal accumulation of Middle East Dust over the North Arabian Sea and the Himalayas Gangetic region on seasonal-to-interannual predictability of the Asian summer monsoon will be discussed.

About the Speaker

Dr. William K. M. Lau is a senior scientist at ESSIC, and adjunct professor of the Department of Atmospheric and Oceanic Sciences, University of Maryland.  He received his B. Sc. (Mathematics and Physics) in 1972, and B. Sc. Special (Applied Mathematics), in 1973 from University of Hong Kong, his PhD in Atmospheric Sciences, 1977, from the University of Washington, Seattle, Washington, U.S.  He was assistant professor at the Naval Postgraduate School, 1978-1980, before joining the NASA Goddard Space Flight center in 1981.  He served as the Head of the Climate and Radiation Branch (1991-2000), Chief, the Laboratory for Atmospheres (2001-2010), and the Deputy Director for Science, Earth Science Division, NASA GSFC (2011-2014).  His research spans 4 decades covering a wide range of topics in climate dynamics, tropical and monsoon meteorology, ocean-atmosphere interaction, aerosol-water cycle interaction, climate variability and change. He frequently visits international research and academic institutions in China, India, Southeast Asia, Taiwan, Hong Kong, North and South America, and Europe to present invited lectures, and keynote speeches in scientific meetings to promote interests and awareness in regional impacts of climate variability and change.