INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2010) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.2124 Large-scale circulations and Tibetan Plateau summer drought and wetness in a high-resolution climate model Oliver Bothe, a * Klaus Fraedrich a,b and Xiuhua Zhu b a KlimaCampus, Meteorologisches Institut der Universit¨ at Hamburg, Grindelberg 5, 20144 Hamburg, Germany b KlimaCampus, Max-Planck-Institut f¨ ur Meteorologie, Bundesstraße 53, 20146 Hamburg, Germany ABSTRACT: Extratropical and tropical influences on Tibetan Plateau severe and extreme dry and wet summer months are investigated focussing on the large-scale circulation and using results of the coupled climate model ECHAM5/MPI- OM. A pre-industrial control run and scenario runs for the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) are considered. Tibetan Plateau precipitation in months of wetness and drought is related to atmospheric circulation anomalies in the North-Atlantic/European sector and to sea surface temperature anomalies in the Tropics. Drought on the Tibetan Plateau is associated with a pronounced wave train bridging Eurasia from the North Atlantic to Asia. Increased transient eddy activity in the North Atlantic storm track has a more south-west to north-east orientation. This supports a high pressure anomaly over the eastern North Atlantic and Scandinavia which excites a cross Eurasian wave train reducing the moisture inflow to the Tibetan Plateau from the Arabian Sea. A concurrent warming in the tropical Indian Ocean increases the low level monsoonal westerlies deviating the moisture transport from the Bay of Bengal towards the Indochinese Peninsula and the Philippines. Wetness on the Tibetan Plateau is dominated by a cooling in the tropical oceans, whereas atmospheric flow is predominantly zonal in the extratropics of North America and Europe. Thus, moisture inflow can reach the Tibetan Plateau via the Arabian Sea, the Bay of Bengal and the mid-latitude westerlies. Future scenarios show little change of atmospheric flow composites for wetness and dryness; the Tibetan Plateau droughts increase by 10% for an A1B-scenario, while extreme wet summer months are reduced by approximately 1%. Copyright  2010 Royal Meteorological Society KEY WORDS Tibetan Plateau; precipitation; teleconnection; ECHAM5/MPI-OM; standardized precipitation index; drought; wetness Received 12 May 2009; Revised 23 September 2009; Accepted 9 February 2010 1. Introduction The Tibetan Plateau is of outstanding relevance for the atmospheric circulation. In particular, it is of primary importance for the Eurasian and Northern Hemispheric climate including the Austral-Asian-monsoon circulation. From an atmospheric point of view its altitude leads to an elevated heat source and its extent (including the Himalayan mountains) presents a mechanical barrier (Flohn, 1968; Raghavan, 1973; Hahn and Manabe, 1975; Tang and Reiter, 1984; He et al., 1987; Li and Yanai, 1996; Wu and Zhang, 1998; Held et al., 2002; Hsu and Liu, 2003; Zhang et al., 2004; Sato and Kimura, 2007; Wang et al., 2008). Considering the water cycle, Tibetan Plateau precipitation feeds the largest Asian rivers (Fekete et al., 1999, 2000; Xu et al., 2008). Thus, Tibetan Plateau precipitation variability and the factors influenc- ing it potentially affect the lives of more than 2 billion people in the bordering countries (Xu et al., 2008). * Correspondence to: Oliver Bothe, KlimaCampus, Meteorologisches Institut der Universit¨ at Hamburg, Grindelberg 5, 20144 Hamburg, Germany. E-mail: oliver.bothe@zmaw.de Assessment of possible future climate changes relies on the ability of coupled ocean–atmosphere general cir- culation models (GCMs) to capture the effects of the Tibetan Plateau and the forcings due to anomalies in the larger environment, say, for example, sea surface temper- atures (SST). The model representation of precipitation depends critically on its resolution and the orography implemented (Dai, 2006). Especially in orographically pronounced regions such as the Tibetan Plateau and its surroundings (e.g. the larger Asian monsoon region), large errors occur in modelled precipitation, its means and variances (Hagemann et al., 2006; Kripalani et al., 2007; Orlowsky et al., 2010). Simmonds et al. (1999) identify the Arabian Sea, the Bay of Bengal, the South China Sea and the mid-latitude westerlies as routes of moisture supply to the Tibetan Plateau. These are modified by the Asian monsoon cir- culation and sea surface temperature conditions (Hong et al., 2008). Regional and remote sea surface tempera- ture (SST) anomaly influences on Tibetan Plateau precip- itation can be classified as ‘direct’ and ‘indirect’ impacts. The Indian Ocean Dipole Mode (IOD, Saji et al., 1999) and El Ni˜ no Southern Oscillation (ENSO, Webster and Copyright  2010 Royal Meteorological Society