The large-scale circulations and summer drought and wetness on the Tibetan plateau

Effects of large-scale atmospheric circulation and surface temperatures on extreme dryness and wetness on the Tibetan plateau in summer are analysed using ERA-40 reanalysis and observed precipitation. The extreme cases of drought and wetness can be associated with circulation anomalies in the North Atlantic/European sector and wave trains bridging the Eurasian continent. Drought in Tibet reveals an intense high pressure anomaly over Scandinavia supported by a more south-west to north-east orientated North Atlantic stormtrack. This creates wave trains crossing Eurasia which, on their southward 'great circle route', reach south-eastern Asia where they modulate the flow north and east of the Tibetan plateau by an anticyclone cyclone dipole suppressing moisture supply from the Bay of Bengal. Wetness in Tibet is characterised by a more zonally oriented cross Atlantic stormtrack creating a low pressure anomaly over central Europe and, associated with it, a northward shift of the sub-tropical westerly and tropical easterly jet; wave trains emerging from the North Atlantic on their equatorward route have now a higher chance to reach the sub-tropical jet entrance (instead of propagating further south). Then the wave trains are re-intensified and, passing the Mediterranean Arabian Sea route to India, interact with the monsoon's western branch to lead to ample moisture supply for Tibet. Surface temperatures give indications for positive (negative) El Nino/Southern Oscillation and Indian Ocean Dipole episodes occurring in years of extreme and severe dryness (wetness) on the Tibetan plateau. A pronounced cold surface temperature anomaly in the tropical North Atlantic precedes and accompanies drought on the plateau. Copyright (C) 2009 Royal Meteorological Society