Experimental reconstruction of large-scale summer monsoon drought over India and the Tibetan Plateau using tree rings from 'High Asia'

Abstract

We investigate the use of a network of 194 tree-ring chronologies from the ‘High Asia’ region of the Himalaya, Karakoram, Tien Shan, and Tibetan Plateau for the reconstruction of large-scale summer monsoon drought variability over India and the Tibetan Plateau. This experiment was carried out to see if the High Asia tree-ring network contains large-scale information related to the north-south climate gradient over this region, which is known to be associated with the development of the Indian summer monsoon. The climate field chosen for experimental reconstruction was a 735-point grid of self-calibrated Palmer Drought Severity Indics (PDSI) calculated from CRU TS 2.1 gridded temperature and precipitation data. This PDSI grid was reconstructed for the summer monsoon season from a subset of tree-ring chronologies covering the period 1600-1989 using a reduced-space multivariate regression method, and both calibration and verification tests indicate that the reconstructions contain significant hindcast skill in substantial sub-regions of the grid, especially over India. Principal components analysis carried out on the almost 400 years of data over the complete grid indicates that the leading spatial mode or EOF of summer monsoon PDSI expresses a dominant north-south alternation in drought and wetness between India and the Tibetan Plateau, a leading mode also present in the instrumental data. Wavelet analysis of the time series expression of this leading EOF also reveals a very stable 40-60 year pattern of multidecadal variability in it. This time scale of variability appears to be related to slowly changing tropical ocean sea surface temperatures. Together, these results validate the use of a geographically restricted set of tree-ring chronologies over High Asia for the reconstruction of large-scale summer monsoon variability over India and the Tibetan Plateau.