Interannual Variability and the Underlying Mechanism of Summer Precipitation over the Yarlung Zangbo River Basin

Based on multiple precipitation and reanalysis datasets, the characteristics of summer precipitation and the associated water vapor transport over the Yarlung Zangbo River basin are examined, focusing mainly on the interannual variability and the underlying mechanism. Results show that: (1) The maximum climatological summer precipitation is seen along the Brahmaputra River in the lower reaches of the valley, which is reasonably reproduced in different datasets. The summer precipitation averaged over the basin is 5.8 mm d^-1 in observations, whereas it is doubled in reanalysis datasets. (2) The climatological water vapor transports are mainly from the Indian Ocean and the Bay of Bengal along the Brahmaputra River to Yarlung Zangbu Grand Canyon. The basin is a moisture sink in summer, with a net moisture convergence of 9.5 mm d^-1, mainly caused by wind convergence and orography. (3) Different reanalysis datasets can capture the basic characteristics of precipitation and moisture transport, but differ in magnitudes. CFSR (NCEP Climate Forecast System Reanalysis) and JRA-25 (Japanese 25-year Reanalysis) are more reasonable than ERA-Interim (European Centre for Medium-Range Weather Forecasts Reanalysis) for precipitation and moisture research over the basin, because of the larger than observed precipitation and moisture convergence in the latter. (4) During 1979-2007, the summer precipitation over the basin exhibits strong interannual variability, with no significant long-term trend. The interannual variability of summer precipitation over the basin is dominated by the cyclonic/anticyclonic water vapor transport over the northern Indian subcontinent generated by anomalous Indian summer monsoon activity. Anomalous southwesterly flow leads to excessive local moisture convergence, of which the meridional wind convergence contributes 83.5%. Different from the climatology, orography contributes little to interannual variability.