Robust response of Asian summer monsoon to anthropogenic aerosols in CMIP5 models

Abstract

"The representation of aerosol processes and the skill in simulating the Asian summer monsoon vary widely across climate models. Yet, for the second half of the twentieth century, the models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) show a robust decrease of average precipitation in the South and Southeast Asian (SSEA) continental region due to the increase of anthropogenic aerosols. When taking into account anthropogenic aerosols as well as greenhouse gases (GHGs), the 15 CMIP5 models considered in this study yield an average June–September precipitation least squares linear trend of −0.20 ± 0.20mm d−1 (50 years)−1, or −2.9%, for all land points in the SSEA region (taken from 75 to 120◦E and 5 to 30◦N) in the years from 1950 to 1999 (multimodel average ± one standard deviation) in spite of an increase in the water vapor path of +0.99 ± 0.65 kg m−2 (50 years)−1 (+2.5%). This negative precipitation trend differs markedly from the positive precipitation trend of +0.29 ± 0.14mm d−1 (50 years)−1, or +4.1%, which is computed for GHG forcing only. Taking into account aerosols both decreases the water vapor path and slows down the monsoon circulation as suggested by several previous studies. At smaller scales, however, internal variability makes attributing observed precipitation changes to anthropogenic aerosols more difficult. Over Northern Central India (NCI), the spread between precipitation trends from individual model realizations is generally comparable in magnitude to simulated changes due to aerosols, and the model results suggest that the observed drying in NCI might in part be explained by internal variability."