Effects of anthropogenic aerosols on the evolution of Indian summer monsoon.

The NCAR Community Earth System Model is used to study the influences of anthropogenic aerosols on the Indian summer monsoon (ISM). We perform two sets of 30-year simulations subject to the prescribed perpetual SST annual cycle. One is triggered by the year 2000 climatology anthropogenic aerosol emissions data over the Indian Peninsula (referred to as AERO), and the other one is by the year 1850 (referred to as CTL). Only aerosol direct effects are included in the experiments. In our results, the transition of ISM in AERO relative to the CTL exhibits a similar ensemble-mean onset date with a larger spread, and more abrupt onset in late spring, and an earlier but more gradual withdrawal in early fall. The aerosols-induced circulation changes feature an upward motion over the northeastern Indian Peninsula and strengthened anticyclonic circulation over the Arabia Sea in the pre-monsoon season, and a northward shift of monsoon flow in the developed monsoon period along with strengthened local meridional circulation over northern India. The strengthened anticyclonic circulation over Arabia Sea caused a 16% increase in natural dust transport from the Middle East in the pre-monsoon season. The elevated aerosol heating over Tibet causes stronger ascending motion in the pre-monsoon period that leads to earlier and more abrupt ISM onset. The earlier monsoon withdrawal is attributed to the aerosol-induced anticyclonic flow within 10°–25°N and cyclonic flow within 0°–10°N over eastern India and Bay of Bengal that resemble the ISM seasonal transition in September. Key Points: Anthropogenic aerosol-radiative heating causes enhanced ascending motion over northern India and Tibet in both pre-monsoon and developed monsoon periods. Aerosol-induced Tibet heating in pre-monsoon period corresponds to earlier and abrupt ISM onset. Aerosol-induced circulation changes in ISM withdrawal period resemble the corresponding seasonal transition over eastern India and Bay of Bengal such that the withdrawal occurs earlier. [ABSTRACT FROM AUTHOR]