Modelling the effect of aerosol and greenhouse gas forcing on the Asian monsoons with an intermediate complexity climate model

The Asian summer monsoons are globally significant meteorological features, creating a strongly seasonal pattern of precipitation, with the majority of the annual precipitation falling between June and September. The stability of such a strongly seasonal hydrological cycle is of extreme importance for a vast range of ecosystems and for the livelihoods of a large share of the world’s population.

 

Simulations are performed with an intermediate complexity climate model, PLASIM, in order to assess the future response of the Asian monsoons to changing concentrations of aerosols and greenhouse gases. The radiative forcing associated with aerosol loading consists of a mid-tropospheric warming and a compensating surface cooling, which is applied to India, Southeast Asia and East China, both concurrently and independently. The primary effect of increased aerosol loading is a decrease in summer precipitation in the vicinity of the applied forcing, although the regional responses vary significantly. The decrease in precipitation is only partially ascribable to a decrease in the precipitable water, and instead derives from a reduction of the precipitation efficiency, due to changes in the stratification of the atmosphere.

 

When the aerosol loading is added in all regions simultaneously, precipitation in East China is most strongly affected, with a quite distinct transition to a low precipitation regime as the radiative forcing increases beyond 60 W/m². The response is less abrupt as we move westward, with precipitation in South India being least affected. By applying the aerosol loading to each region individually, we are able to explain the mechanism behind the lower sensitivity observed in India, and attribute it to aerosol forcing over East China. Additionally, we note that the effect on precipitation is approximately linear with the forcing.

 

The impact of doubling carbon dioxide levels is to increase precipitation over the region, whilst simultaneously weakening the circulation. When the carbon dioxide and aerosol forcings are applied at the same time, the carbon dioxide forcing partially offsets the surface cooling and reduction in precipitation associated with the aerosol response.