Effect of varying aerosol forcing on the Indian summer monsoon in an intermediate complexity climate model

The Indian summer monsoon is a key meteorological event in the Indian calendar, bringing approximately 80% of India’s annual rainfall over the months of June—September. How the monsoon system will react to a changing future climate is of crucial importance for India’s agriculture, industry and economy.

 

Uncertainty remains in the future climate projections of the Indian monsoon,  primarily due to uncertainties in the amount and composition of aerosols over the Asian region. A further complication is that greenhouse gases, expected to increase over the next 50 years and dominate over aerosol forcing, have a directly competing effect on the monsoon. Generally, greenhouse gases act to heat the Earth’s surface, enabling greater moisture uptake and enhancing monsoonal precipitation. In contrast, aerosols have a cooling effect at the surface. The presence of aerosols is analogous to anomalous heating in the mid-troposphere, increasing the static stability of the atmosphere, which is associated with a weakening of the large-scale circulation and thus a weakening of the monsoon. 

 

We use the Planet Simulator (PlaSim), an intermediate complexity climate model, to investigate the interplay between varying aerosol and greenhouse gas forcing, in relation to the Indian summer monsoon. The model is modified to include anomalous heating in the mid-troposphere, which represents the presence of aerosols and effectively cools the surface. Varying the intensity and location of aerosol forcing, as well doubling the amount of carbon dioxide, alters the spatial pattern of precipitation over the Asian region. Increasing the anomalous heating to 150 Watts and applying over the regions of India, East China and Southeast Asia, significantly weakens the large-scale circulation and reduces the summer precipitation to 10-20% of a normal year; essentially a breakdown of the monsoon system. Through modelling sensitivity studies, we define a safe operating space of future climate conditions, where the Indian and East Asian monsoons retain their current regimes. We can also show early warning signals in the precipitation intensity, indicating a phase change of the Indian or the East Asian monsoon.