Projections of South Asian Summer Monsoon under Global Warming from 1.5° to 5°C

The South Asian summer monsoon (SASM) is one of the most crucial climate components in boreal summer. The future potential changes in the SASM have great importance for climate change adaption and policy setting in this populous region. To understand the SASM changes and their link with the global warming of 1.5°-5°C above the preindustrial level, we investigate the changes in the SASM circulation and precipitation based on a large-ensemble simulation conducted with Canadian Earth System Model version 2 (CanESM2). With the global mean surface temperature (GMST) increase, the large-ensemble mean of SASM circulation is projected to weaken almost linearly while the precipitation and precipitable water are projected to enhance quasi-linearly. A double anticyclone along the tropical Indian Ocean is a major anomalous circulation pattern for each additional degree of warming and is responsible for the weakening of the lower-level westerlies. The decreased upper-level land-sea thermal contrast (TC_upper) is the main thermal driver for the weakening of the SASM circulation while the lower-level thermal contrast contributes little. The nonlinearly decreased TC_upper is mainly related to the temperature response to the increased CO₂ forcing and convection-induced latent heat release in the tropics. The increase in the SASM precipitation is mainly due to the quasi-linearly increased positive contribution of the thermodynamic component, while the dynamic component has a negative impact. Both horizontal moisture advection and moisture convergence contribute to the precipitation increase, and moisture convergence plays a dominant role. These results provide new insight that the SASM changes can be roughly scaled by the GMST changes.