Abstract
Stratospheric aerosol geoengineering (SAG) is proposed as a potential climate intervention strategy to counteract global warming. Previous studies have shown that year-round or summer/spring implementation of SAG in one hemisphere would shift the Inter-Tropical Convergence Zone (ITCZ) away from that hemisphere and cause a decrease in precipitation over the tropical summer monsoon regions in that hemisphere. In this paper, our objective is to enhance the tropical summer monsoon precipitation by implementing SAG only in the winter hemisphere. Using the Community Atmosphere Model version 5 (CAM5), we compare a simulation where aerosols are prescribed only over the winter hemispheres in the low and mid latitudes to a simulation where aerosols are prescribed uniformly around the globe year-round, both designed to offset a modest global warming of 1.9°C due to increased CO2 levels.
In the case where SAG is implemented only in the winter hemispheres, the summer ITCZ shifts poleward by 1.06° (0.92°) further into the northern (southern) hemisphere relative to the simulation with no SAG. Consequently, we see an increase in tropical summer monsoon precipitation in both hemispheres but a decrease in tropical annual mean precipitation because of larger tropical cooling. However, a larger decrease in evapotranspiration compared to precipitation leads to an increase in the annual mean surface water availability over tropical land when SAG is implemented over low and mid latitudes of winter hemispheres. Further, SAG implemented in the winter hemispheres would nudge the North Atlantic Oscillation (NAO) toward a positive phase and disrupt mid-latitude winter precipitation in the northern hemisphere. Our study highlights the potential trade-offs associated with SAG strategies for enhancing tropical summer monsoon precipitation which is a lifeline for over half the global population.
