Tropical circulation and monsoon response

Shifts in the seasonal rainfall patterns in the tropics, particularly around the timing and intensity of monsoons, pose a serious risk to agriculture and water security in regions home to billions of people. This is one of the most consequential uncertainties surrounding Stratospheric Aerosol Injection (SAI) because even modest changes could carry outsized consequences for the communities that depend on predictable monsoon patterns.

There are important gaps in the current understanding of the mechanisms by which SAI influences tropical precipitation and monsoons. What current research suggests is that SAI could cause changes in rainfall in the tropics in several ways. First, if SAI is used to cool one hemisphere more than another, the resulting temperature imbalance can displace the Intertropical Convergence Zone (ITCZ) – the band near the equator where trade winds both hemispheres converge and produce the most tropical rainfall – toward the warmer hemisphere, shifting rainfall away from the other hemisphere’s tropics (e.g. Haywood et al., 2013). Second, rising CO₂ reduces rainfall through a pathway that is unrelated to temperature (Allen & Ingram, 2002). SAI, and any method that works by reflecting sunlight back to space, offsets the warming caused by CO₂, but it doesn't remove the CO₂ itself. Because of that, the CO₂-driven reduction in rainfall persists even if SAI is deployed (Bony et al., 2013). Third, by heating the stratosphere, SAI can alter tropical convection (the rising air that produces rainfall) and shift precipitation patterns, potentially reducing tropical rainfall (Simpson et al., 2019).

This research activity will fund multiple research groups over multiple years to develop mechanistic understanding of how SAI affects tropical precipitation and monsoon dynamics and to create assessment frameworks that enable rapid evaluation of these impacts across models, injection strategies, and deployment timelines. This work includes the following components:

1. Mechanistic decomposition of precipitation changes — a decomposition of tropical precipitation changes in existing SAI simulations into distinct physical mechanisms. General circulation models (GCMs) have mean-state biases and biases in how they represent aerosols, including recent aerosol trends, which limit their usefulness on their own for this application. A hierarchy of models — ranging in complexity from simple to fully coupled GCMs — together with observations, will be necessary to tackle this. Researchers will conduct idealized and sensitivity simulations that isolate individual drivers (e.g., local insolation changes, hemispheric energy balance, and land and ocean temperature perturbations) to clarify which mechanisms dominate monsoon responses under different injection strategies. Observation-model comparisons will validate these mechanistic insights against observational constraints on tropical rainfall variability. Disentangling natural variability from forced changes will be particularly challenging but important for this activity.
2. Reproducible, open-source assessment workflow — a tool for rapid assessment of monsoon responses across the multi-model ensemble, standardizing the metrics and decomposition methods developed in the first component and allowing consistent evaluation of monsoon impacts as new SAI simulations become available. For instance, such a tool might include a robust method for bias correction. By creating reusable assessment infrastructure, this component enables future impact studies to quickly evaluate monsoon sensitivity to different injection strategies and deployment timelines, providing policymakers with decision-relevant information about this climate risk.

This area of study will be ongoing, with multiple impact studies over time. This research activity is focused on the initial scope of work needed to meaningfully advance scientific understanding of these processes and make future research faster and more robust. Reflective has funded the following projects in this area of study:

• Stratospheric Aerosol Injection Impacts on the West African Monsoon System: A Mechanistic Understanding from Volcanic Analogs to Climate Intervention Scenarios. Dr. Amadou Thierno Gaye & Dr. Abdou Lahat Dieng, Cheikh Anta Diop University
• Climate Intervention or Regional Risk? Exploring High Latitude Low Altitude Stratospheric Aerosol Injection (SAI) Impacts on West African Monsoon and Climate Extremes. Dr. Nana Ama Browne Klutse, Ghana Environmental Protection Authority & Dr. Kwesi Akumenyi Quagraine, National Center for Atmospheric Research
• Global Drought and Extreme Precipitation Responses to SAI Strategies. Dr. Samantha Stevenson & Dr. Se-Yong Song, University of California, Santa Barbara
• Impacts of High-Latitude/Low-Altitude Stratospheric Aerosol Injection on Surface Temperature and Precipitation in Mainland Southeast Asia. Dr. Pornampai “Ping-Ping” Narenpitak, National Electronics and Computer Technology Center