We study the sensitivity of South Asian Summer Monsoon (SASM) precipitation to Southern Hemisphere (SH) subtropical Absorbed Solar Radiation (ASR) changes using Community Earth System Model 2 simulations. Reducing positive ASR biases over the SH subtropics impacts SASM, and is sensitive to the ocean basin where changes are imposed. Radiation changes over the SH subtropical Indian Ocean (IO) shifts rainfall over the equatorial IO northward causing 1-2 mm/day drying south of equator, changes over the SH subtropical Pacific increases precipitation over northern continental regions by 1-2 mm/day, and changes over the SH subtropical Atlantic have little effect on SASM precipitation. Radiation changes over the subtropical Pacific impacts the SASM through zonal circulation changes, while changes over the IO modify meridional circulation to bring about changes in precipitation over northern IO. Our findings suggest that reducing SH subtropical radiation biases in climate models may also reduce SASM precipitation biases.

It has been proposed that increasing greenhouse gas (GHG)-driven climate tipping point risks may prompt consideration of Solar Radiation Modification (SRM) climate intervention to reduce those risks. Here, we study marine cloud brightening (MCB) SRM interventions in three subtropical oceanic regions using the Community Earth System Model 2 (CESM2) experiments. We assess the response of tipping point-related metrics to estimate the extent to which such interventions could reduce tipping point risk. Both the pattern and magnitude of the MCB cooling depend strongly on location of the MCB intervention. We find the MCB cooling effect reduces tipping point risk overall; however, the distinct pattern effects of MCB versus GHG means it is an imperfect remedy. Indeed, if MCB is applied in certain oceanic regions, it may exacerbate some tipping point risks. It is therefore crucial to carefully assess the potential remote teleconnected response to MCB interventions to reduce unintended climate impacts.

Weakening of the Atlantic Meridional Overturning Circulation (AMOC) with greenhouse gas forcing is a common feature in most Earth System Models (ESMs). Here, we show that uncertainty in regional precipitation projections in CMIP6-class ESMs, particularly over the deep tropics, is linked to uncertainty in how much the AMOC weakens. Specifically, ESMs with greater AMOC weakening exhibit less strengthening (or greater weakening) of monsoonal circulations north of the equator, consistent with a pronounced southward shift in the tropical precipitation maximum. Using an idealized ESM, we show that decreasing the (northward) cross-equatorial ocean heat transport (X-OHT) while doubling CO2 is sufficient to shift the tropical precipitation maximum southward and thereby weaken northern hemispheric monsoon circulations, relative to an experiment where X-OHT remains unchanged. These findings imply that uncertainty in precipitation projections over key regions, particularly in the tropics, may be reduced by better constraining changes in large-scale ocean dynamics with warming.