SRM and tropical Monsoon precipitation

[Govindasamy Bala]

https://academic.oup.com/oocc/article/4/1/kgae016/7737801

Here is a paper published in Oxford Open Climate Change last week. We show that tropical summer MEAN monsoon precipitation CHANGES can be estimated using just two parameters in the case of stratospheric aerosol geoengineering: global mean aerosol optical depth (AOD) and interhemispheric differences in AOD. It is a simple 2-parameter linear model. Instead of AOD, radiative forcing or surface temperature could be also used in this formulation which, I believe, provides a closure to the estimation of tropical mean monsoon precipitation changes not just for SAI but for any forcing.

An investigation of the relationship between tropical monsoon precipitation changes and stratospheric sulfate aerosol optical depth 

Anu Xavier, Govindasamy Bala, Shinto Roose, Usha KH 

Abstract

Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

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With Best Wishes,

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G. Bala
Professor
Center for Atmospheric and Oceanic Sciences
Indian Institute of Science
Bangalore - 560 012
India

Tel: +91 80 2293 3428; +91 80 2293 2505
Email: gb...@iisc.ac.in; bala.gov@gmail.com
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