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Quantification of tropical monsoon precipitation changes in terms of interhemispheric differences in stratospheric sulfate aerosol optical depth

Shinto Roose, Govindasamy Bala, K. S. Krishnamohan, Long Cao, and Ken Caldeira · Climate Dynamics 61, 4243-4258 · 2023

Key finding. Per 0.1 of interhemispheric difference in stratospheric aerosol optical depth, northern hemisphere minus southern, the global mean intertropical convergence zone shifts 1.8 ± 0.0 degrees of latitude and the northern hemisphere monsoon index falls by 6.9 ± 0.4 per cent.

A grouped bar chart of the percentage change in summer monsoon precipitation for four indices — Indian summer monsoon, northern hemisphere, southern hemisphere and tropical — shown three times: per 0.1 of interhemispheric aerosol optical depth difference, per unit of interhemispheric effective radiative forcing, and per unit of interhemispheric temperature difference. In every grouping the Indian and northern hemisphere bars move strongly in one direction, the southern hemisphere bar the other, and the tropical total stays near zero.
The northern and southern bars are near mirror images and the tropical total barely moves — an interhemispheric aerosol asymmetry does not change how much it rains in the tropics so much as move the rain from one hemisphere to the other. The Indian monsoon takes the largest share of that redistribution. Figure 10 from Roose et al. (2023), Climate Dynamics 61, 4243-4258. Reproduced under CC BY 4.0. Extracted from the published PDF and resized for web display.

What question did this research address?

Stratospheric aerosol geoengineering is known to disturb the hydrological cycle, and the monsoons are where that matters most for people. But how much a given aerosol distribution would move monsoon rainfall has not been reducible to a simple number.

This paper asked whether the effect can be expressed as a sensitivity — how much monsoon precipitation changes per unit of asymmetry in how the aerosol is spread between the hemispheres.

What did we find?

The mechanism runs through the intertropical convergence zone. Making one hemisphere more reflective than the other cools it relatively, and the convergence zone — and the rain that comes with it — migrates toward the warmer hemisphere.

The sensitivity is 1.8 degrees of meridional ITCZ shift and a 6.9% reduction in northern hemisphere monsoon precipitation per 0.1 of interhemispheric aerosol optical depth difference.

The same sensitivity is expressed in two other currencies, so it can be applied to studies that report forcing rather than optical depth: 3.5 ± 0.3% change in the monsoon index per watt per square metre of interhemispheric radiative forcing difference, and 5.9 ± 0.4% per degree Celsius of interhemispheric temperature difference.

Effects are largest over India, and equivalent sensitivities are quantified for Indian monsoon precipitation specifically.

Because the relationship is close to linear, the effect of a proposed aerosol distribution on monsoon rainfall can be estimated without running the full model each time.

Why does it matter?

It turns a qualitative warning into a design constraint. That solar geoengineering could disturb the monsoons was already understood; a sensitivity per unit of interhemispheric asymmetry says how much asymmetry is tolerable, which is something a deployment scheme can actually be checked against.

The asymmetry, not the total, is what does the damage. An aerosol loading distributed evenly between the hemispheres moves the convergence zone far less than the same total loading placed unevenly — which makes where the aerosol goes a first-order design question.

It bears directly on who would bear the cost. The monsoon regions most affected are among the most populous and least able to absorb a rainfall shortfall, so the distributional consequence of an aerosol scheme is not incidental to it.

Citation

Shinto Roose, Govindasamy Bala, K. S. Krishnamohan, Long Cao, and Ken Caldeira (2023). Quantification of tropical monsoon precipitation changes in terms of interhemispheric differences in stratospheric sulfate aerosol optical depth. Climate Dynamics 61, 4243-4258.

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