South Asian Summer Monsoon under Stratospheric Aerosol Intervention

doi:10.21203/rs.3.rs-4631758/v1

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South Asian Summer Monsoon under Stratospheric Aerosol Intervention

https://doi.org/10.21203/rs.3.rs-4631758/v1

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The South Asian summer monsoon (SAM) bears significant importance for agriculture, water resources, economy, and environmental aspects of the region for more than 1.5 billion people. To minimize the adverse impacts of global warming, Stratospheric Aerosol Intervention (SAI) has been proposed to lower surface temperatures by reflecting a portion of solar radiation back into space. However, the effects of SAI on SAM are still very uncertain and demand more research. We investigate this using the Stratospheric Aerosol Geoengineering Large Ensemble datasets. Our study reveals a reduction in the mean and extreme summer monsoon precipitation under SAI in this scenario, driven by a combination of the SAI-induced lower stratospheric warming and the associated weakening of the northern hemispheric subtropical jet, changes in the upper-tropospheric wave activities, geopotential height anomalies, and the strength of the Asian Summer Monsoon Anticyclone. Local dust changes that can otherwise be important for SAM rainfall variability under climate change also contribute to changes under SAI. As the interest in SAI research grows, our results demonstrate the urgent need to understand SAM variability under different SAI scenarios, which is essential for sustainable development and disaster preparedness in South Asia.

Earth and environmental sciences/Climate sciences

Earth and environmental sciences/Climate sciences/Climate change

Monsoon

Stratospheric Aerosol Geoengineering

jet stream

atmospheric/tropospheric circulation

extreme precipitation

The South Asian Summer Monsoon (SAM) rainfall contributes to ~ 80% of the total annual rainfall over the Indian region ^1,2. As such, any change in mean or extreme precipitation regionally can substantially impact the economy, agricultural practice, water resources, biodiversity, and the overall ecosystem ^3,4, especially in regions like South Asia and India in the Global South. A drying trend was observed in all-India summer monsoon rainfall from the mid to late 20th century ^5–7. However, recent research reports a revival of monsoon rainfall with an increasing trend in the early 21st century ⁸. Human activity has warmed the world more than 1°C since pre-industrial times, and the destructive impacts are widely felt across the globe. The climate projections studies indicate that the warming will continue and likely exceed the Paris climate target within the next few years ⁹. Recent studies using global coupled models commonly agree that the mean and extreme Indian monsoon rainfall will increase due to global warming in the 21st century^10–12.

The scientific community has recommended rapid emission cutbacks for the reduction of global warming; however, global efforts to date are not providing enough confidence that the ongoing warming trend can be avoided ¹³. In that context, geoengineering (or solar climate intervention) methods are being proposed that could potentially ameliorate global warming ^14,15. Among the proposed methods, stratospheric aerosol intervention (referred to simply as SAI hereafter) is estimated to be more effective and potentially less expensive than other methods ^16,17. Sulfate aerosols have been widely studied for SAI ^18–20. The method was proposed in 1970 ²¹ and brought to the forefront in 2006 ²². It involves injecting sulfur dioxide (SO2) into the stratosphere, which then oxidizes and generates sulfate aerosols ²³. The enhanced stratospheric aerosol layer would scatter some incoming solar radiation and thus cool the Earth’s surface. The method is often viewed as analogous to the influence of large volcanic eruptions. However, SAI could have different impacts and risks since it would have to be applied over a longer time until atmospheric greenhouse gas concentrations have declined sufficiently ²⁰. Concerns exist about the adverse effects of SAI ²⁴, further demanding the need to fully understand the benefits and drawbacks before considering it as a potential climate response option. To understand the impacts and risks of long-term SAI applications, multiple numerical simulations with comprehensive Earth System Models are required as a first step before any SAI applications can be considered ²⁵.

Past modeling studies with the Community Earth System Model (CESM) have demonstrated that under a future high-end global warming scenario, strategic SAI applications could be successful at keeping the global mean surface temperatures and their large-scale gradients at a quasi-present-day level ^18,26–28. However, model studies have found a slowing of the global hydrological cycle under concurrent SAI-induced restoration of the global mean surface temperatures ²⁹, as well as significant regional changes in surface temperature and rainfall ^30,31, including the Asian monsoon region. However, the magnitude and the sign of the regional responses can vary depending on the injection strategy ^32,33. Generally, the regional analysis showed that SAI could change dry regions to become wetter and wet regions to become dryer ³⁰. SAI could further reduce the frequency of heavy and exceptionally heavy precipitation in tropical regions and the number of consecutive dry days in arid regions ^34,35.

While SAI can successfully reduce some risks associated with climate change (e.g., heatwaves, the loss of sea ice in the Arctic, permafrost thaw, and reduction of Greenland Ice Sheet mass) ³⁶, the effects to SAM under SAI are much less understood. Most of the previous studies investigating SAI impacts on the Indian monsoon used idealized experiments, whereby the effect of SAI is approximated by reducing the value of the solar constant or using only a single point SO₂ injection ^31,37,38. In addition, the driving mechanisms behind any change in SAM remain one of the least explored in the context of SAI. In this study, we investigate the impacts of SAI on SAM using a strategic SAI realization that utilizes four SO₂ injection locations and a control algorithm aiming to minimize large-scale surface temperature changes and their gradients. We further support the results with the assessment of the possible drivers behind the simulated SAM changes. This work is intended to help identify detailed processes that lead to changes in SAM precipitation under SAI, which may eventually lead to redesigning SAI injection strategies ^27,28,39,40 to minimize the side effects, including changes in regional precipitation.

The paper is structured as follows: Details of the model simulations used are provided in section 2. Section 3 shows the mean SAM rainfall changes under both climate change with and without SAI, and Sections 4 and 5 discuss the various drivers of the SAI-induced response. Section 6 moves away from the mean responses and contrasts changes in extreme monsoon precipitation under climate change with and without SAI. The main conclusions are summarized in Section 7.

To investigate the response of SAM rainfall to SAI, we use the output from the Geoengineering Large Ensemble Project (GLENS) ²⁸. These model experiments were performed using the Community Earth System Model version 1 (CESM1) with the Whole Atmosphere Community Model (WACCM) as its atmospheric component ⁴¹. This configuration reproduces the observed changes in aerosol optical depth and surface climatic features in response to the Mt. Pinatubo volcanic eruption in 1991, as well as represents the various atmospheric features like the QBO, ozone, and atmospheric water vapour concentration reasonably well compared to observations ⁴¹⁴². A horizontal resolution of 0.9° latitude by 1.25° longitude, along with 70 vertical levels up to 140 km, was used for this model simulation. The Modal Aerosol Module (MAM3) scheme ⁴² was employed to interactively simulate the aerosols in the stratosphere and troposphere. WACCM is fully coupled with the Community Land Model, version 4.5 (CLM4.5), as well as interactive ocean and sea-ice models. More details about the model configuration, simulation details, and parametrization process can be found in past literatures ^28,43.

GLENS ²⁸ consists of a 20-member ensemble of the CMIP5 RCP 8.5 scenario over the period 2010–2030 as the control period (CTRL hereafter). Three CTRL simulations continue till 2097 and serve as the baseline future RCP8.5 scenario without SAI (RCP 8.5 hereafter). The SAI scenario (SAI thereafter) is also a 20-member ensemble; it spans 2020 to 2097 and aims to counter the warming of RCP8.5. In particular, using a feedback control algorithm, SAI simulations inject SO₂ into the lower stratosphere (~ 5 km above tropopause) at four latitudes (15°N/S and 30°N/S), maintaining the global mean surface temperature and the interhemispheric and Equator-to-pole temperature gradients close to the CTRL levels ^18,28.

To maintain consistency in the calculation when contrasting simulations with and without SAI, we only use the first three members of each ensemble. We analyze the period 2070–2090 for both RCP and SAI and compare them against the quasi-present-day CTRL (i.e., 2010–2030). Table 1 summarizes the model simulations analyzed.

Table 1

Detail of simulations used in this study.
Simulation	Abbreviation	Time period	No of Ensemble
Present day Climate	CTRL	2010–2030	3
Future Climate without SAI	RCP 8.5	2070–2090	3
Future Climate with SAI	SAI	2070–2090	3

Figure 1 depicts the spatial mean precipitation anomalies for the SAM regions under the future RCP8.5 and SAI scenarios. Please note that during the summer monsoon season, the precipitation is mainly liquid rain ⁴⁴ ; hence, we use the words interchangeably in the manuscript. The model rainfall is evaluated against the observation datasets and provided in Fig S1.

SAM summer (June-July-August, JJA) mean rainfall under a global warming scenario increases significantly over the Indian subcontinent, especially in the Himalayas, northeast of the Bay of Bengal, and the west coast of India (Fig. 1a). Similar findings are reported under extreme global warming scenarios (SSP5-8.5 and RCP-8.5) using Coupled Model Intercomparison Project Phase (CMIP) 5 and 6 models ^10,12. Modeling studies agree that such an increase in the summer monsoon rainfall over South Asia is primarily attributed to an increase in the moisture-holding capacity of the atmosphere under a warmer climate ^12,45. In contrast, in response to SAI, rainfall significantly declines over most of northern and central India compared to CTRL (Fig. 1b). More than 1.5 mm day^− 1 reduction in rainfall is simulated in the core central Indian region (as defined in Goswami et al., 2006) in 2070–2090, with a further extension of the rainfall reduction into the regions of the Bay of Bengal to the east of the Indian continent. Unlike the overall increase in precipitation over the SAM region under RCP 8.5, the rainfall changes simulated under SAI do not decrease everywhere in the region. Instead, positive rainfall anomalies are found over the southern parts of India, with more rainfall increases extending northeast from India into parts of China. Notably, the mechanisms through which SAI can impact SAM and the associated precipitation patterns are relatively unknown. In the subsequent Section 4, we investigate the possible drivers behind the rainfall anomalies.

Past studies have highlighted that SAI can affect the mean climate of the stratosphere ^28,46,47. Sulfate aerosols located in the stratosphere are effective at reflecting and scattering incoming solar radiation and, therefore, act to cool the surface and the free troposphere, but also absorb infrared radiation ⁴⁸, resulting in a warming of the lower tropical stratosphere ^46,49,50. In the specific SAI scenario used here, 30–40 Tg-SO₂/yr injections were needed to counter the RCP8.5 warming of 3–4 degrees C by 2070–2090 in this model. The resulting lower stratospheric warming that maximizes in the tropics reaches more than > 8 degrees Celsius at ~ 50 hPa (Fig. 2a).

The strong anomalous warming of the lower stratosphere imposed by SAI leads to tropical tropospheric circulation changes ^32,47,51, a weakening of the subtropical jets in both hemispheres (Fig. 2b) and a modification in the wave activity in the subtropics (Fig. 2c). Independently of SAI, previous studies have linked upper tropospheric waves to Indian and Asian monsoons ^44,52,53, whereby summertime planetary wave activity at ~ 200 hPa across Indian longitudes strongly correlates with SAM rainfall ^54–56. Here, we identify a similar connection between the 200 hPa meridional wind anomalies and rainfall changes. SAI-induced lower stratospheric heating and weakening of the northern subtropical jet modulate the mean summertime 200 hPa meridional winds (Fig. 2c), with the anomalies resembling a structure similar to the stationary Rossby wave ^44,56. The meridional wind response to SAI in the South Asian region basically reinforces the climatological wave pattern (Fig. S2) and results in an anomalous cyclonic circulation at 200 hPa, which consists of anomalous southward flow to the west of India and anomalous poleward flow to the east of India (Fig. 2c). The positive anomaly of the 200 hPa meridional winds is aligned with the reduction in rainfall over the central and northern parts of India.

These changes are also aligned with changes in the climatological Asian Summer Monsoon Anticyclone (ASMA) in the upper troposphere (Figure S3a) that forms across the Asian Summer Monsoon region and is linked to the heating of the Tibetan plateau and deep convective activities over the head of the Bay of Bengal ⁵⁷.

Under SAI, the ASMA decelerates, which is illustrated by the anomalously cyclonic flow compared to the control simulation (Fig. 2d). We also observe an anomalous splitting of ASMA into two parts, one over India and the other over the Middle East and North Africa region (Fig. 2d). The deceleration of ASMA contributes to the reduction in the strength of the summer monsoon circulation in the lower troposphere (Fig. 2e; see Fig. S3b for the climatology) and the simulated SAI-induced drying over central and northern India.

The changes in the upper atmospheric wave activities and weakening of ASMA are also reflected in the associated changes in vertical velocities (Fig. 2f). Changes in the large-scale vertical motions often indicate the surface pressure patterns. For example, anomalous sinking air motion usually corresponds to anomalous high-pressure regions ⁴⁴. Under the SAI scenario, an anomalous downward motion (red in Fig. 2f), indicating reduced upwelling over the longitudes of the core monsoon region, is found; this is also illustrated by the anomalous increase in geopotential height (here illustrated at 500 hPa, Fig. 3a) and sea-level pressure (Fig. 3c) over vast areas of India, Pakistan, and the Middle East. The anomalous subsidence of dry air over the subcontinent inhibits convection and rainfall over the Indian landmass. It also contributes to the anomalous warming at 850 hPa over large parts of Indian subcontinents (> 1ºC over the core monsoon region, Fig. 3b) to the reduction in adiabatic cooling. Additionally, slight anomalous upward motion (blue) over the longitudes of the Southern part of the Indian subcontinent (Fig. 3f) seems to support the increasing precipitation in that region. The nature of the phase of upper atmospheric wave activities and pressure velocity anomalies seems sufficient to explain the patterns of mean monsoon rainfall ^53,56, as discussed in Section 1.

Other contributors to the reduction in rainfall simulated under SAI are possible. First, SAI-induced warming in the lower stratosphere (Fig. 2a) increases the static stability of the tropical troposphere ^30,32, potentially inhibiting convection in the core Monsoon region (Fig. 2f). A similar mechanism has been proposed to explain the role of temperature anomalies associated with the different phases of the Quasi-Biennial Oscillation on the tropical convection in observations and idealized models ^58,59. Second, the substantial reductions in the low-level wind speed at 850 hPa level over the Arabian Sea (anomalous easterly winds in Fig. 2e; compared with the climatology in Fig. S3b) prevents moisture convergence (Fig S4) to the mainland, resulting in a reduction of the mean Indian summer monsoon rainfall ^44,60. Third, an anomalous northwesterly wind over north India and an anticyclonic feature over the monsoon trough region (over the head Bay of Bengal) do not allow the continental ITCZ to move northward ⁵¹, hindering the monsoon strength and moisture inflow from the Bay of Bengal branch.

Finally, past studies have reported that a global warming hiatus scenario could further weaken the pressure gradient between the Mascarene High and the northern hemisphere landmass ⁶¹. Our findings show similar results under the SAI scenario (Figure S5). Such weakening of Mascarene High intensity could have further weakened the high-level cross-equatorial winds in the western Indian Ocean and monsoon hydrology.

4.1 Cloud cover anomalies

Clouds play a crucial role in the energy and hydrological cycles of the Earth–-atmosphere system ^62,63. High clouds dominate the SAM region during monsoon months, whereas low clouds are more frequent ⁶⁴. For the SAI scenario, a significant and widespread reduction in low and high cloud cover is simulated over the Indian region. A notable reduction (> 5%) in high cloud cover is found over a large part of the Indian landmass aligned to the maximum rainfall reduction (Fig. 1). A similar signature is also observed in total cloud cover (Fig. 4c). On the contrary, the low cloud anomalies possess a spatial pattern (north-south dipole) similar to precipitation anomalies.

The moisture deficit over land, reduced convection, and the lower and mid-tropospheric anomalous high-pressure system are possible reasons behind such widespread reduction in cloud cover under the SAI scenario.

Dust particles warm the atmosphere while reducing sunlight reaching the surface, leading to atmospheric warming and surface cooling ⁶⁵. The Middle East and North Africa region is the major summertime dust source in South Asia ^60,66. Observational studies indicate that dust-induced warming (clear sky) is highest over the Arabian Sea compared to many dust-laden regions across the globe ⁶⁷. Based on modelling estimates, local dust-induced radiative forcing could be as large as ~ 36 Wm^− 2 over the Arabian Sea during the summer monsoon ⁶⁰. Such a large radiative forcing over the Arabian Sea has been found to trigger large-scale convergence over the North Africa/Arabian Peninsula region and act as feedback to strengthen the northward component of the summer monsoon westerly winds over the Arabian Sea ⁶⁰. Consequently, this enhances moisture convergence and precipitation over the Indian region ^66,68.

We find a substantial reduction in the dust burden under the SAI scenario. Such negative changes in dust have previously been reported from these simulations under both the SAI and RCP scenarios (slightly larger for the SAI scenario), and are due to the weakening of the dust hotspot emissions from the sources of the Middle East, along with changes in soil water, precipitation patterns, and leaf area index ⁶⁹.

In agreement with the previous analysis, we find the most substantial negative changes in regions that are climatologically high in dust (Fig. 5). Such a substantial reduction in dust load over the Arabian sea and adjacent regions might further contribute to the simulated rainfall reduction in the monsoon region under SAI. In addition, an increase in the dust burden is found over regions of the Indo-Gangetic plane, which could be the result of the anomalous SAI-induced monsoon drying over the mainland and a reduction in the strength of wet removal processes, a dominant feature during the monsoon season. Such change in dust over the North Indian regions may have further implications on radiative balance and regional climate change.

The annual global economic losses from floods have surpassed $30 billion over the previous decade, with a significant portion of these costs attributed to high rainfall events in Asia (International Disaster Data Base, http://www.emdat.be ). In addition, the floods attributed to extreme events in India contribute to 10% of the global economic loss. India witnessed 268 reported cases of flooding between 1950 and 2015, impacting around 825 million individuals. These floods displaced 17 million people and caused the loss of 69,000 lives (International Disaster Data Base).

Despite declining trends in mean rainfall (10–20% over the central Indian region), widespread heavy rain events in central India have increased from 1950 to 2015 ^70,71. Such a rise in extreme rainfall events over India is linked to the moisture contribution from the Arabian Sea and the Bay of Bengal ⁷². We have investigated the total rainfall in different percentile bands for the control, the future RCP85, and future SAI conditions (Fig. 6) over Central India.

Under the high GHG emission RCP8.5 scenarios, the precipitation is increasing in all the bins. On the other hand, a notable reduction in extreme precipitation (90-99th percentile) is simulated under the SAI scenario compared to the present day (CTRL). Hence, this SAI scenario may help reduce precipitation extremes causing devastating floods; however, it may also reduce water availability due to the concurrent reductions in the mean rainfall (Section 3).

7. Summary and conclusion

The continuing global warming trend due to climate change will likely lead to more irregular precipitation patterns with an increasing risk of both flooding and drought. Using a three-member ensemble from Geoengineering Large Ensemble (GLENS) simulations, our study investigates the effects of stratospheric aerosol intervention (SAI) on the South Asian summer monsoon (SAM). Unlike many of the past SAI studies, which mainly focused on the response to SAI, we have investigated the possible mechanisms behind the rainfall changes in the SAM region, summarized as follows:

A reduction in both mean and extreme SAM precipitation was simulated for the SAI scenario compared to both a near present-day period and the same future period with RCP8.5 in the absence of SAI.

The use of sulfate aerosols for SAI results in a heating of the lower tropical stratosphere and wave-induced weakening of the northern hemispheric subtropical jet stream.

We observe a strong relationship between the SAI-induced phase of wave activities, geopotential height anomalies, the strength of the Asian Monsoon Anticyclone, lower atmospheric pressure gradients, and wind circulations. These changes play a decisive role in lowering summertime rainfall over the core monsoon region.

The weakening of the strength of the Asian summer monsoon Anticyclone with SAI dampens the convective activities in the core monsoon region. The positive anomalous pressure in the lower troposphere is further associated with reduction in convection and cloud cover. In addition, the heating in the lower tropical stratosphere acts to increase the tropospheric static stability, which may further reduce tropical convection.

A substantial reduction in dust is found over the Arabian Sea. Since local changes in dust can be a significant contributor to the SAM rainfall variability under both present-day and future climate change, such regional dust change might play an additional role in amplifying the SAI effects on SAM hydrology.

The different processes described above may contribute to some extent in driving the SAI-induced changes in the location and magnitude of SAM and its rainfall. More detailed investigations using additional model experiments are needed to understand and quantify the relative roles of different mechanisms, in particular under different SAI scenarios ⁷² and strategies ³². In addition, our results are based on only one climate model, so more work is needed to apply a similar analysis to other models ^73,74, identifying inter-model differences and potential reasons behind them.

It is worth mentioning that the results reported here do not advocate for any deployment of stratospheric aerosol geoengineering but rather attempt to improve our understanding of its consequences on the regional climate system like SAM. Before considering SAI as a feasible approach working alongside strong greenhouse gas emission reductions to mitigate global warming, it is essential to thoroughly investigate its impact on the Earth system and its consequences for both natural and human systems. As such, our work constitutes a crucial first step to understand SAM variability under different SAI applications, which would be essential for sustainable development and disaster preparedness in South Asia.

Author contributions

Author contributions: A. A. and S.T. designed the research; A. A. and S.T. performed the data analysis. A.A., S.T., E.B., and S.F. performed research and wrote the paper.

Competing interests

The authors declare no competing interest.

Acknowledgments

The authors thank NOAA Climate Program Office Earth’s Radiation Budget Awards 820 Number 03-01-07-001 and NA22OAR4310477. EMB also acknowledges support from the National Oceanic and Atmospheric Administration (NOAA) cooperative agreement NA22OAR4320151 and the Earth’s Radiative Budget (ERB) initiative. The CESM project is supported primarily by the National Science Foundation. This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the NSF under Cooperative Agreement No. 1852977. Computing and data storage resources, including the Cheyenne supercomputer (doi:10.5065/D6RX99HX), were provided by the Computational and Information Systems Laboratory (CISL) at NCAR.

Data availability

All simulations were performed on the Cheyenne high-performance computing platform (https://doi.org/10.5065/D6RX99H) and are available on the Earth System Grid. More details can be found at https://www.cesm.ucar.edu/community-projects/glens.

Code availability

All the code used in this paper can be obtained at a reasonable request to the corresponding author.

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You are reading this latest preprint version

South Asian Summer Monsoon under Stratospheric Aerosol Intervention

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Version 1

Abstract

Figures

1. Introduction

2. Methods

3. Mean precipitation response to SAI and climate change

4. Drivers of the SAM rainfall changes under SAI

4.1 Cloud cover anomalies

5. Dust Feedback

6. Impact of SAI on extreme precipitation

7. Summary and conclusion

Declarations

Author contributions

Competing interests

Acknowledgments

Data availability

Code availability

References

Additional Declarations

Supplementary Files

Status:

Version 1