Will the South Asian monsoon overturning circulation stabilize any further?

Krishnan, R.; Sabin, T. P.; Ayantika, D. C.; Kitoh, Akio; Sugi, Masato; Murakami, Hiroyuki; Turner, Andrew G.; Slingo, Julia; Rajendran, Kavirajan

doi:10.1007/s00382-012-1317-0

Cited by 151 publications

(88 citation statements)

References 79 publications

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“…Meanwhile, a recent comparison of trends in four different gridded rainfall data sets for India noted area-averaged rainfall decreases since the 1950s (Bollasina et al, 2011), although with considerable spatial differences (especially in north-eastern peninsular India) and relatively few gridpoints yielding statistically significant trends. Negative trends in the orographic rainfall near the Western Ghats on the west coast of India in APHRODITE gridded gauge data have been attributed to weakening of the monsoon circulation (Krishnan et al, 2013).…”

Section: Published By Copernicus Publications On Behalf Of the Europementioning

confidence: 99%

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

2015

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Abstract. Comparison of single-forcing varieties of 20thcentury historical experiments in a subset of models from the Fifth Coupled Model Intercomparison Project (CMIP5) reveals that South Asian summer monsoon rainfall increases towards the present day in Greenhouse Gas (GHG)-only experiments with respect to pre-industrial levels, while it decreases in anthropogenic aerosol-only experiments. Comparison of these single-forcing experiments with the all-forcings historical experiment suggests aerosol emissions have dominated South Asian monsoon rainfall trends in recent decades, especially during the 1950s to 1970s. The variations in South Asian monsoon rainfall in these experiments follows approximately the time evolution of inter-hemispheric temperature gradient over the same period, suggesting a contribution from the large-scale background state relating to the asymmetric distribution of aerosol emissions about the equator.By examining the 24 available all-forcings historical experiments, we show that models including aerosol indirect effects dominate the negative rainfall trend. Indeed, models including only the direct radiative effect of aerosol show an increase in monsoon rainfall, consistent with the dominance of increasing greenhouse gas emissions and planetary warming on monsoon rainfall in those models. For South Asia, reduced rainfall in the models with indirect effects is related to decreased evaporation at the land surface rather than from anomalies in horizontal moisture flux, suggesting the impact of indirect effects on local aerosol emissions. This is confirmed by examination of aerosol loading and cloud droplet number trends over the South Asia region. Thus, while remote aerosols and their asymmetric distribution about the equator play a role in setting the inter-hemispheric temperature distribution on which the South Asian monsoon, as one of the global monsoons, operates, the addition of indirect aerosol effects acting on very local aerosol emissions also plays a role in declining monsoon rainfall. The disparity between the response of monsoon rainfall to increasing aerosol emissions in models containing direct aerosol effects only and those also containing indirect effects needs to be urgently investigated since the suggested future decline in Asian anthropogenic aerosol emissions inherent to the representative concentration pathways (RCPs) used for future climate projection may turn out to be optimistic.In addition, both groups of models show declining rainfall over China, also relating to local aerosol mechanisms. We hypothesize that aerosol emissions over China are large enough, in the CMIP5 models, to cause declining monsoon rainfall even in the absence of indirect aerosol effects. The same is not true for India.

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Section: Published By Copernicus Publications On Behalf Of the Europementioning

confidence: 99%

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

2015

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“…Despite the large number of recent studies, the relative roles of anthropogenic greenhouse warming vs. aerosols on Asian monsoon (ASM) climate change remain mired as a subject of debate and continued intense research (Ramanathan et al 2001;Chung and Ramanathan 2006;Lau et al , 2008Meehl et al 2008;Ramanathan and Carmichael 2008;Wang et al 2009Wang et al , 2012Wang et al , 2017Li et al 2010Li et al , 2016Bollasina et al 2011;Lee and Wang 2012;Turner and Annamalai 2012;IPCC 2013;Krishnan et al 2013;Song et al 2014;Jin and Wang 2017). One of the main reasons for the slow progress is that ASM climate change is an immensely complex phenomenon, involving global-scale forcing, coupled to regional-and local-scale feedbacks arising from natural variability in the atmosphere-ocean-land system, as well as human activities.…”

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Changing circulation structure and precipitation characteristics in Asian monsoon regions: greenhouse warming vs. aerosol effects

2017

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Using model outputs from CMIP5 historical integrations, we have investigated the relative roles of anthropogenic emissions of greenhouse gases (GHG) and aerosols in changing the characteristics of the large-scale circulation and rainfall in Asian summer monsoon (ASM) regions. Under GHG warming, a strong positive trend in low-level moist static energy (MSE) is found over ASM regions, associated with increasing large-scale land-sea thermal contrast from 1870s to present. During the same period, a mid-tropospheric convective barrier (MCB) due to widespread reduction in relative humidity in the mid-and lower troposphere is strengthening over the ASM regions, in conjunction with expanding areas of anomalous subsidence associated with the Deep Tropical Squeeze (Lau and Kim in Proc Natl Acad Sci 12: [3630][3631][3632][3633][3634][3635] 2015). The opposing effects of MSE and MCB lead to enhanced total ASM rainfall, but only a partial strengthening of the southern portion of the monsoon meridional circulation, coupled to anomalous multi-cellular overturning motions over ASM land. Including anthropogenic aerosol emissions strongly masks MSE but enhances MCB via increased stability in the lower troposphere, resulting in an overall weakened ASM circulation with suppressed rainfall. Analyses of rainfall characteristics indicate that under GHG, overall precipitation efficiency over the ASM region is reduced, manifesting in less moderate but more extreme heavy rain events. Under combined effects of GHG and aerosols, precipitation efficiency is unchanged, with more moderate, but less extreme rainfall.© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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“…A few recent studies have also pointed out the increasing frequency of extreme rainfall events over the South Asian monsoon domain, due to a warming climate 3 . However, quite a few other studies indicate that the monsoon rainfall [4][5][6] and the Hadley and Walker circulations [7][8][9] are weakening over the South Asian domain during the past half century (since 1950s). Some of these studies suggest that, though the extreme rainfall events have increased over some regions 3 , the frequency of moderate-to-heavy rainfall events has decreased over the subcontinent 8 .…”

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“…However, quite a few other studies indicate that the monsoon rainfall [4][5][6] and the Hadley and Walker circulations [7][8][9] are weakening over the South Asian domain during the past half century (since 1950s). Some of these studies suggest that, though the extreme rainfall events have increased over some regions 3 , the frequency of moderate-to-heavy rainfall events has decreased over the subcontinent 8 . In addition, several recent studies based on the Coupled Model Inter-comparison Project phase 5 (CMIP5) indicate a significant weakening of the large-scale Asian summer monsoon circulation, especially in the middle to upper atmosphere [10][11][12] .…”

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Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

et al. 2015

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There are large uncertainties looming over the status and fate of the South Asian summer monsoon, with several studies debating whether the monsoon is weakening or strengthening in a changing climate. Our analysis using multiple observed datasets demonstrates a significant weakening trend in summer rainfall during 1901-2012 over the central-east and northern regions of India, along the Ganges-Brahmaputra-Meghna basins and the Himalayan foothills, where agriculture is still largely rain-fed. Earlier studies have suggested an increase in moisture availability and land-sea thermal gradient in the tropics due to anthropogenic warming, favouring an increase in tropical rainfall. Here we show that the land-sea thermal gradient over South Asia has been decreasing, due to rapid warming in the Indian Ocean and a relatively subdued warming over the subcontinent. Using long-term observations and coupled model experiments, we provide compelling evidence that the enhanced Indian Ocean warming potentially weakens the land-sea thermal contrast, dampens the summer monsoon Hadley circulation, and thereby reduces the rainfall over parts of South Asia.

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Will the South Asian monsoon overturning circulation stabilize any further?

Cited by 151 publications

References 79 publications

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models

Changing circulation structure and precipitation characteristics in Asian monsoon regions: greenhouse warming vs. aerosol effects

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

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