Observations show that South Asia underwent a widespread summertime drying during the second half of the 20th century, but it is unclear whether this trend was due to natural variations or human activities. We used a series of climate model experiments to investigate the South Asian monsoon response to natural and anthropogenic forcings. We find that the observed precipitation decrease can be attributed mainly to human-influenced aerosol emissions. The drying is a robust outcome of a slowdown of the tropical meridional overturning circulation, which compensates for the aerosol-induced energy imbalance between the Northern and Southern Hemispheres. These results provide compelling evidence of the prominent role of aerosols in shaping regional climate change over South Asia.

The South Asian haze builds up from December to May, is mostly of anthropogenic origin, and absorbs part of the solar radiation. The influence of interannual variations of absorbing aerosols over the IndoGangetic Plain in May on the Indian summer monsoon is characterized by means of an observational analysis. Insight into how the aerosol impact is generated is also provided.It is shown that anomalous aerosol loading in late spring leads to remarkable and large-scale variations in the monsoon evolution. Excessive aerosols in May lead to reduced cloud amount and precipitation, increased surface shortwave radiation, and land surface warming. The June (and July) monsoon anomaly associated with excessive May aerosols is of opposite sign over much of the subcontinent (although with a different pattern) with respect to May. The monsoon strengthens in June (and July).The analysis suggests that the significant large-scale aerosol influence on monsoon circulation and hydroclimate is mediated by the heating of the land surface, pursuant to reduced cloudiness and precipitation in May. The finding of the significant role of the land surface in the realization of the aerosol impact is somewhat novel.

The Northern Hemisphere monsoons are an integral component of Earth's hydrological cycle and affect the lives of billions of people. Observed precipitation in the monsoon regions underwent substantial changes during the second half of the twentieth century, with drying from the 1950s to mid-1980s and increasing precipitation in recent decades. Modeling studies suggest that anthropogenic aerosols have been a key factor driving changes in tropical and monsoon precipitation. Here we apply detection and attribution methods to determine whether observed changes are driven by human influences using fingerprints of individual forcings (i.e., greenhouse gas, anthropogenic aerosol, and natural) derived from climate models. The results show that the observed changes can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. Anthropogenic aerosol, not greenhouse gas or natural forcing, has been the dominant influence on Northern Hemisphere monsoon precipitation over the second half of the twentieth century.

The late twentieth century response of the South Asian monsoon to changes in anthropogenic aerosols from local (i.e., South Asia) and remote (i.e., outside South Asia) sources was investigated using historical simulations with a state-of-the-art climate model. The observed summertime drying over India is replaced by widespread wettening once local aerosol emissions are kept at preindustrial levels while all the other forcings evolve. Constant remote aerosol emissions partially suppress the precipitation decrease. While predominant precipitation changes over India are thus associated with local aerosols, remote aerosols contribute as well, especially in favoring an earlier monsoon onset in June and enhancing summertime rainfall over the northwestern regions. Conversely, temperature and near-surface circulation changes over South Asia are more effectively driven by remote aerosols. These changes are reflected into northward cross-equatorial anomalies in the atmospheric energy transport induced by both local and, to a greater extent, remote aerosols.

Anthropogenic aerosols are a key driver of changes in summer monsoon precipitation in the Northern Hemisphere during the 20th century. Here we apply detection and attribution methods to investigate causes of change in the West African and South Asian monsoons separately and identify the aerosol source regions that are most important for explaining the observed changes during 1920-2005. Historical simulations with the GFDL-CM3 model are used to derive fingerprints of aerosol forcing from different regions. For West Africa, remote aerosol emissions from North America and Europe (NAEU) are essential in order to detect the anthropogenic signal in observed monsoon precipitation changes. The changes are significantly underestimated in the model, however. While natural (volcanic) forcing seems to also play a role, the dominant contribution is found to come from aerosol-induced changes in the interhemispheric temperature gradient and associated meridional shifts of the Intertropical Convergence Zone. For South Asia, in contrast, changes in observed monsoon precipitation cannot be explained without local emissions. Here the findings show a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975, and since then by the increase in local emissions offsetting the decrease of NAEU emissions. The results show that the aerosol forcing from individual emission regions is strong enough to be detected over internal variability. They also underscore the importance of the spatial pattern of global-aerosol emissions, which is likely to continue to change throughout the expected near-future decline in global emissions.

[1] The viability of the elevated heat pump hypothesis, a mechanism proposed by for absorbing aerosols' impact on South Asian summer monsoon hydroclimate, is assessed from a careful review of these authors' own analysis and others since then. The lack of appreciation of the spatial distribution of the aerosol-related precipitation signal over the Indian subcontinent, its east-west asymmetric structure, in particular, apparently led to the development of this hypothesis. Its key elements have little observational support, and the hypothesis is thus deemed untenable. Quite telling is the observation that local precipitation signal over the core aerosol region is negative, i.e., increased loadings are linked with suppressed precipitation and not more as claimed by the hypothesis. Finally, motivated by the need to address causality, the analysis of contemporaneous aerosol-monsoon links by Bollasina et al. (2008) is extended by examining the structure of hydroclimate lagged regressions on aerosols. It is shown that findings obtained from contemporaneous analysis can be safely interpreted as representing the impact of aerosols on precipitation, not vice versa. The possibility that both are shaped by a slowly evolving, large-scale circulation pattern cannot however be ruled out.Citation: Nigam, S., and M. Bollasina (2010), "Elevated heat pump" hypothesis for the aerosol-monsoon hydroclimate link:

The impact of the late twentieth century increase of anthropogenic aerosols on the Indian monsoon onset was investigated with a state‐of‐the‐art climate model with fully interactive aerosols and chemistry. We find that aerosols are likely responsible for the observed earlier onset, resulting in enhanced June precipitation over most of India. This shift is preceded by strong aerosol forcing over the Bay of Bengal and Indochina, mostly attributable to the direct effect, resulting in increased atmospheric stability that inhibits the monsoon migration in May. The adjusted atmospheric circulation leads to thermodynamic changes over the northwestern continental region, including increased surface temperature and near‐surface moist static energy, which support a stronger June flow and, facilitated by a relative warming of the Indian Ocean, a vigorous northwestward precipitation shift. These findings underscore the importance of dynamical feedbacks and of regional land‐surface processes for the aerosol‐monsoon link.

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