Edinburgh Research Explorer
Anthropogenic Aerosols and the Weakening of the South Asian
Summer Monsoon
Citation for published version:
Bollasina, MA, Ming, Y & Ramaswamy, V 2011, 'Anthropogenic Aerosols and the Weakening of the South
Asian Summer Monsoon', Science, vol. 334, no. 6055, pp. 502-505.
https://doi.org/10.1126/science.1204994
Digital Object Identifier (DOI):
10.1126/science.1204994
Link:
Link to publication record in Edinburgh Research Explorer
Document Version:
Peer reviewed version
Published In:
Science
Publisher Rights Statement:
Author final draft as submitted for publication.
___________________________________________________________
Cite As: Bollasina, MA, Ming, Y & Ramaswamy, V 2011, 'Anthropogenic aerosols and the weakening of the
south asian summer monsoon' Science, vol 334, no. 6055, pp. 502-505.
___________________________________________________________
The final version was published by the American Association for the Advancement of Science (2011).
___________________________________________________________
General rights
Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)
and / or other copyright owners and it is a condition of accessing these publications that users recognise and
abide by the legal requirements associated with these rights.
Take down policy
The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer
content complies with UK legislation. If you believe that the public display of this file breaches copyright please
contact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately and
investigate your claim.
Download date: 10. Aug. 2022
Anthropogenic aerosols and the weakening of the
South Asian summer monsoon
Massimo A. Bollasina,
1
Yi Ming,
2∗
V. Ramaswamy
2
1
Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ 08542
2
Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ 08542
∗
To whom correspondence should be addressed; E-mail: Yi.Ming@noaa.gov.
Observations show that South Asia underwent a widespread summertime dry-
ing during the second half of the 20th century, but it is still unclear whether
this prolonged trend was due to natural variations or human activities. Here
we use a series of climate model experiments to investigate the South Asian
monsoon response to natural and anthropogenic factors. We find that the ob-
served precipitation decrease can be attributed almost entirely to man-made
aerosols. The drying is a robust outcome of a slowdown of the tropical merid-
ional overturning circulation, which compensates for the aerosol-induced en-
ergy imbalance between the northern and southern hemispheres. These results
provide compelling evidence of the prominent role of aerosols in shaping re-
gional climate change over South Asia.
The South Asian summer monsoon provides up to 80% of the annual mean precipitation for
most regions of India, and has tremendous impacts on agriculture, health, water resources, econ-
omy, and ecosystems throughout South Asia (1). It is also an important part of the global-scale
1
atmospheric circulation, as its vigorous ascent dominates the boreal summer tropical merid-
ional overturning (the Hadley circulation) (2), and has profound remote influences (3). A possi-
ble long-term (decadal to centennial) shift in monsoon rainfall associated with climate change
could have even more far-reaching consequences for the region than natural variations. A num-
ber of observational studies have investigated the multi-decadal trend of monsoon rainfall over
India, and found a persistent drying trend during the second half of the 20th century (4–7). Yet,
the root cause of this trend remains unclear.
Both aerosols and greenhouse gases can affect the South Asian summer monsoon. The in-
crease of aerosols and associated decrease in surface solar radiation (“dimming”) over South
Asia have been well documented (4, 8). Climate model experiments suggested that sulfate
aerosol may significantly reduce monsoon precipitation (9). Recent studies, some of which fo-
cused specifically on absorbing aerosols (4, 8, 10, 11), postulate as possible mechanisms both
surface cooling from reduced surface solar radiation (and consequent reduction of the merid-
ional thermal contrast between the northern and southern Indian Ocean (5)) and atmospheric
heating due to absorption of solar radiation (8).
The warming caused by increased greenhouse gases, meanwhile, may also play a role. An-
nual mean tropical sea surface temperatures (SST) have increased on average by ∼0.5 K since
the 1950s. This warming is particularly notable over the Indian Ocean (12). Tropical circulation
(particularly its zonal component) is expected to weaken in response to an increase in surface
temperature since global-mean precipitation, which is controlled by the overall atmospheric en-
ergy balance, cannot increase as fast as the lower tropospheric water vapor concentration (the
thermodynamical scaling argument) (13, 14). Despite a weakening of the monsoon circula-
tion, most studies projected an increase of the seasonal monsoon rainfall under global warming,
partly owing to more abundant water vapor (15).
We utilize several long-term observational datasets of precipitation to identify possible re-
2
cent trends in the South Asian monsoon (16). The boreal summer (June-September) climatolog-
ical precipitation has a widespread maximum over central-northern India, which includes part of
the vastly irrigated Indo-Gangetic Plain. Our initial focus is on this analysis region (76
◦
-87
◦
E,
20
◦
-28
◦
N), whose location and size are similar to those in (22). The CRU data show a marked
reduction from the 1950s to the end of the 20th century (Fig. 1). The linear trend of -0.95 mm
day
−1
(50 years)
−1
is statistically significant at the 95% confidence level (p = 0.04) (23). Com-
parable drying trends are also seen for UDEL and PREC/L. This finding is broadly consistent
with the previous studies (5, 7, 25, 26), which made use of additional datasets. Although the
IMR data show a small downward trend, it is not statistically significant enough. The decrease
amounts to 9-11% of the total monsoon rainfall received by the region for CRU, UDEL and
PREC/L, and 2% for IMR (see Fig. S4 and SOM).
Even more interestingly, a coherent large-scale pattern emerges: a drying over central-
northern India and most of Southeast Asia (consisting of Indochina and the Maritime Continent)
coinciding with a wettening over southern India and over northwestern India and Pakistan (Fig.
2). This distinct spatial structure is consistent among the majority of the datasets (see Fig. S5
and SOM). Averaged over the whole country, the Indian summer rainfall underwent a reduction
of 4-5% over 50 years (4,6,7).
Was this observed change of the South Asian monsoon precipitation caused by natural vari-
ability or human interference? If the latter, what were the relative contributions of aerosols
and greenhouse gases, the two most important anthropogenic climate forcing agents? Answer-
ing these questions pose a challenging test case on our fundamental understanding of the core
working of the Earth’s hydrological cycle, and holds the key to a more reliable projection of
regional climate change.
Ensemble simulations with a state-of-the-art coupled atmosphere-ocean global climate model
(GCM) provide us a means to attribute the observed long-term trend. The model used here is the
3
U.S. National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynam-
ics Laboratory (GFDL) CM3 GCM, which includes an explicit treatment of the aerosol-cloud
interactions and aerosol indirect effects (27). The model data analyzed in this study cover the
period 1951-1999 and come mainly from three sets of historical simulations (1860-2005): (1)
a five-member ensemble with all the forcings, natural (solar variations and volcanoes) and an-
thropogenic (well-mixed greenhouse gases, ozone, aerosols and land use) alike (ALL F), (2) a
three-member ensemble with greenhouse gases and ozone forcings only (WMGGO3), and (3)
a three-member ensemble with aerosol forcing only (AERO). The ensemble simulations forced
with all the natural forcings only and with all the anthropogenic forcings only are also examined
(see SOM).
The all-forcing ensemble (ALL
F) captures the drying trend over central-northern India rea-
sonably well (Fig. 1). The model also simulates a large-scale drying over the eastern Indian
Ocean and Southeast Asia, and a moistening over the northern Arabian Sea and western equa-
torial Indian Ocean, in agreement with the observed pattern over adjacent lands (Fig. 2). These
changes are determined to be of anthropogenic origin, as the naturally forced ensemble cannot
reproduce the observed pattern (Figs. S6 and S7). Deficiencies in simulating the dynamical in-
teraction of the monsoon flow with the elevated orography of the Tibetan Plateau, owing to the
model’s relatively coarse horizontal resolution, are likely to be responsible for the substantial
disagreement over southern China.
The individual ensembles forced with different forcing combinations clearly indicate that
the drying over central-northern India can be attributed to aerosols (AERO) (Fig. 1). The region
would become wetter if the model is driven only by greenhouse gases and ozone (WMGGO3).
Note that WMGGO3 appears to amplify the trend in AERO, indicating nonlinearity in the total
response to both forcings (29,30). Aerosols (AERO) also give rise to the changes in the latitudi-
nal direction over Southeast Asia in the all-forcing ensemble (Fig. 2). The reduced precipitation
4