Stier, Philip, Susan C van den Heever, Matthew W Christensen, Edward Gryspeerdt, Guy Dagan, Stephen M Saleeby, Massimo Bollasina, Leo J Donner, Kerry A Emanuel, Annica M L Ekman, Graham Feingold, Paul Field, Piers M Forster, Jim M Haywood, Ralph A Kahn, Ilan Koren, Christian Kummerow, Tristan L'Ecuyer, Ülrike Lohmann, Yi Ming, Gunnar Myhre, Johannes Quaas, Daniel Rosenfeld, Bjørn H Samset, Axel Seifert, Graeme L Stephens, and Wei-Kuo Tao, August 2024: Multifaceted aerosol effects on precipitation. Nature Geoscience, 17, DOI:10.1038/s41561-024-01482-6. Abstract
Aerosols have been proposed to influence precipitation rates and spatial patterns from scales of individual clouds to the globe. However, large uncertainty remains regarding the underlying mechanisms and importance of multiple effects across spatial and temporal scales. Here we review the evidence and scientific consensus behind these effects, categorized into radiative effects via modification of radiative fluxes and the energy balance, and microphysical effects via modification of cloud droplets and ice crystals. Broad consensus and strong theoretical evidence exist that aerosol radiative effects (aerosol–radiation interactions and aerosol–cloud interactions) act as drivers of precipitation changes because global mean precipitation is constrained by energetics and surface evaporation. Likewise, aerosol radiative effects cause well-documented shifts of large-scale precipitation patterns, such as the intertropical convergence zone. The extent of aerosol effects on precipitation at smaller scales is less clear. Although there is broad consensus and strong evidence that aerosol perturbations microphysically increase cloud droplet numbers and decrease droplet sizes, thereby slowing precipitation droplet formation, the overall aerosol effect on precipitation across scales remains highly uncertain. Global cloud-resolving models provide opportunities to investigate mechanisms that are currently not well represented in global climate models and to robustly connect local effects with larger scales. This will increase our confidence in predicted impacts of climate change.
Wilcox, Laura J., Robert J Allen, Bjørn H Samset, Massimo Bollasina, Paul T Griffiths, James Keeble, Marianne T Lund, Risto Makkonen, Joonas Merikanto, Declan O'Donnell, and David J Paynter, et al., August 2023: The Regional Aerosol Model Intercomparison Project (RAMIP). Geoscientific Model Development, 16(15), DOI:10.5194/gmd-16-4451-20234451-4479. Abstract
Changes in anthropogenic aerosol emissions have strongly contributed to global and regional trends in temperature, precipitation, and other climate characteristics and have been one of the dominant drivers of decadal trends in Asian and African precipitation. These and other influences on regional climate from changes in aerosol emissions are expected to continue and potentially strengthen in the coming decades. However, a combination of large uncertainties in emission pathways, radiative forcing, and the dynamical response to forcing makes anthropogenic aerosol a key factor in the spread of near-term climate projections, particularly on regional scales, and therefore an important one to constrain. For example, in terms of future emission pathways, the uncertainty in future global aerosol and precursor gas emissions by 2050 is as large as the total increase in emissions since 1850. In terms of aerosol effective radiative forcing, which remains the largest source of uncertainty in future climate change projections, CMIP6 models span a factor of 5, from −0.3 to −1.5 W m−2. Both of these sources of uncertainty are exacerbated on regional scales.
Liu, Zhen, Yi Ming, Lin Wang, Massimo Bollasina, M Luo, and Ngar-Cheung Lau, et al., August 2019: A Model Investigation of Aerosol‐Induced Changes in the East Asian Winter Monsoon. Geophysical Research Letters, 46(16), DOI:10.1029/2019GL084228. Abstract
The response of the East Asian winter monsoon (EAWM) circulation to aerosols is studied using a coupled atmosphere‐slab ocean general circulation model. In the extratropics, the aerosol‐induced cooling in the mid‐latitudes leads to an intensified subtropical jet stream, a deepened East Asian trough, and thus an enhanced EAWM. In the tropics, the local Hadley circulation shifts southward to compensate for the interhemispheric asymmetry in aerosol radiative cooling. Anomalous subsidence at around 10°N leads to a salient anticyclone to the southwest of the Philippines. The associated southwesterlies advect abundant moisture to South China, resulting in local precipitation increase and suggesting a weaker EAWM. The EAWM response to aerosol forcing is thus driven by a competition between tropical and extratropical mechanisms, which has important implications for the future monsoon evolution as aerosol changes may follow different regional‐dependent trajectories.
Undorf, S, D Polson, Massimo Bollasina, and Yi Ming, et al., May 2018: Detectable impact of local and remote anthropogenic aerosols on the 20th century changes of West African and South Asian monsoon precipitation. Journal of Geophysical Research: Atmospheres, 123(10), DOI:10.1029/2017JD027711. Abstract
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 inter‐hemispheric temperature gradient and associated meridional shifts of the Inter‐tropical Convergence Zone. For South Asia, in contrast, changes in observed monsoon precipitation can not 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.
The late 20th 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 pre-industrial 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.
Bollasina, Massimo, and Yi Ming, February 2013: The general circulation model precipitation bias over the southwestern equatorial Indian Ocean and its implications for simulating the South Asian monsoon. Climate Dynamics, 40(3-4), DOI:10.1007/s00382-012-1347-7. Abstract
Most of current general circulation models
(GCMs) show a remarkable positive precipitation bias over
the southwestern equatorial Indian Ocean (SWEIO), which
can be thought of as a westward expansion of the simulated
IO convergence zone toward the coast of Africa. The bias
is common to both coupled and uncoupled models, suggesting
that its origin does not stem from the way boundary
conditions are specified. The spatio-temporal evolution of
the precipitation and associated three-dimensional atmospheric
circulation biases is comprehensively characterized
by comparing the GFDL AM3 atmospheric model to
observations. It is shown that the oceanic bias, which
develops in spring and reduces during the monsoon season,
is associated to a consistent precipitation and circulation
anomalous pattern over the whole Indian region. In the
vertical, the areas are linked by an anomalous Hadley-type
meridional circulation, whose northern branch subsides
over northeastern India significantly affecting the monsoon
evolution (e.g., delaying its onset). This study makes the
case that the precipitation bias over the SWEIO is forced
by the model excess response to the local meridional sea
surface temperature (SST) gradient through enhanced nearsurface
meridional wind convergence. This is suggested by
observational evidence and supported by AM3 sensitivity
experiments. The latter show that relaxing the magnitude
of the meridional SST gradient in the SWEIO can lead to a
significant reduction of both local and large-scale
precipitation and circulation biases. The ability of local
anomalies over the SWEIO to force a large-scale remote
response to the north is further supported by numerical
experiments with the GFDL spectral dry dynamical core
model. By imposing a realistic anomalous heating source
over the SWEIO the model is able to reproduce the main
dynamical features of the AM3 bias. These results indicate
that improved GCM simulations of the South Asian summer
monsoon could be achieved by reducing the springtime
model bias over the SWEIO. Deficiencies in the
atmospheric model, and in particular in the convective
parameterization, are suggested to play a key role. Finally,
the important mechanism controlling the simulated precipitation
distribution over South Asia found here should
be considered in the interpretation and attribution
Bollasina, Massimo, and Yi Ming, November 2013: The role of land-surface processes in modulating the Indian monsoon annual cycle. Climate Dynamics, 41(9-10), DOI:10.1007/s00382-012-1634-3. Abstract
The annual cycle of solar radiation, together with the resulting land–ocean differential heating, is traditionally considered the dominant forcing controlling the northward progression of the Indian monsoon. This study makes use of a state-of-the-art atmospheric general circulation model in a realistic configuration to conduct “perpetual†experiments aimed at providing new insights into the role of land–atmosphere processes in modulating the annual cycle of precipitation over India. The simulations are carried out at three important stages of the monsoon cycle: March, May, and July. Insolation and SSTs are held fixed at their respective monthly mean values, thus eliminating any external seasonal forcing. In the perpetual May experiment both precipitation and circulation are able to considerably evolve only by regional internal land–atmosphere processes and the mediation of soil hydrology. A large-scale equilibrium state is reached after approximately 270 days, closely resembling mid-summer climatological conditions. As a result, despite the absence of external forcing, intense and widespread rains over India are able to develop in the May-like state. The interaction between soil moisture and circulation, modulated by surface heating over the northwestern semi-arid areas, determines a slow northwestward migration of the monsoon, a crucial feature for the existence of desert regions to the west. This also implies that the land–atmosphere system in May is far from being in equilibrium with the external forcing. The inland migration of the precipitation front comprises a succession of large-scale 35–50 day coupled oscillations between soil moisture, precipitation, and circulation. The oscillatory regime is self-sustained and entirely due to the internal dynamics of the system. In contrast to the May case, minor changes in the land–atmosphere system are found when the model is initialized in March and, more surprisingly, in July, the latter case further emphasizing the role of northwestern surface heating.
Bollasina, Massimo, Yi Ming, and V Ramaswamy, July 2013: Earlier onset of the Indian Monsoon in the late 20th century: The role of anthropogenic aerosols. Geophysical Research Letters, 40(14), DOI:10.1002/grl.50719. Abstract
The impact of the late 20th 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 thermodynamical 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.
Bollasina, Massimo, and S Nigam, June 2011: Modeling of regional hydroclimate change over the Indian subcontinent: Impact of the expanding Thar Desert. Journal of Climate, 24(12), DOI:10.1175/2010JCLI3851.1. Abstract
The Thar Desert between northwestern India and Pakistan is the most densely populated desert region in the world, and the vast surrounding areas are affected by rapid soil degradation and vegetation loss. The impact of an expanded desert (implemented by changing vegetation type and related greenness fraction, albedo, surface roughness length, emissivity, among others) on the South Asian summer monsoon hydroclimate is investigated by means of 7-month, 4-member ensemble sensitivity experiments with the Weather Research and Forecasting model.
It is found that extended desertification significantly affects the monsoon at local and large scales. Locally, the atmospheric water cycle weakens because precipitation, evaporation, and atmospheric moisture convergence all decrease; soil moisture and runoff reduce too. Air temperature cools because of an increase in albedo (the desert makes the area brighter) and a reduction of surface turbulent fluxes; the cooling is partially offset by adiabatic descent, generated to maintain thermodynamic balance and originating at the northern flank of the low-level anticyclone forced by desert subsidence. Regionally, an anomalous northwesterly flow over the Indo-Gangetic Plain weakens the monsoon circulation over northeastern India, causing precipitation to decrease and the formation of an anomalous anticyclone in the region. As a result, the middle troposphere cools because of a decrease in latent heat release, but the ground heats up because of a reduction in cloudiness. At larger scale, the interaction between the anomalous circulation and the mountains leads to an increase in precipitation over the eastern Himalayas and Indochina.
The findings of this study reveal that the expansion of the Thar Desert can lead to a pronounced and large-scale impact on summer monsoon hydroclimate, with a potential to redistribute precious water over South Asia.
Bollasina, Massimo, and S Nigam, September 2011: The summertime ‘‘heat’’ low over Pakistan/northwestern India: evolution and origin. Climate Dynamics, 37(5-6), DOI:10.1007/s00382-010-0879-y. Abstract
A deep low in sea-level pressure is present
from May to September over Pakistan and northwestern
India (hereafter, the Pak–India low). It is often referred as
the ‘‘heat’’ low to convey the significance of surface thermal
effects reckoned to be important for its origin. The
present analysis, rooted in observations and diagnostic
modeling, suggests that the Pak–India low is influenced
both by regional and remote forcing. Regionally, the
influence of Hindu Kush mountains is found to be stronger
than the impact of land-surface heating and attendant
sensible heating of the planetary boundary layer, questioning
the suitability of the ‘‘heat’’ label in canonical
references to this circulation feature. Observational analysis
indicates that the notable May-to-June deepening of the
Pak–India low and its further deepening in July, however,
arises from remote forcing—the development of monsoon
deep-convection over the Bay of Bengal and eastern India
in June and July. It is hypothesized that the associated
upstream descent over Iran–Turkmenistan–Afghanistan
(i.e. east of the Caspian Sea) and related low-level northerlies
over the Elburz–Zagros–Hindu Kush mountains
contribute to the strengthening of the Pak–India low in
June (and July) from interaction with regional orography.
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.
Nigam, S, and Massimo Bollasina, April 2011: Reply to comment by K. M. Lau and K. M. Kim on '. Journal of Geophysical Research: Atmospheres, 116, D07204, DOI:10.1029/2010JD015246.
