Anthropogenic Aerosols - Model Projections

The atmospheric brown cloud that has been associated with South Asia, is made up of a number of aerosols. A significant part of the brown cloud is formed of black carbon (Ramanathan and Crutzen, 2003). Black carbon is primarily produced by soot from fossil fuel combustion and biomass burning (Collins et al., 2002). Thus, it is no surprise that black carbon may have an impact on the Indian monsoon.  Meehl etal., (2008) have completed a six-member ensemble of 20^th Century simulations, using a variety of black carbon scenarios. The CCSM3 model (see Collins et al., 2006) produced the following outputs. Increased black carbon increases lower-tropospheric heating over South Asia and reduces the amount of solar radiation reaching the surface during the dry season. Surface temperatures are subsequently reduced in the Bay of Bengal, Arabian Sea, and over India. Prior to the monsoon in March-April-May precipitation increases, which is followed by reduced precipitation during the active monsoon period.

Similar results were produced by Collier and Zhang (2009) using the NCAR Community Atmosphere Model, CAM3. The high-resolution model is run for a 16 month period to understand the implications of increased aerosols (soil dust, black and organic carbons and sulphate). Like Meehl et al., (2008) the model suggests a decrease in surface temperatures and consequently increases in precipitation during the pre-monsoon months. Furthermore, the presence of aerosols induces tropospheric shortwave heating over central India, which decreases convection and precipitation, and increases evaporation. Increased evaporation during pre-monsoon months weakens the near-surface cyclonic circulation, and consequently reduces precipitation during the active monsoon months. The spatial distribution of changes in precipitation remains uncertain, which therefore requires assistance from observational data.

The two studies highlighted here present different results from previous observational research. The observational data provides specific areal changes of precipitation, whereas the models mainly focus on total precipitation. The models fail to identify areas of intense precipitation and so more research is required to understand why the models fail to project that yet the observations have witnessed it. 

Anthropogenic Aerosols and the Indian Monsoon: Observational Research

One of the areas of the world with higher Aerosol concentration is South Asia, due to the recent rapid urbanization and population growth. Ramanathan et al., (2005) suggests the aerosol forcing in over populated regions at the surface and in the atmosphere can be an order of magnitude greater than those of anthropogenic greenhouse gases, as in the case for the Indo-Asian haze. Subsequently concerns have been raised that anthropogenic aerosols are impacting monsoon rainfall and mechanisms. Bollasina and Nigam (2008) have produced some observational research indicating the impacts. Their work is aligned with the vast quantity of other observed research linked to aerosols and the Indian monsoon.

The authors statistically analysed the relationship between distribution and variability of aerosols (derived from Total Ozone Mapping Spectrometer) with cloud cover, surface heating, surface shortwave and longwave radiation and monthly precipitation.  The research produced two important conclusions:

1.     Increased anthropogenic aerosols present in May leads to reduced cloud cover and precipitation, increased surface wave radiation, and land surface warming. These changes are attributed to the evaporation of the cloud layer from the absorption of solar radiation by aerosols and subsequent heating of the air – also known as the “semi direct” effect.

2.     As the monsoon progresses, the monsoon intensifies leading to increased rainfall in June and July over India. The authors argue that the enhancement of the monsoon results from the increased thermal contrast (originated in May).

Fundamentally the authors identify that large-scale anthropogenic 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 following post shall look summate the relationship between aerosols and the Indian monsoon based on models. 

Impacts of Land Use and Land Cover Change

Southern Asia, predominantly the Indian subcontinent region, has shown dramatic land use land cover changes in recent times in response to the growing food and fiber requirements of a fast increasing population (Foley et al., 2005). Landscape changes in the region, in recent decades, has primarily occurred because of urbanisation (Kishtawal et al., 2010) and intensive agriculture/irrigation practices (Douglas et al., 2009). Such changes are illustrated through population changes in figure 1. Recent research has illustrated that human-induced landscape changes can affect atmospheric processes from local to regional weather patterns (Cotton and Pielke, 2007; Alpert et al., 2006) and climate variability (Pielke et al., 2007; National Research Council, 2005). Subsequently this post will look to summarise 3 journals illustrating these impacts.

Figure 1. (a) Population of India form 1901 to 2001 based on official census reports. (b) Density of population
(persons per square kilometre, values shown are natural log of actual numbers) over the Indian summer
monsoon region for the year 2000. (c) Difference of the density of population (natural log of numbers shown)
between the years 1990 and 2000. Source: Niyogi et al., (2010). p. 2.

The first was produced by Niyogi et al., (2010). Using satellite data the authors attempted to link daily rainfall observations with monthly satellite land surface data, to illustrate the connection between land use change and monsoonal rainfall. Using genetic algorithms (GA), empirical orthogonal functions (EOF), and causal discovery algorithms (CDA) a range of patterns were identified. Firstly, the EOF and GA analysis identified decreasing rainfall in the monsoon season in north/northwest India, which coincided with regions of agricultural intensive land use but highlighted further analysed is required. Additionally, correlations and the CDA suggest that pre-monsoon (March-April) vegetation affects July month precipitation over Peninsular India. In particular, a negative relationship exists. These results suggest that an increase in agriculture has possibly weakened the early monsoon rainfall. The journal highlights how agricultural intensification could be reducing summer monsoon rainfall over certain regions of India.

Douglas et al., (2006) have focused on changes in irrigation on land-atmosphere interactions and Indian monsoon precipitation. Douglas et al used the Regional Atmospheric Modelling System (RAMS). Four scenarios were adopted: (1) a control – observed NDVI (satellite measure of vegetation productivity); (2) irrigated crop scenario; (3) non-irrigated crop scenario; and (4) a scenario of natural vegetation growth. The model indicated that under active monsoon conditions, surface energy and moisture flux over India are sensitive to irrigation intensity and this effect is more pronounced than the other scenarios. Irrigation was proven to increase moisture flux, which in turn modified the convective potential energy. This caused a reduction in the surface temperature and led to a modified regional circulation pattern and changes in mesoscale precipitation.

Similarly to Douglas et al., Lee et al., (2009) focused on the effects of irrigation but relied upon observational data like Niyogi et al., (2010). The authors examined the effects of land cover change over the Indian subcontinent during pre-monsoon season (March, April and May - MAM) on early summer monsoon rainfall using NDVI and July precipitation between 1982 and 2003. MAM NDVI has increased and the increases are significantly correlated with increases in the irrigated area, not preceding rainfall. July rainfall significantly decreased in central and southern India, and the decrease is statistically related to the increase in the preceding MAM NDVI. The authors highlight that decreased July surface temperatures (an expected result of increased evapotranspiration due to irrigation and increased vegetation) leads to a reduced land-sea thermal contrast, which is one of the factors driving the monsoon, and therefore weakens the monsoon circulation. A weak early monsoon is partially a result of irrigation and the resultant increased vegetation and crop activity prior to the monsoon.

The 3 journals reviewed here use statistical evidence to suggest increased land cover land use change is weakening parts of the Indian monsoon. Importantly, each journal highlights that results are more robust over northern and Peninsular India; so further research is required to understand the impacts in different regions. At a socio-economic level the research illustrates how unsustainable, increase in agricultural intensification may begin to have a negative feedback and the relationships proved throughout the research should be a primary focus of future climate simulations, particularly throughout India. 

The 3 main References:

Douglas, E., Niyogi, D., Frolking, S., Yeluripati, J., Pielke, A., Niyogi, N., Vorosmarty, C. and Mohanty, U. (2006) Changes in moisture and energy fluxes due to agricultural land use and irrigation in the Indina Monsoon Belt. Geophysical Research Letters. 33:1-5.

Lee, E., Chase, T., Rajagopalan, B., Barry, R., Wiggs, T. and Lawrence, P. (2009) Effects of irrigation and vegetation activity on early Indian summer monsoon variability. International Journal of Climatology. 29: 573-581. 

Niyogi, D., Kishtawal, C., Tripathi, S. and Govindaraju, R. (2010) Observational evidence that agricultural intensification and land use change may be reducing the Indian summer monsoon rainfall. Water Resources Research. 46: 1-17. 

See each journal for other references used. 

Recent Trends in the South Asian Monsoon

Changes to the SAM have gained a lot of attention in recent years, particularly as climate change science has developed. The ability to measure the monsoon is rather complex, but in terms of impacts on populations it is generally determined by precipitation patterns. Anthropogenic forcing’s, such as greenhouse gases, have been associated with recent changes. The changes to the amount of precipitation derived from the monsoon is illustrated in figure 1. In general there has been a decreasing trend in precipitation since the early to middle 1900s. The figure outlines the results from a series of models and recorded data, thus helping provide an evaluation of changes to the amount of precipitation.

Figure 1: Time series of mean summer (June-September) precipitation averaged over land points. AIR, CRU and IMD illustrate the observed data. The remainder of data is that of climate models. Of particular importance is the inset, which compares the recorded data. The faint thin black line illustrates the observations without smoothing. For those interested the modelled changes following the present day are the projection for an A1B scenario (2000-2100). The graph is from Turner and Annalamalai (2012).  

However, in truth, the amount of precipitation is not a fair reflection of changes to the monsoon. Much of society throughout Southern Asian not only rely on the amount of precipitation but the stability and consistency of rainfall. Many journals highlight how the total precipitation has only decreased slightly but the intensity of rainfall has become far more sporadic throughout the affected regions. Subsequently, leading to droughts in some areas and catastrophic flooding in others. As would be expected there is a degree of natural variability, especially at the decadal level, however, there is large concern over the stability of monsoon rainfall, particularly throughout India.  Turner and Slingo (2009) have produced a review journal of the extremes of precipitation and active-break cycles of the SAM. The journal provides a lengthy overview of multiple recorded and modelled studies that have highlighted the change to rainfall in terms of intensity. The authors focus particularly on active-break cycles; the period of rainfall, followed by a break in rainfall. Unlike previous records, different parts of the impacted region are experiencing greater active-break cycles, meaning some areas witness less active periods than before and others witnessing more, thus leading to extremes in climatic conditions. 

For a far more in depth analysis of changes to the monsoon see:

Turner, A. and Slingo, J. (2009) Subseasonal extremes in precipitation and active-break cycles of the Indian summer monsoon in a climate-change scenario. Quarterly Journal of the Royal Meteorological Society. 135: 549-567.

And

Turner, A. and Annamalai, H. (2012) Climate change and the South Asian summer monsoon. Nature. 2:587-595.

What? When? Where? Why?

To set the scene for this scientific blog I thought it would be appropriate to outline what the South Asian Monsoon is? When and where the monsoon occurs? And what the driving mechanisms of the monsoon are?

Fig1. Map showing average onset (Monsoon arrival) dates and
wind directions prevalent during the SAM.

The term ‘monsoon’ originated from the Arabic ‘mausim’ which means season. Ramage (1971) was the first to define and use the word ‘monsoon’ in terms of an annual reversal wind regime and a contrast between a rainy summer and a dry winter. Local inhabitants of monsoon-affected regions often refer to the monsoon as the rainy seasons as the it brings excessive rainfall over short periods from the beginning of June to end of September (see Fig. 1 and 2) with primarily dry conditions throughout the winter.  The focus of this blog is the South Asian Monsoon (SAM), which predominantly impacts the Indian subcontinent: Bangladesh, Bhutan, India, Nepal, Pakistan and Sri Lanka.

Fig 2. The average Indian rainfall. Over 80% occurs during the
monsoon season (June to September). 

The explanation of the SAM and many other monsoons are often over-simplified in many media articles due to the complexity of processes. The following explanation of the monsoon is from a June 2012 paper by A. Turner and H. Annamalai. As one of the most up to date journals in this field and as a review, it provides what I feel is a very detailed but an easily understandable description of the driving mechanisms.

At the most basic level, the seasonal cycle of solar heating through spring warms the land regions surrounding South and Southeast Asia faster than the adjoining oceans, owing to differences in heat capacity, and develops a large-scale Meridional surface temperature gradient. Subsequently, this forms a surface heat low over northern India in late spring; the north-south pressure gradient then induces a cross-equatorial surface flow and return flow aloft. The Himalayan and Tibetan Plateau ensure that sensible heating during spring occurs aloft, meaning that the large-scale Meridional temperature gradient exists not just at the surface but over significant depth in the troposphere, anchoring the monsoon onset, and intensity. The intense solar heating in late spring and summer gives thermodynamic conditions favouring the occurrence of convection poleward of the Equator, allowing the monsoon to be viewed as a seasonal migration of the Intertropical Convergence Zone. The north-northwest migration of winter convection from the equatorial region and its interaction with circulation leads to a positive feedback and deeper monsoon trough, enhancing the cross-equatorial flow in the lower troposphere that feeds moisture to the monsoon, as well as the Tibetan anticyclone and easterly jet with a return cross-equatorial flow at upper levels. The north-south-oriented East African Highlands anchor the low-level cross-equatorial flow and the Earth’s rotation aids in the formation of the low-level westerly jet as it approaches South Asia from across the Arabian Sea. The rapid intensification of rainfall and circulation during the onset can be attributed to wind-evaporation feedback as well as feedback between extratropical eddies and the tropical circulation.

The next blog shall look to provide some relevant statistics regarding the SAM, with a particular focus on recent changes to the monsoon. This shall provide a setting for a series of blogs about anthropogenic activities and whether or not these are the cause of changes to the monsoon.