Message

Indian Summer Monsoon

Feedback mechanisms

Indian summer monsoon with mean and regular precipitation within the season

  • The Moisture-advection feedback (regional, well established): According to most studies the moisture-advection feedback mechanism is a unique and single mechanism in the system. It can be perceived as the strongest feedback mechanism that has the greatest influence on monsoon precipitation in the region. Thus the uncertainty of the influence of this mechanism to the monsoon system is low. The strength of this mechanism relates to the main atmospheric processes (latent heat, land to ocean pressure gradient, advection etc.) for the monsoon precipitation to occur. The release of latent heat from precipitation over land leads to increase in the temperature difference between land and ocean. Latent heat is heat that is taken up and stored when a substance changes state from a solid to a liquid, from a liquid to a gas, or from a solid directly to a gas. This enhances the land to ocean pressure gradient that determine in which direction and at what rate the pressure changes around the Indian peninsula. The increased pressure gradient leads to stronger winds and pushes more moist air northward from the ocean onto the continent. The stronger flows on shore increase landward advection of moisture eventually forming rain clouds which leads to increased precipitation and associated release of latent heat. Rain clouds are especially likely when the continental areas have higher elevations (Himalaya mountains) because the humid sea air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas. 
  • The solar radiation-sea surface temperature feedback (regional, speculative): This mechanism is perceived as speculative due to ongoing discussions whether it could increase the monsoon intensity and/or variability or even have a negative impact on the initial regime. The increasingly warming atmospheric temperature from green house gas emissions has been linked with the increasing sea surface temperatures. It is well proved that evaporation increases with fluids warming up, therefore with higher sea surface temperatures enhance vapour amount in atmosphere. Atmospheric warming also increases the amount of irrigation necessary for agriculture use. In hot and dry weather open source water bodies are exposed to sunlight allowing for evaporation to occur thus increasing the vapour amount in atmosphere (Douglas et al. 2006). Sprinkler irrigation also is exposed to increased evaporation as wind and high air temperatures provide the necessary energy for the water droplets to evaporate (Playán et. al 2005). This leads to increased advection process forming rain clouds and more precipitation. Increased precipitation results in moister soils therefore decreasing biomass burning and aerosol exposure into atmosphere. As a result the Brown cloud formation is reduced allowing more solar radiation to reach surface. The feedback is closed when the increased amount of solar radiation continues to warm up both atmospheric and sea surface temperatures. The discussions about the positive or negative nature of this mechanism towards maintaining this regime begin with sea surface temperature anomalies. Palmer et al. (1992) has pointed out that enhanced convection associated with the warm SST anomalies in the tropical eastern Pacific Ocean can induce anomalous subsidence of the east–west Walker circulation over the Indian landmass. Thereby this would suppress the monsoon rainfall. Convection is described with the transfer of heat by the actual movement of the warmed matter. Walker circulation is an ocean-based system of air circulation that influences weather on the Earth. Another cause for discussions is the fact that unsustainable irrigation management can still cause droughts even if precipitation is maintained. This can occur due to rapidly increased groundwater use for crop irrigation causing the groundwater table to decrease even after mean monsoon precipitation year. Insufficient irrigation after the groundwater level has decreased to far can cause drought. 

Indian summer monsoon with weak and irregular precipitation within the season

  • The vegetation-surface albedo feedback (regional, well established): Increasing demand for food production is increasing the demand for agricultural land in India, leading to deforestation in regional scale (Sinha et al. 1991). Analysis indicates that the impact of deforestation often extends beyond the deforested regions themselves (McGuffie et al. 1995). Deforestation leads to reduction in vegetation cover and reduces rainfall through an increase in surface albedo (Knopf et al. 2008). Therefore the monsoon circulation is weakened as the amount of reflected solar radiation increases due to high albedo, thus decreasing the temperature difference between ocean and land. This in turn affects the moisture-advection feedback loop – a weaker pressure gradient leads to weaker winds pushing less moist air onto the continent. As the flows onshore are weaker landward advection of moisture decreases, which leads to decreased precipitation. This reinforces the lack of vegetation cover as the soil moisture decreases resulting in droughts and increased biomass burning occurs which decreases vegetation cover even further. This mechanism is considered to determine the occurrence of this regime shift. Once it begins to operate in the system it very much links to the moisture –advection feedback mechanism as most of the variables in both mechanisms overlap. This results in change in moisture-advection mechanism that is paramount to the previous regime. When the driving demand for food production results in deforestation that causes an increase in surface albedo, it begins to change the processes also in the moisture-advection mechanism. Therefore this mechanism can be perceived as the initiator for the regime shift. 
  • The Moisture-advection feedback (regional, well established): According to most researchers the moisture-advection feedback mechanism alongside vegetation-surface albedo mechanism is the main cause of abrupt changes in monsoon dynamics (Zickfeld et al. 2005). The approved importance of this mechanism in the initial regime determines that change in its processes will lead to regime shift. Thus the uncertainty of the influence of this mechanism to the monsoon system is well established as other feedback mechanism also closely relate to the moisture-advection feedback mechanism (see Fig.1). The lack of release of latent heat from weak precipitation over land leads to a decrease in the temperature difference between land and ocean. Latent heat is heat that is taken up and stored when a substance changes state from a solid to a liquid, from a liquid to a gas, or from a solid directly to a gas. This weakens the land to ocean pressure gradient that describes in which direction and at what rate the pressure changes around the Indian peninsula. The reduced pressure gradient leads to weaker winds and pushes less moist air northward from the ocean onto the continent (Rickenbach et al. 2009). As the flows on shore are weaker landward advection of moisture decreases, so that rising air cools less and eventually forms fewer rain clouds which leads to decreased precipitation and associated release of latent heat (Levermann et al. 2005). 
  • The dust-precipitation feedback (regional, speculative): This feedback mechanism has to be studied in more detail as some of the links between the variables are considered speculative. The strength of the feedback mechanism mostly depends on lack of vegetation cover and precipitation for the latter being dependent on moisture advection mechanism. If the influence of these two variables is weak then this is also present in the mechanism itself. According to research lack of precipitation causes the soil to dry rapidly thus decreasing soil moisture. This is an active component in the evolution of the monsoon system as it can enhance Indian summer monsoon precipitation (Fennessy et al.1994). Lack of precipitation and dried soils cause droughts that result in increasing frequency of biomass burning. This process even further decrease vegetation cover resulting in an increased amount of dust in the atmosphere (see Fig.1). Dust and biomass burning are one of the major sources of aerosols in the atmosphere. The latter is recognized as a source of large concentrations of small cloud condensation nuclei, which lead to the formation of a high concentration of small cloud droplets and therefore to an increased cloud albedo (Rosenfeld et al. 2001). The increased levels of aerosols from biomass burning and industrial pollution create atmospheric brown clouds. These clouds are basically layers of air pollution consisting of aerosols such as black carbon, organic carbon, and dust that absorb and scatter solar radiation. The absorption together with the scattering leads to a large reduction of Ultra Violet and visible wavelength solar radiation at the surface, alternately referred to as dimming. In addition, aerosols nucleate more cloud drops that enhance scattering of solar radiation and contribute to additional dimming. The nucleation of clouds by aerosols also reduces the precipitation efficiency of clouds. Atmospheric brown clouds have such a large effect on the monsoon primarily because the forcing simultaneously impacts many components of the monsoon system in various mechanisms, including the solar heating of the surface–atmosphere system while blocking the solar radiation, the sea surface temperatures due to lack of solar radiation on surface, the convective instability of the troposphere, evaporation due to decrease of SST. These are factors that have fundamental influences on the monsoon rainfall. Increased reflectivity of the atmosphere under both clear-sky and cloudy conditions leads to a reduced fraction of solar energy entering the systems (Knopf et al. 2008). Aerosol particles can absorb or reflect incoming solar radiation to exert a large radiative cooling at the earth's surface (Satheesh and Ramanathan, 2000). This leads to a weakening of the land-ocean temperature contrast and, in turn, the monsoon circulation (Knopf et al. 2008). The influence of this mechanism is still questionable as IPCC projections do not agree that sulphate aerosols would dampen the strength of Indian Summer Monsoon (IPCC 2007).

Drivers

Important shocks (eg droughts, floods) that contribute to the regime shift include:

  • Droughts (regional, well established): This event is closely linked with oscillations in precipitation and temperature change. It is understood that after vegetation cover decrease and surface albedo increase due to deforestation in the region, it affects the main mechanism – the moisture-advection feedback. This cause the lack of precipitation leading to droughts. Extreme droughts further reduce vegetation cover as the frequency of biomass burning has been increased.

 The main external direct drivers that contribute to the shift include:

  • Deforestation (regional, well established): This main direct driver is associated with the indirect driver of increased food production. Deforestation causes substantial changes in the monsoon circulation system and decreases the precipitation. As a consequence of the vegetation loss due to deforestation the dust and aerosol levels in the atmosphere increase significantly. Aerosols are the main source of the formation of Brown clouds that in order impede the amount of solar radiation reaching the surface. Brown clouds also alter the monsoon circulation system by decreasing the sea surface temperatures as the surface receive less solar radiation. All this leads to change in the main monsoon circulation mechanism that is responsible for precipitation in the region (see Fig.1). This driver is regional to local as it covers the Indian subcontinent and depends on the activity of local loggers and the distribution of forests.
  •  CO2 emmissions (global, well established): The increasing concentration of CO2 in atmosphere is a driver that is being speculated about whether it maintains the current regime or pushes the system towards a regime with weaker precipitation. Researchers are still arguing which case is most likely and it depends on the source to present the influence of this driver. IPCC report (2007) project that carbon release from anthropogenic sources will continue increasing during the coming decades. There are studies indicating that emissions of greenhouse gases that alter the heat budget of the system and therefore the land-sea temperature contrast, could increase the monsoon intensity and/or variability (Knopf et al 2008; Kripalani et al. 2007).They present research arguing that increased CO2 will increase the interannual variability of daily precipitation and contribute to an increase in average global temperatures. Increased temperatures could lead to increased moisture and precipitation in the Indian monsoon system. Nevertheless Palmer et al. (1992) pointed out that enhanced convection (the transfer of heat by the actual movement of the warmed matter) associated with the warm SST anomalies in the tropical eastern Pacific Ocean can induce anomalous subsidence of the east–west Walker circulation over the Indian landmass and thereby suppress the monsoon rainfall. The Walker circulation is an ocean-based system of air circulation that influences weather on the Earth. The Walker circulation is the result of a difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean. Normally, the tropical western Pacific is warm and wet with a low pressure system, and the cool and dry eastern Pacific lie under a high pressure system. This creates a pressure gradient from east to west and causes surface air to move east to west, from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up in the atmosphere, west-to-east winds complete the circulation. Recent study indicates that there was strong hydrological deficit in July 2002 due to the impact of the perturbed Walker circulation in the African and Indian Ocean region. The reduced moisture transport in the Arabian Sea and India resulted in large deficit monsoon rainfall over India (Fasullo 2005). After these discussions the influence of this driver on the regime shift is speculative. As the input of green house gases in atmosphere is global, therefore it also regards to CO2 increase in atmosphere as a driver. 

The main external indirect drivers that contribute to the shift include:

  • Food production (regional, well established): The demand for food will likely continue to increase as populations in the region and globally continue to grow adding another 2 to 4 billion people by the mid 21st century (Cohen et al. 2003). Therefore this will also put pressure on the use of freshwater on irrigation. Sprinkler irrigation is exposed to increased evaporation as wind and high air temperatures provide the necessary energy for the water droplets to evaporate (Playán et. al 2005). These processes are thought to enhance the monsoon precipitation. Nevertheless processes in other mechanisms undermine this perception. Irrigated agriculture does not universally increase precipitation. In the more humid parts of southern India, latent heat flux from pre-conversion tropical forests was greater than from contemporary agriculture, even though roughly half the cropland in Tamil Nadu is irrigated. Thus the region has had an overall reduction in latent heat flux that results in weaker precipitation as agriculture has expanded. However, in the drier north and north central parts, conversion to predominantly irrigated agriculture has led to significant increases in vapour and latent heat fluxes (Douglas et al. 2006). This indirect driver also aggravates the necessity for deforestation in order to increase agricultural area. Continuing "business as usual" would take several decades for the vegetation to recover. Change from forest to cropland cause an increase in surface albedo as the darker and bigger surface area due to leaves of trees is changed to crops that reflect more sunlight. Furthermore the increased albedo affects the main feedback mechanism of monsoon circulation thus decreasing the monsoon precipitation (see Fig.1).

Slow internal system changes that contribute to the regime shift include:

  • Atmospheric temperatures (global; well established): This variable mainly affects ice volume variations throughout the year and in case of continuous depletion of Arctic sea ice it triggers various mechanisms that maintain the new regime of Arctic without summer sea ice.

Key thresholds

  •  Temperature difference between land and ocean: Threshold at which strong winds and moisture transport towards continent maintain mean monsoon precipitation.
  •  Vegetation cover: Threshold at which the presence of vegetation cover maintains rainfall through an decrease of surface albedo. Therefore reflected solar radiation decreases thus increasing the temperature difference between ocean and land.

Leverage points

  • Vegetation cover (regional, well established): Increasing vegetation cover would decrease the albedo thus increasing the temperature difference between land and ocean which is essential for the creation of monsoon precipitation. Sustainable logging and fire prevention could increase vegetation cover.
  •  Albedo (regional, well established) Increasing vegetation cover would decrease the albedo that directly would increase the temperature difference between land and ocean.
  •  Biomass burning (regional, well established) Fire prevention may be the most cost-effective and efficient mitigation program that an agency or community can implement to avoid vegetation loss through biomass burning. This would maintain the necessary differences in temperature between land and ocean. 
  •  Atmospheric temperature (global, well established): It is crucial to manage increasing atmospheric temperatures as this event is altering the other processes that occur in this regime shift. Temperature increase could be managed by decreasing the CO2 emissions that affect this variable. 
  • Soil moisture (regional/local, contested): Preventing loss of soil moisture is essential to avoid potential fires and soil erosion that would increase aerosol and dust concentration in atmosphere. Therefore it is necessary to regulate groundwater extraction so that an adequate share and amount of it is used for drinking water and irrigation purposes. This could be achieved by notifying certain areas where groundwater extraction for private use is forbidden. One way to minimise the loss in soil moisture is to monitor continually the conditions in drought-prone regions, particularly in the wake of poor rainfall, and provide early warning to prevent or minimise the effects of an impending drought. Prevention, mitigation preparedness, institutional infrastructure, capacity building and logistic of relief material for a quick response should be planned and reserved according to the drought vulnerability level of the regions. Water trading nationwide and also worldwide could become a reality in order to maintain soil moisture (Yang et al. 2008). This trading system would need to be coupled with government regulation, control, and protection for the poor thus turning against corruption and private buying. Usage of rain barrels when the rainy season allows to store water for the case if drought might occur. These barrels should be sealed thus avoiding evaporation.

Ecosystem service impacts

Ecosystem services linked with the initial regime are freshwater, food crops, livestock, wild animal and plant foods, timber, fuel and fiber crops and hydropower.