doi:10.1038/nindia.2015.88 Published online 3 July 2015
Global warming and local factors such as rising temperatures and decreasing forest cover are altering India’s lifeblood, the monsoon.
According to a series of recent studies, extreme rainfall events are increasing, and low to moderate events decreasing, leading to upsetting India’s agriculture and lives of farmers.
Importantly, three-fourths of India’s annual rainfall comes during the summer monsoon months, June to September. India being predominantly rural, more than 70% of the fields are rain-fed during the summer monsoon, and more than 700 million people are engaged in agriculture-related activity.
Over the last 60 years, during peak season (July to August), the average rainfall over Central India has reduced from approximately 10mm/day to about 9 mm/day. However, the day to day variability in rainfall during these months has increased leading to periods of heavy rainfall or prolonged dry spells.
A widely-cited study1 suggests “significant rising trends in the frequency and magnitude of extreme rain events and a significant decreasing trend in the frequency of moderate events over central India during the monsoon seasons from 1951 to 2000.”
Bhupendra Nath Goswami, former director of Indian Institute of Tropical Meteorology (IITM) and lead author says the probable cause of increasing trend of extreme events is global warming. “As the temperature is increasing, the atmosphere can hold more water vapour. Water vapour being lighter than dry air, the potential for convective instability in the atmosphere increases. Stronger instability leads to stronger convective events and stronger extreme rain events,” he explains.
Severe thunderstorms that result in these extreme rain events have small spatial scale. Rainfall occurs when moist air converges thereby drying the neighborhood temporarily. As it takes some time for the atmosphere to replenish, the potential for small and medium rain events decreases, he adds.
Goswami notes that increasing amount of pollution in the atmosphere also has a potential for invigorating convective precipitation and may have some contribution towards the increasing trend of extreme rain events.
Examining extreme rainfall events between 1901 and 2004, scientists have also found2 that the “frequency of extreme rainfall events shows significant inter-annual and inter-decadal variations in addition to a statistically significant long term trend of 6% per decade”. The study attributes the increasing trend of extreme rainfall events in the last five decades to the increasing trend of sea surface temperatures (SST) and surface latent heat flux over the tropical Indian Ocean, which refers to heat released by water evaporation and absorbed by water condensation.
“When the western Indian Ocean becomes warmer, there is more evaporation and hence the water vapour increases over India, making the atmosphere unstable and leading to more extreme rainfall events,” says Jayaraman Srinivasan of the Center for Atmospheric and Ocean Science (CAOS) at the Indian Institute of Science (IISc).
It is not easy to explain the year to year variation since monsoon is a complex system. This year (2015), there is a warming of the ocean in the equatorial Central Pacific, 9000 km away. “A warmer Pacific Ocean causes more deep clouds. This leads to more sinking motion over India,” Srinivasan observes.
Large-scale factors such as the ‘Pacific Walker’ circulation – a broad-scale circulation pattern in the atmosphere above the tropical Pacific – also influence the monsoon. In future, if the world is 4 degrees warmer than today, monsoons are predicted to fail more often3. Simulations with a global climate model show that while today’s summers bring abundant rain, under greenhouse-gas induced warming, weak monsoon years could be the norm.
Jacob Schewe, a physicist at the Potsdam Institute for Climate Impact Research, Germany and lead author, says, “Interestingly, average monsoon rainfall first increases under the global warming scenario, and only after a few decades begins to decrease drastically.”
The Pacific Walker circulation is related to the El Nino-Southern Oscillation phenomenon and interacts with the ocean through sea surface temperatures. “During an El Nino, which occurs naturally every few years, the Walker circulation is weaker than normal; and vice versa during a La Nina. These changes often have substantial impacts for example, droughts or excessive rainfall and floods, in many countries especially in the tropics,” Schewe explains.
“Now, under global warming, the Walker circulation in the model weakens so that its mean state becomes similar to an El Niño. This leads to a more frequent subsidence of dry air and suppression of rainfall over the Indian region,” he adds.
The fear of El Nino leading to monsoon failure stalks India’s weathermen and policy makers. However, all El Nino years don’t produce drought in India. Krishna Kumar Kanikicharla, a climate scientist formerly with the IITM and coordinating lead author of one of the reports of the Intergovernmental Panel on Climate Change (IPCC), has analysed 132 years of rainfall records. Along with colleagues, he shows that the warmest sea surface temperatures in the central equatorial Pacific are more effective in the failure of Indian monsoon than the SST in eastern Pacific4. “The impact of El Nino is decreasing. However, due to global warming, monsoon will be stronger in the short term,” he says.
The IPCC special report warns that intensity and frequency of extreme rainfall will increase in the next half of this century, leading to more floods and droughts5.
Building on previous studies of extreme events in India, Deepti Singh, a graduate student at Stanford University and her colleagues analysed6 daily rainfall records since 1951. “We found significant changes in increase in the severity of wet spells and in the frequency of dry spells.” Although the average rainfall has reduced by 1mm per day over the last 60 years, Singh notes, “the day to day variability in rainfall during these months has increased leading to periods of heavy rainfall or prolonged dry spells.”
At smaller spatial-scales, however, local effects may influence extreme rainfall events more than large-scale phenomena7. Arpita Mondal, a post-doctoral research associate at IISc and her colleague Pradeep P. Mujumdar, have examined intensity, duration, and frequency of extreme rainfall as the function of three physical factors – global average temperature, local temperature, and El Nino index.
“We have tried to identify which of the physical factors are important for each of these characteristics. We found that local temperatures are way more important than global average temperature or the El Nino index.”
1. Goswami, B. N. et al. Increasing trend of extreme rain events over India in a warming environment. Science 314, 1442-1445 (2006)