- From 1931 to 2015, the average rainfall in the northern Western Ghats has increased by 2 percent per decade while in the southern region it has decreased by 3 percent per decade.
- The observed trends are explained by a northerly shift in the low-level jetstream (fast-moving winds in the lower troposphere), which is triggered by an abnormal rise in sea surface temperatures in the Arabian Sea and northern India.
- However, other factors can also influence the movement of the jetstream like changes in rainfall itself.
Every summer, Indian farmers eagerly await the arrival of the southwest monsoon, which comprises up to 90 percent of the country’s annual rainfall. The Western Ghats (WG), a 1600-km mountain range lying parallel to the west coast of India, are the first to experience the monsoon rains.
But the amount of rain in the region varies considerably with factors such as elevation playing a role. Now, a recent study reveals that over the past 85 years, rainfall patterns within the mountain-stretch have changed; the northern parts have become wetter while the southern region has been getting drier.
The research team attributes a northerly shift in the low-level jet stream (LLJ) — fast-moving moisture-laden westerly winds in the lower troposphere (the lowest part of our atmosphere) — triggered by a rise in the sea surface temperature over the northern Arabian Sea. The LLJ, which moves across the Arabian Sea towards the west coast of India, is one of the main sources of moisture for the southwest monsoon.
“Although the Western Ghats are getting enough rainfall (~300 cm/year) and more than 80 percent of this rainfall is contributed by the southwest monsoon, the southern region shows a significant reduction in rainfall, which is alarming,” said Hamza Varikoden, lead author of the study, who is a scientist at the Centre for Climate Change Research in the Indian Institute of Tropical Meteorology in Pune. “The state governments concerned need to take actions for appropriate collection of rainwater in the form of micro-irrigation projects, mini-hydral projects and so on.”
To unveil regional trends in rainfall in the Western Ghats, the team examined daily gridded IMD (India Meteorological Department) rainfall data from 1901 to 2015. And to shed light on the causes of the changes in rainfall patterns, they analysed wind and temperature data obtained from the National Centre for Environmental Prediction-National Centre for Atmospheric Research (NCEP-NCAR) from 1948 to 2015. In addition, sea surface temperature data spanning the period from 1931 to 2015 from the Hadley Centre was combined in their analysis.
Wetter north, drier south
The analysis revealed that from 1931 to 2015, the northern WG experienced an increase in average rainfall of 2 percent per decade (+0.3 mm per day per decade) while rainfall in the southern region decreased at 3 percent per decade (-0.39 mm per day per decade).
These trends, the researchers point out, are restricted to the upstream flow of the low-level westerlies (winds originating from the west). When the westerlies encounter the mountainous barrier of the WG, they are forced to rise and form organised clouds in the windward side, which pour down as rain, explained Varikoden. “Any modification to these flows can significantly affect the orographic clouds and hence the rainfall pattern of the region.”
In the northern windward region, the increased wetting is notable in the low-altitude plains (elevation less than 200 m), which receive the maximum amount of rainfall. As you move south on the coastal plains, rainfall decreases, and it is also more variable.
When looking at trends in anomalous rainfall, the team noted that the southern region had excess rainfall in the past decades but deficit rainfall in the recent decades. However, the opposite trend was observed for the northern region where earlier decades had deficit rainfall and recent ones saw surplus rainfall.
Why is this happening?
According to the team, the reasons for the contrasting trends in average rainfall lie in a 5 degrees (550 km) northerly shift of the core of the low-level jet stream (LLJ)—high-speed westerly winds in the lower troposphere peaking at 1.5 km.
“This low-level flow starts from the easterly trade winds and crosses the equator at Somali region with strong winds in order to call it as LLJ, then it moves towards north and turns towards the Indian peninsular region,” said Varikoden. Along this route, the LLJ picks up moisture from the surrounding oceans and seas, which ultimately falls as rain on the Indian landmass.
But why has the LLJ moved northward? The study points to an abnormal increase in the lower tropospheric temperature over northern continental India and rising sea surface temperatures in the northern Arabian Sea—particularly over the Oman upwelling region—in the recent decades.
“The hotter region leads to the formation of low-pressure zones. Due to these low-pressure regions, the core of the winds shows a tendency to move towards that zone. This would facilitate movement of LLJ towards north and can be attributed to the heating,” explained Varikoden.
As the core moved northward, the westerly winds over the southern WG weakened, reducing moisture transport over the region and lowering the conditions for the development of organised monsoon clouds.
“The shift in LLJ is expected, but such a magnitude of shift is unexpected and the tropospheric warming over the Arabian Sea is also alarming,” said Varikoden. In a 2014 study, another research team had reported a poleward shift of the LLJ by 1.5 to 2 degrees in latitude. The researchers think that this smaller shift could be because of the team’s shorter period of analysis
Other factors play a role
Raghu Murtugudde, a professor of atmospheric, oceanic, and earth system science at the University of Maryland and visiting professor at IIT Bombay said it is interesting to look at past trends in rainfall. But he cautioned that both studies are incomplete in explaining the cause and effect.
“Rainfall is a heat source and it can have feedbacks. So when rainfall has changed, claiming that it is because of the shift in the jet may not be the entire story,” he said. He explained his point with an example, “If Eurasian snow cover has a trend and that created a large scale ocean -land temperature gradient trend and that changed the rainfall, then the associated heat source changes will adjust the jet as well.”
He pointed out that the changes in temperature and the response “cannot always be separated easily in a complex coupled climate system especially when land, ocean and atmosphere are so tightly coupled.”
Essentially, the 2014 study along with this one, are “explaining that there is a consistency in the rain change and the jet change,” he said.
The relation to Kerala floods
There is still a long way to go to explain the reason for the Kerala floods earlier this summer, said Murtugudde. “There were multiple heavy rain events over Kerala and this required the jet to tilt back towards the south. Does that mean that LLJ is shifting back?” he asked. “We cannot know that for another decade since to see any trend we need to wait.”
Varikoden said that they did not analyse the recent anomalous rainfall in Kerala. “That was an extreme event and has to be studied in detail to uncover the exact reasons.”
CITATION
Varikoden., H., Revadekar, J.V., Kuttippurath, J., Babu., C.A. (2018). Contrasting trends in southwest monsoon rainfall over the Western Ghats region of India. Climate Dynamics, doi.org/10.1007/s00382-018-4397-7
Banner image: Western Ghats. Photo by Rakdept/Wikimedia Commons.
