Every year, the entire country awaits the onset and evolution of monsoon with baited breath. Each year tends to be different – and this year has managed to produce a rather unique onset and evolution thus far.
The onset this season was delayed by unforeseen interactions between typhoons and cyclones. Cyclone Biparjoy was born after the onset and lingered for longer than normal to delay the arrival of the monsoon over Mumbai by nearly two weeks. The city finally saw the monsoon arrive together with Delhi for the first time in over half a century. The monsoon trough thus ended up with an exaggerated curvature over northwest India.
The deficit due to the delayed onset has been all but wiped out but the distribution of rainfall remains as patchy as ever, with excess rainfall over the northern Western Ghats into Northwest India and deficits extending in a horseshoe pattern from Uttar Pradesh into Odisha and back to the east into Chhattisgarh, Madhya Pradesh, and Maharashtra. Extreme heat has also been reported in parts of Himachal Pradesh, even as some areas of the state received heavy rainfall.
What is normal and what is not in this smorgasbord of heat, drought, and floods? The impact of climate change has always been of great interest, but it is worth remembering that everything today happens in a warmer world that is also more humid. As the old adage goes, climate is what we expect and weather is what we get. With global warming, a warm and humid atmosphere acts like a steroid for the weather.
Every weather event now has some contribution from global warming. One must pay close attention to the weather patterns that emerge due to other factors as well. The El Niño has been grabbing most of the headlines this year and yet it is not clear that the monsoon mayhem thus far has had much to do with the El Niño.
What is not getting noticed as much is that the wildfires thus far this year have burned over three-times the normal area and have also emitted about three times as much carbon dioxide so far. This has also had a contribution to the warming.
What else could be driving this weird summer monsoon?
Excess rainfall over northwest India is consistent with the Arabian Sea having warmed by about 1.5 degrees Celsius since January. This was expected, according to a study last year that the author was part of.
June contributes only about 15% of the rainfall to the seasonal total. Monsoon rainfall distribution always tends to be patchy. The Indian subcontinent is like a popcorn kettle that gets heated up as the Sun crosses over into the northern hemisphere in March. Rainfall is like the kernels of corn popping randomly around the kettle.
The instabilities in the atmosphere that drive convection are not strong enough to drive large-scale rainfall during the pre-monsoon season.
Rainfall this pre-monsoon was above normal due to a combination of the warm Arabian Sea and an unusually high number of western disturbances. As a result, the soils were left moister than normal, which in turn affected the evolution of the monsoon. However, the mystery is that, despite averaging rainfall over a month, a season or even multiple seasons, rainfall distribution remains uneven. Disuniform terrain and heterogeneous land-use patterns are the likely culprits.
The Atlantic Ocean and the upper atmospheric circulation also tinker with the monsoon. The entire Atlantic Ocean has been warmer than normal since March. While the so-called Atlantic Niño, with a warm tropical Atlantic, generally tends to suppress monsoon rainfall, it is not clear what the impacts are when the entire Atlantic is as warm as it has been this year.
The strongest winds that occur in the upper atmosphere can spontaneously break into clockwise and anticlockwise patterns, especially when they run into mountainous terrain, such as the Himalaya. Strong clockwise winds, with air flowing out from the centre, in the upper atmosphere demand an anticlockwise circulation near the surface, in order to feed the upper level outflow. Such a convergence near the surface can drive excess rainfall.
Finally, the warming over the Himalaya has not been uniform either. Some parts of the mountain chain are amplifying global warming, leading to rapid local warming. Irregular weather patterns during the monsoon superpose on these local features as a result of the winds expanding or compressing as they race up and down the narrow valleys. The results can be cloudbursts, heavy rains or even heatwaves – depending on the local flow patterns. Such disparate weather patterns can occur side by side as well.
The conclusion is that the Indian subcontinent is a veritable popcorn kettle that can throw up many surprises. Everything is not directly attributable to global warming – even as every little weather event is happening in a warmer and wetter world. Only improved forecasts with sufficient granularity in space and time can reduce the element of surprise resulting from these weather monsters.
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.