A visualisation of all tropical cyclone tracks over South and Southeast Asia that formed in 1985-2005, in six-hour intervals. The colour scheme denotes wind intensities on the Saffir-Simpson scale. Storms remnants are not shown.
| Photo Credit: Nilfanion/NASA, Public domain
Numerous studies have reported trends in various climate variables over the Indian subcontinent. A decreasing trend in the amount of monsoon rainfall for more than six decades is one. Others include intensifying trends in the occurrence of extreme rainfall events, droughts, heatwaves, and cyclones. The period over which these trends have been estimated vary but global warming has always been invoked as the prime suspect.
A question that isn’t getting as much attention as it deserves to in this milieu is: are these really trends, or are they shifts or decadal cycles?
(A shift is a jump from one state to another, such as a quick transition from one amount of rainfall to another. The best example is seasonal monsoon rainfall: it tends to remain above the long-period average (LPA) for about 20 years and then shifts to a state of less rainfall than the LPA for a similar duration.)
The question matters because trends, shifts and decadal cycles portend important differences in the way we plan the use of our resources, including water, crops, energy, etc.
Scientists’ glossary of change
A common term used by climate scientists these days is ‘anthropogenic trend’. ‘Trend’ of course implies that there are climate variables moving in one direction, like the continuous increase in temperature. The ‘anthropogenic’ suffix presumes that these trends are occurring within human lifetimes. As such, the duration over which a variable needs to evolve for its behaviour to be called a ‘trend’ is not always clear.
Climate scientists also use the term ‘secular trend’, which is to say that a variable has been increasing for a certain period within a longer span, such as for 30 years in a 100-year period.
Then there is ‘decadal variability’, a common term that isn’t entirely distinct from a shift. Decadal variability refers to an oscillation from a positive to a negative phase on the order of tens of years. On the other hand, a shift can mean an irreversible jump or just a rapid transition that will later return to a prior/older state.
On the whole, without observing a particular variable for sufficiently long periods of time, climate scientists tend to be less than rigorous about their choice of descriptor to describe the variable’s observed behaviour.
In this context, it is critical to understand whether cyclones are becoming common and/or more intense, if they are a part of a decadal oscillation or if their numbers have jumped to a new state.
A rapid increase
A study just published in the journal Climate and Atmospheric Science (of which I am a part) reported a sharp change in the potential for cyclones to form over the Arabian Sea during the late 1990s.
Cyclone-genesis – or cyclogenesis – is an indicator that denotes the chance of a cyclone forming. It depends on some parameters, including the sea surface temperature, the ocean heat content, change in winds from the surface into the upper atmosphere (or the vertical shear), and rotation of winds near the surface. If these conditions line up, they will sow the seed for a cyclone, but we still don’t fully understand why some seeds sprout and grow into cyclones and some don’t.
This said, all these factors except for wind rotation have seemingly favoured a higher cyclone formation potential since the 1990s. The crucial question is why this switch – a rapid increase – occurred around this time. The present study notes that the rapid increase in the cyclogenesis potential over the Arabian Sea coincides with a shift in the so-called ‘Warm Arctic, Cold Eurasian’, or WACE, pattern. Again: a shift rather than a trend.
WACE is a pattern of warm surface temperatures over the Arctic and a large blob of cold surface temperatures over Eurasia. This pattern is associated with upper level circulation changes that reach into the Indian Ocean sector.
Global warming also experienced a slowdown around the same time (although this continues to be debated). More interestingly, scientists have argued that a so-called ‘regime shift’ occurred in the same period as well. Such shifts are not unheard of; a similar event was noted in the mid-1970s.
A new why
The causes of such changes are not fully understood – but they do raise some remarkable questions for India. Irrespective of whether they are ‘shifts’ or ‘decadal cycles’, it is important that we understand whether the decrease in the monsoon’s intensity, increase in the amount of extreme rainfall, and greater prevalence of heatwaves are trends that will continue in the coming decades – or if they are parts of longer processes that will revive the monsoon, reduce the number of cyclones, and ameliorate heatwaves and extreme rainfall.
The expectations with which we invest in resources to adapt to future climate risks are vexed by many difficulties, including those arising from uncertainties in climate risk at the level of specific regions across the country, vis-a-vis sea-level rise, heavy rain, drought, heatwaves, and cyclones. Of course, given our limited financial resources, climate adaptation remains a considerably monumental socioeconomic and political challenge.
As a result, climate scientists have our work cut out for us. Instead of always focusing on predicting what climate change will look like in 2100 or training our tunnel vision on global warming targets, we need to better understand the natural variability in our own neighbourhood – especially since natural variability itself is modulated by global warming.
As the new study indicates, a monsoon decadal cycle that used to last for around 20 years earlier may now last for longer. Why?
Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.
