A satellite view of Cyclone Tauktae before it made landfall over South Gujarat on May 17, 2021. Image: NASA Earth Observatory
- Uncertainty is the name of the game. As Niels Bohr famously said, forecasting is always difficult, especially about the future.
- This said, we can expect a weaker monsoon circulation in 2022 due to global warming as well as the La Niña winter of 2021-2022.
- The result will be weaker vertical shear and warm sea surface temperatures, which will favour extremely severe cyclones during this pre-monsoon.
- Even though warming and wind changes may increase the so-called cyclone genesis potential, the actual number of cyclones that will be born is not fully understood.
Late season Cyclones Tauktae and Yaas in 2021 caused damages estimated at almost Rs 34,000 crore, with Tauktae also costing 198 lives. Much has been written about the enhanced cyclones activity in both the pre-and post-monsoon seasons over the north Indian Ocean due to global warming.
As we inch towards the 2022 pre-monsoon cyclone season, it is fair to ask if the number of cyclones likely to occur prior to the onset of the monsoon can be forecasted. A reliable prediction of the frequency of cyclones would greatly help prepare for, mitigate and recover from cyclones. India’s cyclone forecasts have improved remarkably and the disaster preparation, management and recovery have also led to a steady drop in the number of lives lost.
The cyclone risk still looms large each year but the honest answer for the question of forecasting of cyclone frequency is unfortunately an emphatic ‘no’. A total of about 80 cyclones occur each year across the global tropics with the total number varying less than 10% from year to year and decade to decade. The evidence thus far is that the total number of cyclones across all oceans have not shown any increase in response to global warming. But the number of highest category cyclones have ticked upward with rapid intensification occurring in all cyclone regions.
The north Indian Ocean is unique in its rapid and monotonic warming due to the heat that is received from the warming atmosphere as well as the heat poured in from the Pacific through the Indonesian Seas. Even the Southern Ocean pipes in warm waters into the Indian Ocean below about 1 km. The surface warming increases the so-called potential intensity of cyclones – the maximum strength they can reach and the strongest winds and rains cyclones can bring.
The actual number of cyclones however depends on many other factors, such as the amount of heat available in the surface ocean, the vertical shear, or the change in winds from the surface to the upper atmosphere, and the near-surface air rotation, or vorticity generated by convective activity (rain events). For example, warming of the surface ocean may be associated with reduced volume of warm water and/or increased vertical shear, both of which can inhibit growth of cyclones.
The end result is that even though the warming and wind changes may increase the so-called cyclone genesis potential, the actual number of cyclones that will be born is not fully understood, hence not easily predictable.
The country has barely recovered from the whiplash of the 2021 monsoon, which put up a mind-bending show exceeding all expectations of an extreme monsoon. But we must get ready again for what may come in terms of the pre-monsoon cyclones and a monsoon that may be deficit. This is because the La Niña winter of 2021-2022 appears to be transitioning to an El Niño-neutral summer. The winter of 2020-2021 was also a La Niña event, but then the unpredicted arrival of the Atlantic Niño and the record warming over the Arctic delivered a rollercoaster monsoon with heavy rainfall from late August into October.
The onset of the 2021 monsoon started with the devastating Cyclones Tauktae and Yaas, which pulled the monsoon trough forward for a nearly on-time onset. But the Atlantic Niño, the little brother of the Pacific El Niño, delivered a weak June-July rainfall with a greatly reduced number of monsoon depressions. Record Arctic warming and sea ice loss altered the late monsoon producing deluges that extended into October due to the resurgence of the La Niña. Summer and winter monsoons merged into one and the circulation changes suppressed the post-monsoon cyclone activity completely.
The post-monsoon cyclone season of 2021 with no cyclones for the first time in three decades indicates that a warm ocean alone is not a reliable predictor of cyclone frequency. The north Indian Ocean has seen more severe and extremely severe cyclones during both the pre- and the postmonsoon cyclone seasons, but the post-monsoon of 2021 is a clear example of how this can change on a dime.
The La Niña winter has brought the cold air penetrating deep into peninsular India, with an unusual dust storm that veered much further south, into Mumbai and Pune. We can now expect a weaker monsoon circulation in 2022 due to global warming as well as the La Niña winter of 2021-2022. The result will be weaker vertical shear and warm sea surface temperatures, which will favour extremely severe cyclones during this pre-monsoon.
As Niels Bohr famously said, forecasting is always difficult, especially about the future. Considering the risk associated with cyclones, one must make the best possible guess despite these uncertainties. Prepare for the worst and hope for the best.
Raghu Murtugudde is a professor of atmospheric and oceanic science and Earth system science at the University of Maryland. He is currently focusing on developing climate courses as an open source for his Climate Academy.