Cyclones over Arabian Sea on the rise due to global warming: Hiroyuki Murakami
The unusually high number of cyclones in the Arabian Sea, some of them extremely severe with wind speeds higher than 167 kmph, are a result of warming seas and changes in sea surface temperature distribution, says Hiroyuki Murakami, a climate scientist at the US National Oceanic and Atmospheric Administration’ Geophysical Fluid Dynamics Laboratory. His paper in the journal Nature in 2017 was among the first to make these linkages. According to India Meteorological Department, the ratio of cyclones in the Arabian Sea, over which the severe cyclonic storm Nisarga formed this week, and the Bay of Bengal, where cyclone Amphan originated on May 20 , is normally 1:4. But this is set to change. Edited excerpts from an email interview :
We have been seeing a rise in cyclonic storms over the Arabian Sea in the past few years. What could be the reasons?
Attached figure shows the number of extremely severe cyclonic storms (ESCSs) over the Arabian Sea since 1998. You can clearly see that ESCSs are increasing especially during the post-monsoon season (October-December). Our climate models suggest this increase is largely due to global warming.
What is the normal frequency of cyclonic storms expected in Arabian Sea?
The average number of observed cyclones (including both weak and intense storms) is two per year between 1998-2019 in the Arabian Sea.
Why have the frequency of intense/severe cyclonic storms increased in the post-monsoon period over the Arabian Sea?
Regarding the increase in ESCSs during the post-monsoon season, we found three factors may be related. One is the rising sea surface temperature, especially over the Arabian Sea. This warm ocean contributed to intensifying cyclones, leading to more severe storms in the Arabian Sea. The second factor is that onset of winter monsoon is delaying, leading to a longer storm season. The last factor is increases in anthropogenic aerosols, leading to changes in sea surface temperature distribution that in turn change monsoon circulation, resulting in more active storms.
What is the impact of climate change on tropical cyclones?
In general, when global warming proceeds, the surface ocean gets warmer. This warm condition is favourable for intensification of tropical cyclones. But increasing greenhouse gasses also cause warming in the upper atmosphere, that makes the atmosphere more stable. This stable atmosphere is unfavourable for tropical cyclones. Therefore, the frequency of tropical cyclone genesis would decrease when global warming proceeds. But once a storm generates, the storm can develop into an intense storm due to the warmed surface ocean. Above discussion is generally applied to global tropical cyclones. But when we discuss regional tropical cyclones, it is more complex. Some regions get more active storms by changing circulation whereas some regions get less active. Expecting changes in regional tropical cyclones contain a lot of uncertainty.
Are we to expect more such severe cyclonic storms to impact the west coast of India?
Yes, especially during the post-monsoon season. Our climate models suggest an increasing frequency of severe storms over the Arabian Sea during the post-monsoon season, as we have reported in our paper.