The recent cyclone first detected on June 5 entered the record books as the longest-ever in documented history of the Arabian Sea. On June 6, it was ominously named Biporjoy, which means calamity in Bengali. Categorised as a very severe cyclone, it had a total life span of 13 days and three hours. The average life of a cyclonic storm of this category during monsoon over the Arabian Sea is six days and three hours.
Among the longer ones was the extremely powerful cyclonic storm Kyarr (October, 2019) over the Arabian Sea which had a life span of nine days and 15 hours. The longest duration of a cyclone was over the Bay of Bengal 46 years ago.
It had a span of 14 days and six hours in November 1977 in the North Indian Ocean region, which includes the Bay of Bengal and the Arabian Sea. Scientists largely attribute the extended span of cyclones to the sea surface temperature (SST) going up due to global warming. In many ways, Biporjoy caught many Indian scientists by surprise. Besides the longest span, it developed during the onset phase of the Southwest monsoon. It was also recurving, as it changed its path nine times and covered 2,525 km (depression to depression) before making landfall near Jakhau port in Gujarat.
Rising waves in Arabian Sea
According to the IMD, about 66 cyclonic storms (with Maximum Sustained Wind ≥ 62 kmph) developed over the Arabian Sea during 1965-2022. Of them, eight crossed the Gujarat coast during the period.
A 2021 study on the ‘Changing status of tropical cyclones in the North Indian Ocean’, found that the frequency, duration and intensity of cyclones in the Arabian Sea have increased significantly.
There has been a 52% increase in the number of cyclones in the Arabian Sea, while ‘very severe cyclones’ have increased by 150%.
Moreover, the intensity of cyclones has increased in the Arabian Sea by about 20% (post-monsoon) to 40% (pre-monsoon). Further, there has been an 80% increase in the total duration of cyclones in the Arabian Sea during the last two decades. The duration of very severe cyclones has increased by 260%. Increase in cyclones in the Arabian Sea is a matter of concern, as the region feeds moisture to the Southwest monsoon, which is the backbone of the economy.
Since cyclones take away the moisture, they end up weakening the monsoon. This year, El Nino, a condition characterised by warm waters in the Pacific that weaken the monsoon, has already got stronger, further negatively impacting the monsoon. “This time, a weak and delayed monsoon onset, along with an exceptionally warm Arabian Sea, made it favourable for the formation of Cyclone Biparjoy. The Arabian Sea surface temperatures were 2-4°C above normal, supplying extra heat and moisture necessary for an intense cyclone,” said Dr Roxy Mathew Koll, Climate Scientist at the Indian Institute of Tropical Meteorology.
Weakening of monsoon
Climate change has been causing a rise in cyclogenesis in the Indian Ocean, resulting the weakening of monsoon. The threshold value for the formation of a cyclone is 26 degree Celsius, but at the time of Biporjoy cyclogenesis, the SSTs were in the range of 30-32 degrees Celsius.
Scientists attribute it to rising ocean heat due to climate change. Also, the accumulated cyclone energy (ACE)—a measure of the total wind energy during a cyclone’s lifetime in the Arabian Sea has almost tripled. It adds to the cyclone’s disaster potential after landfall.T