• Thursday, Feb 20, 2020
  • Last Update : 09:08 pm

A cyclone in a changing climate

  • Published at 12:03 am November 10th, 2019
They’ll keep on coming BIGSTOCK

The intensity of extreme cyclones could increase in the future

Tropical cyclone Matmo is reborn as Bulbul, and the rare overland passage by this cyclone brings a threat to India and Bangladesh. The international media is calling it Matmo, since it originated from the remnant of a Western Pacific tropical storm in the South China Sea in late October of 2019. 

Bulbul was expected to strengthen to become a very severe tropical storm yesterday. It was centred about 500km south of Kolkata, India, moving north-northeast. a red alert for Matmo/Bulbul was issued. 

However, interaction with, and landfall somewhere along, the mouths of the Ganges is likely today, near Kolkata. By now, winds should be below gale force, with a consequently smaller storm surge. Heavy rain is still likely. 

The story of Matmo is very interesting, and should be a research topic for now and the foreseeable future.

Matmo was formed in the Philippine Sea on October 24, and became a tropical storm on October 30. As a tropical storm, it continued west onto the coast of Vietnam on October 31. At that point, winds were recorded at 112kmph and 200mm of rain fell on the city. 

Then it started to move westward overland. During this overland journey, tropical cyclones are normally diminished, as there is no seawater on land (note that the fuel for any tropical cyclone comes from sea water at a temperature above 27 degrees Celsius). 

The Philippines Sea was at 30C and the South China Sea was at 29C when Matmo originally formed. It does not matter how warm the land is, land cannot evaporate enough moisture into the storm to maintain the strength of TCs. 

So, Matmo should have died overland. But, apparently magically, that didn’t happen and the remnants of Matmo, over the Andaman Sea (currently, sea water is at 30C), regained energy, spin, cloud, and rain potential, and continued to drift westwards and was reclassified as a tropical storm at the start of Thursday.

It is therefore very unusual that TC MATMO-19/BULBUL-19 has just revived itself after travelling about 1,800km overland.

Bangladesh perspective

It is important to note that major cyclones have drastically increased in recent decades. 

During the period of 1978-2019, a total of approximately 133 tropical cyclones formed in the Bay of Bengal, showing annual mean occurrence of 3.6 TCs. Bangladesh was hit by 33 tropical cyclones, which constitute about 25% of the total number of cyclones formed during that period. 

According to the current statistics, an average of 1.15 tropical cyclones hit Bangladesh per year. It has also been seen that Noakhali and Chittagong, including the eastern part of Meghna, the estuary was hit by about 26% of the cyclones of Bangladesh and the south-eastern coast of Cox’s Bazar, Teknaf, and adjacent areas by 29.5%. 

The south-central and south-western coastal zones were hit by 16% and 28% respectively.

It is evident from these 23 year’s tracks, that the tropical cyclones hit the Meghna estuary and south-eastern coast mostly in May (pre-monsoon) and the central and western coast in October-November (post-monsoon). 

From a historical perspective, it has been observed that many major cyclones that hit Bangladesh were either a year of El Niño or La Niña. 

Most of the major cyclones that hit Bangladesh in different periods were either a year of El Niño or La Niña event, or a transitioning period from one to other events.

So, the question can be raised now -- do El Nino/La Nina events intensify the threat of cyclone activities in Bangladesh? Based on the available findings, the answer is yes!

Currently the ENSO response to global warming differs strongly from model to model, and to a certain extent the science here is as yet inconclusive. 

However, recent findings suggested that extreme El Nino events (ie, 1982-83, 1997-98, and 2015-16) could double in the future due to greenhouse warming, and projected that it can occur roughly every 10 years instead of every 20. 

Compared to 1950-1980 (data related to El Niño/ La Niña events is not available before 1950), the numbers of El Niño/La Niña events have considerably increased in the recent past, and it is likely to continue in future; and the anticipated effects of a changing climate could be the reason for an increase in the intensity of extreme cyclones in Bangladesh. 

Md Rashed Chowdhury is currently working as the Principal Research Scientist (Graduate Faculty) of the Pacific ENSO Applications Climate Center (PEAC). He can be reached [email protected]