According to the National Oceanic and Atmospheric Administration (NOAA), Climate Prediction Center's (CPC) ENSO diagnostic discussion, La Niña has ended and ENSO-neutral conditions are expected to continue.
Some of the Global Climate Model forecasts favour ENSO-neutral to continue through the spring, with El Niño forming during the summer of 2023 and persisting through the fall. Other models favour ENSO-neutral through the summer of 2023, with elevated chances of El Niño developing afterward. In any case, we seem to be heading to El Niño during the summer of 2023 and/or afterwards.
Note that the El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. El Niño (warming of the ocean surface/tropical Pacific sea surface temperatures: SSTs) and La Niña (cooling of the ocean surface) are the extreme phases of the ENSO cycle; between these two phases there is a third phase called ENSO-neutral (neither El Niño nor La Niña -- often tropical Pacific SSTs is generally close to normal or average).
The ENSO transition is the time when one of the extreme phases of ENSO (El Niño or La Niña) weakens and becomes neutral and then shows signs of changing to another extreme phase.
For example, there was a triple-dip La Niña from 2000-23 and this La Nina ended in March 2023, and an ENSO-neutral condition is now visible in the ocean and atmosphere. Furthermore, the ocean and atmosphere are behaving like we are heading to El Niño. This is what we call an ENSO transition phase.
This transition started in March 2023 and is likely to continue during the summer or afterward.
So, now the question is whether the ENSO transition intensifies the threat of cyclones in Bangladesh. Before we provide any definite answers, let us try to explore some diagnostic records of cyclones and El Niño, La Niña, and ENSO transition years in Bangladesh.
Presented below are the top cyclones (since 1960) and the activities of El Niño, La Niña, and transition years:
May 22, 2016: Tropical cyclone Roanu where the maximum sustained winds were 80-90kmph with gusts of nearly 110kmph, killed 21 and displaced half a million people. This was an ENSO transition year (from a strong El Niño in 2015 to ENSO-neutral to La Niñain 2016).
May 27, 2009: Tropical cyclone Aila, which had a maximum wind speed of 110kmph, killed 339 and left more than one million people homeless. This was also an ENSO transition year (from La Niña to ENSO-neutral).
April 14-15, 2009: Cyclonic storm Bijli which had a maximum wind speed of 90kmph and killed four. This was the same ENSO transition year (La Niña to ENSO-neutral).
November 15, 2007: Category-4 Cyclone Sidr had a maximum wind speed of 215kmph killed thousands, and affected 650,000 people.
November 29-30, 1997: A cyclone hit Chittagong. The maximum wind speed was 224kmph. A strong El Niño event was very active during that time.
April 29-30, 1991: A cyclone hit Bangladesh late at night on April 29. The storm originated in the Pacific Ocean and reached the Bay of Bengal coast after 20 days. The maximum wind speed reached 225kmph. The official death toll was 150,000. A moderate El Niño event was active during that time. The onset of that El Niño event was in the later part of 1990 and affected Bangladesh's climate for about two full years.
May 24-25, 1985: A severe cyclone hit Chittagong, Cox's Bazar, Noakhali, and coastal islands causing a 10 to 15 feet high tidal surge. The death toll was 11,069. A moderate La Niña event, the onset of which started in the later part of 1984, was very active during that period. This La Niña continued to affect Bangladesh's climate up to the end of 1985, and then the opposite phase of the ENSO (ie El Niño) event started in 1986.
November 12, 1970: A cyclone hit the entire coast of Bangladesh with a maximum wind speed of 222kmph causing a 10 to 20 feet high tidal surge. The official death toll was 500,000, but the unofficial figure was about 120,000. A moderate-to-strong La Niña event was active during that time. The onset of La Niña was in the later part of 1970, which continued to affect Bangladesh's climate up to 1972.
May 28-29, 1963: A severe cyclonic storm lashed Chittagong, Noakhali, Cox's Bazar, and the coastal islands of Sandwip, Kutubdia, Hatiya, and Maheshkhali, killing about 11,520 people. A moderate El Niño event was active in the Pacific basin during that time. The onset of that El Niño event was in the earlier part of 1963, which continued to influence the climate of Bangladesh for about three full years.
The above chronology shows that most of the major cyclones that hit Bangladesh in different periods were either the year of El Niño or La Niña or during the ENSO transition period. Similarly, we are currently in the ENSO transition phase and heading to El Niño. Therefore, there are reasons to believe that the ongoing transition and the forthcoming El Niño may intensify the threat of cyclones in Bangladesh.
Now the next question is: How does El Niño or La Niña relate to tropical cyclones? The well-documented causal hypothesis is that ENSO alters the global atmospheric circulation affecting tropical cyclone frequencies by changes in the lower troposphere vortices and the vertical wind shear (differences in the wind between upper and lower levels).
The response of the different basins to ENSO varies; some show changes in the frequency of events, while others have shifted in the initial location of the tropical cyclones. There are many reasons for these different responses: The relation between the time of year of peak tropical cyclone activity to the ENSO annual cycle, the location of the basin about the eastern equatorial Pacific, and the climatological characteristics of each basin.
While the Pacific region's tropical cyclone activity can be forced directly by the SST changes, the other basins are affected only indirectly through teleconnections.
Dr Md Rashed Chowdhury is an Affiliate Research Faculty at Department of Atmospheric Sciences/ University of Hawaii at Manoa.
