Cyclone Sidr: Impact of climate change?

Tropical cyclone Sidr hits Bangladesh

Cyclone Sidr is the fourth named storm of the 2007 North Indian Ocean cyclone season. The storm formed in the central Bay of Bengal, and quickly strengthened to reach peak sustained winds of 215 km/h (135 mp/h), which would make it a Category-4 equivalent tropical cyclone on the Saffir-Simpson Scale. The storm eventually made landfall near Bangladesh on November 15. The name Sidr was contributed by Oman; it is an Arabic name of a tree belonging to the Ziziphus family, specifically the Ziziphus spina-christi.
Cyclone Sidr, El Nino-Southern Oscillation (ENSO) cycle, and climate change
During the past several decades, scientists have discovered that the global weather patterns are linked to El Niño- Southern Oscillation (ENSO) climate cycle. Please note that the terms El Niño and La Niña represent opposite extremes of the ENSO cycle. El Niño reflects periods of exceptionally warm sea surface temperatures across the eastern tropical Pacific while La Niña represents periods of below-average sea-surface temperatures across the eastern tropical Pacific. In general, El Niño indicates drier than average climate (like the drought in 1982-83 and 1998) and La Niña indicates wetter than average climate (like the flooding in 1988 and 1998) in Bangladesh.
Now the basic question is 'does ENSO (El Niño or La Niña) intensify the threat of Cyclone in Bangladesh'? Before we provide any definite answer, let us try to explore some diagnostic records of cyclones and El Niño/La Niña in Bangladesh. Presented below are the top five cyclones (since 1960) and the activities of El Niño/La Niña events during those years:
May 28- 29 1963: A severe cyclonic storm lashed Chittagong, Noakhali, Cox's Bazar and 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.
November 12, 1970: A cyclone hit the entire coast of Bangladesh with a maximum wind speed of 222-km per hour causing 10 to 20 feet high tidal surge. The official death toll was 500,000, but the unofficial figure was about 120,0000. A moderate-to-strong La Niña event was active during that time. Actually, the onset of that La Niña was in the later part of 1970, which continued to affect Bangladesh's climate up to 1972.
May 24 -25, 1985: A severe cyclone hit Chittagong, Cox's Bazar, Noakhali and coastal islands causing 10 to 15 feet high tidal surge. Death toll: 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 ENSO (i.e., El Niño) event started in 1986.
April 29 -30, 1991: A cyclone hit Bangladesh at late night of April 29. The storm originated in the Pacific Ocean and reached the Bay of Bengal coast after 20 days. The maximum wind speed reached 225-km per hour. Official death toll: 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.
November 29 -30, 1997: A cyclone hit Chittagong. The maximum wind speed was 224-km per hour. A strong El Niño event was very active during that time.
The above chronology shows that most of the major cyclones that hit Bangladesh in different periods were either a year of El Niño or La Niña event. Similarly, for the year 2007-08, a La Niña event is now well established in the central and eastern Pacific, which, in fact, started to develop from January 2007. However, it should also be remembered that destructive cyclone activities may also be seen in any other non-ENSO year too.
Now based on the above findings, there are strong reasons to believe that 'the El Niño and La Niña events intensify the threat of cyclones in Bangladesh'. The next question is 'how does El Niño or La Niña relate to tropical cyclone'? The well documented causal hypothesis is that ENSO alters the global atmospheric circulation affecting tropical cyclone frequencies by changes in the lower troposphere vorticity sources and the vertical wind shear (differences in wind between upper and lower levels). The response of the different basins to ENSO vary, some show changes in the frequency of events, while others have shifts in the initial location of the tropical cyclones. There are many reasons for these different responses: the relation between time of year of peak tropical cyclone activity to the ENSO annual cycle, the location of the basin in relation to the eastern equatorial Pacific and the climatological characteristics of each basin. While the Pacific region tropical cyclone activity can be forced directly by the sea surface temperature changes, the other basins are affected only indirectly through teleconnections.
Now the most important question: 'is there any teleconnections between ENSO and global climate variability and change? The answer is yes; and how? At first, let's explore the variability part. While ENSO is a natural part of the Earth's climate, the question of its intensity or frequency is of great importance and interest. Because the ENSO signal tends to be strongest during the winter months, its most direct impacts tend to correspond with the global climate anomalies (rainfall, flooding, drought, cyclones etc) in the fall, winter, and spring seasons. The intensity of these global climate anomalies is heavily tied to the intensity of the ENSO event itself. This is also true in the case of the greater Ganges-Brahmaputra-Meghna basin areas. Therefore, we have already observed in the past that Bangladesh is very dry in any strong El Niño year and very wet in any strong La Niña year. We have also seen that El Niño or La Niña events intensify the cyclone activities in Bangladesh.
So, the final question is 'how does ENSO relate to climate change'? The simplest answer is, as compared to 1950-1975, the number of ENSO years has considerably increased in 1976-2000. Therefore, we observe very frequent extreme weather events in the recent decades. Although alarming, but the forecast is that this trend is likely to continue further, may be with more frequent ENSO events. Scientists theorise that the increased level of ENSO (El Niño and La Niña) events observed over the last few decades is, at least in part, the result of global climate change.
So, the question now being asked after Cyclone Sidr is, are extreme weather events becoming more frequent, and is this linked to climate change? The answer is yes!
Dr. Rashed Chowdhury is research scientist of the Pacific ENSO Applications Centre. He is also a graduate faculty of the University of Hawaii (USA) and is primarily responsible to develop ENSO-based climate forecasts for the small island countries in the Asia-Pacific region (rashed@hawaii.edu)