Currently we are facing a weak El Niño year. The traditional meaning of El Niño has been limited to dry (or drought) conditions in the South-Asian countries, which means that Bangladesh and the greater Ganges-Brahmaputra-Meghna (GBM) basin systems will receive lower-than-normal rainfall. The low rainfall may cause the whole region to dry out or even face drought (e.g., droughts in 1982, 1997, and 2015-16). While El Niño-driven drought is the likely scenario to prevail in Bangladesh and other South-Asian countries (e.g. Nepal, India), we are watching a full-fledged basin-wide flooding. Why is this happening now? Here is an explanation of this flooding situation from the perspective of El Niño-Southern Oscillation (ENSO) climate cycle.
The ENSO cycle directly affects rainfall distribution in the tropics and can have a strong influence on weather across the world. El Niño and La Niña are the extreme warm and cold phases of the ENSO cycle. El Niño is caused by major warming of the equatorial waters in the Pacific Ocean (Fig. 1) and La Niña is caused by major cooling of the same equatorial waters (Fig. 1).
In the last couple of years, there have been significant improvements in the understanding of ENSO dynamics, particularly the warm phase El Niño component. We now understand that in addition to the traditional El Niño (i.e., 'Cold Tongue El Niño: CTE), there is another type of El Niño called Warm Pool El Niño (WPE) (i.e., El Niño Modoki in Japan), which also plays a dominant role across the Pacific and Indian Ocean. The CTE event is characterised by warming of waters in the eastern Pacific Ocean while the WPE is associated with warming of waters in the western Pacific (fig. 2). In general, the CTEs are known as strong events (e.g., 1982-83, 1997-98, and 2015-16) and WPE are known as weak to moderate events (e.g., 1977-78, 1990-91, 1994-95, 2004-05, 2009-10, and 2018-19).
While the drought impacts of the CTE are well known, the impacts of WPE are relatively less explored or, in some cases, unexplored. Interestingly, in some cases (or in some countries), the WPE, as compared to CTE, displays quite opposite impacts. This means that in places where CTE events are associated with drier than normal conditions, the WPE events are associated with significantly wetter than normal conditions.
What we are currently watching in South-Asia (e.g., floods) is the impact of the WPE. The1987 floods occurred during a moderate strength El Niño year. While in all traditional El Niño years, Bangladesh and the whole GBM basin system received lower than normal rainfall and, in some years, faced severe drought, the year 1987 brought the opposite climate fingerprints. Bangladesh was flooded in 1987. Approximately 55,000 square kilometres area was inundated in the whole country and a part of Dhaka city went under water for a few days. The critical question is why the El Niño of 1987 caused flooding in Bangladesh when the expected pattern was drought? This was an open question in 1987 and we did not have any logical explanation until recently.
The ENSO knowledge base has improved significantly over the years and we have a better understanding of the spatial (basin-wide/region-wide/country-wide) pattern of both traditional and non-traditional El Niño impacts. We now know that 1987 started as a WPE and ended as a WPE as well (note that some events begin as a WPE and finally mature into a traditional type El Niño). Besides 1987, there are several other WPE years that caused flooding in Bangladesh. The period 2004-05 can be stated here as an example of this as well. With long-term increasing temperature and rainfall, the recent model-based study provided a strong message that El Niño events are in the process of becoming more intense in the future, particularly, with more frequent WPE events. Therefore, the ENSO-based climatic disruption is a major concern that needs immediate attention in Bangladesh.
Dr. Md. Rashed Chowdhury is the Principal Research Scientist of the Pacific ENSO Applications Climate Center (PEAC), University of Hawaii, USA.