Forecasting floods: La Niña Impact
Submerged crop field in moderate flooding.
LAST year I wrote an article (published on August 7, 2010 in The Daily Star) with a title 'how likely is flooding this year'. That article was actually prepared in April 2010, when the onset of La Niña was clearly visible. I mentioned that some readers' will be surprised to read this article as I was writing about a probabilistic outlook of monsoon flooding when the country was extremely dry and hot with a serious shortage of rainfall. Readers were surprised, but they admitted that a flood of moderate strength did occur last year. It didn't inundate the urban thoroughfares of Dhaka, but it inundated many suburban and rural areas. This is the beauty of probabilistic forecasting techniques where you can make forecasts for at least 3-6 months in advance with reasonable accuracy. Based on the science of seasonal forecasts (i.e., correlations between La Niña and flooding in Bangladesh) a moderate flood was likely to occur in Bangladesh during August-September of 2011; and it is occurring. This was clearly visible from last April; some scientist may argue that it was well perceived even from January 2011. However, the weakening phase of La Niña created an uncertainty about the strength of flooding (either normal or above normal). I hope that the related flood forecasting, warning, and disaster management agencies in Bangladesh were fully aware about this flooding well ahead of time. After two years of La Niña (2009-11) (weak-to-moderate strength), we are now having ENSO-neutral phase. While as per definition the phase is neutral, the atmospheric component (i.e., trade winds) is still active. This is not very normal, but we still have some lingering effect of La Niña. This is causing more rainfall in the greater Ganges-Brahmaputra-Meghna (GBM) basin system, which, in turn, flooding some areas in the following few days. So, the country is experiencing a moderate flood this year. CAUSAL CONNECTION BETWEEN LA NIÑA AND SEASONAL FLOODINGThe climate (i.e., rainfall, stream-flows, and sea level) of Bangladesh are sensitive to El Niño-Southern Oscillation (ENSO) climate cycle. In a strong El Niño year, the whole basin experiences lower than normal rainfall. The deficiency of rainfall causes the Bangladeshi rivers to become drier because of low-flow and, as a result, the country faces severe drought. On the other hand, when the year is La Niña then there is a significant increase in rainfall over the greater Ganges-Brahmaputra-Meghna (GBM) basins causing flooding along the whole catchments. This, in turn, severely floods Bangladesh, as it is the lowest riparian country in these basins. Other than a few exceptions, it is clear that Bangladesh faces major catastrophic flooding during the La Niña years. For example, the highest flood years recorded were 1955, 1974, 1987, 1988, 1998, 2004, and 2007. It is also noticeable that, other than 1987 and 2004, all were La Niña years. The two El Niño years (i.e., 1987, 2004) were of moderate to weak strength and the reasons for flooding during these two years require further research. It is possible that the onset and the maturity stages of the ENSO cycle and other local and global oceanic and atmospheric factors may have contributed to this flooding. Similarly, the driest years recorded were: 1977-78, 1982-83, 1985, 1986, 1989, 1990-92, 1994, 1997, and 2001. It is also evident here that, except for 1985, 1989, and 2001, all were El Niño years. The three La Niña years (i.e., 1985, 1989, and 2001) were of very weak strength (or ENSO neutral phase). This is also again a subject for further research and similar onset and the maturity stages of the ENSO cycle may be responsible too. FLOOD FORECASTS AND WARNING SYSTEM
At present, Bangladesh has a flood forecasting lead-time of approximately only 3-7 days. The Flood Forecasting and Warning Center (FFWC) of the Bangladesh Water Development Board (BWDB) is responsible for monitoring flooding in the country. The forecasting procedure at the FFWC is comprised of several stages; these include real-time rainfall and water level data collection, meteorological forecasting and boundary estimation, flood forecasting by numerical modeling, and flood warning dissemination by daily bulletin. The quality of the 3-7 days lead time forecasts has been recognized as positive for hazards management during the flooding season. The dissemination network of the FFWC is also very instrumental. However, the short lead-time of these forecasts made it of limited use for the agricultural sector. To better use this for the agricultural sector, the most viable solution is to produce long-lead seasonal forecasts, the demand of which is significantly increasing in Bangladesh, and disseminate these through the appropriate channels. Although the seasonal forecasts have been used successfully for hazards management and increased agricultural productivity in one-quarter of the globe, the scientific research on these forecasts is just beginning in Bangladesh. It is essential to adopt a comprehensive approach for developing a knowledge base on seasonal forecasts; the sooner is better! CONCLUSIONS
Scientific advances have led to the ability to generate skilful ENSO forecasts, and substantial progress has been made in understanding regional impacts of the ENSO cycle. However, as compared to 1950-1975, the number of ENSO (El Niño and La Niña) events has considerably increased in 1976-2000 and extreme weather events have frequently been observed in recent decades. This trend is likely to continue further, may be with more frequent ENSO (El Niño/La Niña) events. At this stage there is no easy answer on 'how to face the future challenges', but some immediate responses are necessary. As an immediate response, ENSO-based seasonal climate forecasts can play an important role to help meet some of these future challenges. In this regard, the writer would like to highlight his experience arising from the operational seasonal forecasting schemes in the US-Affiliated Pacific Islands (USAPI). Like Bangladesh, the climate variability and change in the USAPI region is also sensitive to ENSO (El Niño/La Niña) events. Currently, an operational 'climate forecasting and warning response' scheme is fully instrumental in the USAPI region. It started in 1994 and is fully operational since 2004. This operation scheme has considerably reduced the vulnerability to climate hazards for the island communities. It has successfully empowered the underrepresented people in atmospheric sciences. In Bangladesh, where structural solutions to flooding are not sustainable, the current flood forecasting system needs to be enhanced with ENSO-based seasonal climate information. This will offer an environmentally friendly, non-structural, and cost-effective decision options regarding its flood management and agricultural adaptations.
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