How likely is flooding this year?
The on-going La Niña and seasonal flood forecasts in Bangladesh
What is seasonal forecasts: Some readers will be a bit surprised to read this article as when the country is dry and hot with a serious shortage of rainfall, I am writing about a probabilistic outlook of flooding for the next few months. This is the beauty of probabilistic forecasting techniques where you can make advance forecasts for at least 3-6 months in advance. In the case of short-range and deterministic forecasts, we can generate information 3-7 days in advance, but the probabilistic seasonal forecasts can produce information for about 3-6 months in advance with reasonable accuracy. However, there is a problem here while the deterministic forecasts are mostly accurate as these are on shorter time scales, the probabilistic forecasts have some uncertainties as these are on longer time scales.
So, when you read a probabilistic flood forecast in three tercile format like 40:40:20, it means the possibility (or probability) of higher-than-normal flooding is 40-percentage (upper tercile), the possibility for normal flooding is 40-percentage (middle tercile), and the possibility for lower-than-normal flooding is 20-percentage (lower tercile). So, even we forecast 40-percentage possibility for a higher-than-normal flooding, there is still a 20-percentage possibility for a lower-than-normal flooding. As compared to deterministic forecasts, this is the kind of limitation probabilistic forecasts have. On the contrary, while the deterministic method can produce forecasts only during the occurrence of an event [for example, the 'Flood Forecasting and Warning Center' of the Bangladesh Water Development Board (BWDB) produces information on daily rise/fall in river water-level in a flooding season], the probabilistic method can produce forecasts in a hot/dry spring about a rainy summer and monsoon flooding.
What is the causal connection between El Niño/La Niña and seasonal flooding in Bangladesh: There is evidence of teleconnections between the strength of El Niño/La Niña and seasonal climate anomalies (e.g., rainfall, flood, and cyclone) in Bangladesh. The prime objective of this article is to provide an improved description of the El Niño/La Niña related seasonal flooding in Bangladesh.
Bangladesh normally faces a deficit of rainfall during any major El Niño event. Many previous major El Niño years like 1951, 1958, 1972, 1983, 1993, 1997, and most recently, the El Niño of 2009 recorded significant deficits in monsoon seasonal-average (Jul-Aug-Sep) rainfall. For example, 1951, 1958, and 1972 recorded about 38-, 48-, and 10-percentage of less rainfall (data for other years are not available to me now). Similarly, all the major (strong) La Nina years (1964, 1973, 1988 and 1998) recorded excessive rainfall the increase was: 4-percentage in 1964, 8-percentage in 1973, 30-percentage in 1988, and 10-percentage in 1998. In particular; Bangladesh is always found to be wetter than normal during any La Niña years (weak, moderate, or strong); however, Bangladesh is not always drier than normal during any El Niño years (for example, the year 1987 was a moderate El Niño year and the country was not dry, rather it was a flooding year).
During strong El Niño years, the trade-wind weakens and as a result the whole Ganges-Brahmaputra-Meghna (GBM) basin experiences less rainfall. The deficiency of rainfall causes rivers to be drying because of low-flow and the country faces severe drought. On the other hand, during any La Niña year, the trade-wind strengthens and as a result there is significant increase of rainfall along the greater GBM basins causing flooding along the whole catchments. This, in turn, severely floods Bangladesh, as it is the lowest riparian country in these basins.
How likely is flooding in Bangladesh during the La Niña year 2010: La Niña refers to the appearance of colder-than-average sea surface temperatures (SSTs) in the central or eastern equatorial Pacific region (the opposite to conditions during El Niño). It is a cold event where the SSTs become anomalously colder compared to the long-term average for the central and eastern equatorial Pacific. La Niña episodes also feature large-scale changes in the atmospheric winds across the tropical Pacific, including increased easterly (east- to- west) winds across the eastern Pacific in the lower atmosphere, and increased westerly (west-to-east) winds over the eastern tropical Pacific in the upper atmosphere.
These conditions reflect an enhanced strength of the equatorial Walker Circulation. When this Walker circulation is strong, the upper tropospheric winds in the Australasian region are easterly and consequently the tropical disturbances are transported westward into the Bay of Bengal. Therefore, rainfall becomes very active in the region of western Pacific.
According to NOAA (National Weather Service) Climate Prediction Center (CPC) (available at http://www.cpc.noaa. gov/products/analysis_monitoring/enso_advisory/index.shtml), the La Niña conditions will continue to develop during July-August 2010. The majority of models now predict La Niña conditions to develop during June-August and to continue through early 2011. Over the past several months most of the statistical and coupled model forecasts have tended towards cooler conditions in the tropical Pacific through late-2010. The spread of some of the statistical and coupled model forecasts (ENSO-neutral to weak La Niña to) indicated some uncertainty in the outlooks. However, current conditions (stronger-than-average easterly winds over the central equatorial Pacific) and recent cooling trends in observed oceanic conditions support weak La Nina conditions (may gain strength to become moderate) in the tropical Pacific during the next three months. However, as of today, there is no model forecast for a rapid transition to strong La Niña by August-October of 2010.
So, it looks like a weak-to-moderate La Niña is developing, which may cause a slight enhanced rainfall during the next 2-3 months in the greater GBM basins, which will cause a basin-wide flooding, and will ultimately cause flooding inside of Bangladesh.
How does the flooding look like in a probabilistic scale: As discussed above, the current forecast for flooding in August-September-October (ASO) of 2010 can be presented in three tercile format as 40:40::20, which means there is 40-percentage probability (or possibility) for higher-than-normal flooding, 40-percent probability for normal flooding, and 20-percentage probability for lower-than-normal flooding .This probabilistic forecast is based on monitoring of the ocean and knowledge of how the atmosphere has responded in the past to similar La Niña in Bangladesh, with a variety of lag times. Also keep in mind that this forecast can change anytime in the future if the interaction between the ocean and atmosphere changes.
Percentage likelihood of flooding in 2010: Above-normal flooding(40%); Near-normal flooding (40%); Below-normal flooding (20%).
How helpful is the seasonal forecasts in Bangladesh: Although the seasonal products are used widely and successfully for hazards management in one-quarter of the globe, the scientific research in Bangladesh relating to seasonal products is just beginning. The government and water experts in Bangladesh will decide how effectively they can use the products of seasonal forecasts. In addition to short-term deterministic forecasts, the medium-to-long term seasonal forecasts are essential for developing a real-time response plan for hazards management. It is extremely helpful to enhance the agricultural decision support system in Bangladesh.