An alternative for flood management in Bangladesh

Schematic view of sea surface temperature and tropical rainfall (Left, Figures courtesy of NOAA, Climate Prediction Centre)

For any type of response planning (i.e., relief operation at the government level and crop plantation at the individual level), there is a demand for long-range (month to season) flood forecasts. The seasonal forecast is a type of long-range forecast. Seasonal forecasting is the outcome of a shift from deterministic predictions (e.g., 0.2 mm of rain will fall tomorrow) to probabilistic forecasting schemes. Here the emphasis is on forecasting the probability that a particular climate variable will be significantly above or below a mean state over a time-averaged period (usually ranging from a month to a season) (e.g., there is a 20% probability that the seasonal flooding in the northern part of Bangladesh will be higher than normal). Primary observations revealed that there is teleconnections between the strength of El Nino/La Nina and the climate variability (magnitude of flooding, for example) in Bangladesh.

What is El Nino/La Nina?
The term El Niño was first coined more than 100 years ago to describe the unusually warm waters that would occasionally form along the coast of Ecuador and Peru. This phenomenon typically occurred late in the calendar year near Christmas, hence the name El Niño (spanish for "the boy child", referring to the Christ child). Today the term El Niño is used to refer to a much broader scale phenomenon associated with unusually warm water that occasionally forms across much of the tropical eastern and central Pacific. The time between successive El Niño events is irregular but they typically tend to recur every 3 to 7 years.

La Niña is the counterpart to El Niño and is characterised by cooler than normal sea-surface temperatures (SSTs) across much of the equatorial eastern and central Pacific. A La Niña event often, but not always, follows an El Niño and vice versa. Once developed, both El Niño and La Niña events tend to last for roughly a year although occasionally they may persist for 18 months or more. El Niño and La Niña are both a normal part of the earth's climate and there is recorded evidence of their having occurred for hundreds of years.

Although El Niño and La Niña events are characterised by warmer or cooler than average SSTs in the tropical Pacific, they are also associated with changes in wind, pressure, and rainfall patterns. In the tropics where El Niño and La Niña form, rainfall tends to occur over areas having the warmest SSTs. Under normal conditions (see figure 1) the warmest water is found in the western Pacific, as is the greatest rainfall. Note that the dark arrows in the figure indicate the direction of air movement in the atmosphere with upward arrows associated with clouds and rainfall and downward-pointing arrows associated with a general lack of rainfall. Notice that under normal conditions winds near the ocean surface travel from east to west (these winds are called "easterlies") across the Pacific. Under El Niño conditions, the easterlies weaken, warmer than average SSTs cover the central and eastern tropical Pacific, and the region of heaviest rainfall has moved eastward as well. La Niña conditions could be thought of as an enhancement of normal conditions. During these events unusually cold ocean water extends westward to the central Pacific, the easterlies near the ocean surface are stronger than usual, and the warm SSTs in the western Pacific are accompanied by heavier than usual rainfall.

What is El Nino-Southern Oscillation (ENSO)?
While the tropical ocean affects the atmosphere above it, so too does the atmosphere influence the ocean below it. In fact, the interaction of the atmosphere and ocean is an essential part of El Niño and La Niña events (the term "coupled system" is often used to describe the mutual interaction between the ocean and atmosphere). During an El Niño, sea level pressure tends to be lower in the eastern Pacific and higher in the western Pacific while the opposite tends to occur during a La Niña. This see-saw in atmospheric pressure between the eastern and western tropical Pacific is called the Southern Oscillation, often abbreviated as SO. A standard measure of the Southern Oscillation is the difference in sea level pressure between Tahiti and Darwin, Australia (see figure 2). Since El Niño and the Southern Oscillation are related, the two terms are often combined into a single phrase, the El Niño-Southern Oscillation, or ENSO for short. Often the term "ENSO Warm Phase" is used to describe El Niño and "ENSO Cold Phase" to describe La Niña.

Why do we care so much about what goes on in the tropical Pacific with El Niño and La Niña?

Once developed, El Niño and La Niña events typically persist for about a year and so the shifted rainfall patterns associated with them typically persist for several seasons as well. This can have a significant impact on people living in areas of the tropical Pacific since the usual precipitation patterns can be greatly disrupted by either excessively wet or dry conditions. In addition, the shifting of tropical rainfall patterns during El Niño and La Niña not only affects the tropical Pacific region but areas away from the tropical Pacific as well. This includes many tropical locations as well as some regions outside the tropics in both the Northern and Southern Hemispheres. Why this occurs is related to how rainfall (associated with sea surface temperatures) in the tropics affects wind patterns in the atmosphere. In the tropics, air that rises to form clouds and precipitation at a certain location must subside somewhere else (what goes up must come down). This is how one tropical region that is persistently wet, for example, can lead to another region being persistently dry. Shifts in tropical rainfall and winds can also affect regions outside of the tropics by altering prevailing wind patterns that circulate around the globe.

How El Nino/La Nina years are defined?
Given that there are typical characteristics of El Nino and La Nina, how are specific 'ENSO events' defined? How large must the value of the index be, and for how long must it persist in order for an El Nino or La Nina to be identified as strong or moderate? Any definitive objective procedure for classifying intensity is yet to be explored. However, a common method used for this purpose is based on the Nino 3.4 sea-surface temperatures (SST) index. (see figure-3)

In this method, an El Nino or La Nina event is identified if the five-month running average of the Nino 3.4 index exceeds +0.5Êdeg. C (for El Nino) and 0.5 deg. C (for La Nina) for at least six consecutive months. According to this multivariate ENSO index, seven major El Nino years are 1997-98, 1991-92, 1986-87, 1982-83, 1972-73, 1965-66 and 1957-58, and seven major La Ni-a years are 1998-99, 1988-89, 1975-76, 1973-74, 1970-71, 1964-65, 1954-55 and 1949-50.

This ranking may even vary if based on an averaged Nino 3.4 index over different seasons. Also, it was observed that the relative ranking of events would vary if the ranking were based on an index other than Nino 3.4. For example, the classification system in the Western Regional Climate Centre (WRCC) that is based on the average value of SOI for the months of June-November provides a different ranking of events. With this WRCC approach, the ENSO phase is determined by atmospheric quantities (SOI) (value of SOI = -1.0: Strong El Nino, SOI = -0.5: Moderate El Nino, SOI = + 0.5.: Moderate La Nina, and SOI = +1.0: Strong La Nina). Another classification that is based on cold (La Ni-a) and warm (El Ni-o) episodes is also available. This has been compiled to provide a season-by-season breakdown of cold and warm conditions in the tropical Pacific. However, as compared to other methods, the Nino 3.4 SST ranking method is said to be the most reliable and widely used.

El Nino/La Nina and seasonal flooding in Bangladesh
Approaches to seasonal forecasting can broadly be divided into two categories: empirical/statistical techniques, and numerical/dynamical modeling, of which the former have historically been more widely developed. Unfortunately, research in Bangladesh relating to seasonal flooding has just begun. Therefore, considering the present research level in Bangladesh, Bangladesh can be benefited by using simple statistical methodology to develop seasonal flood forecasts. It can be added here that, based on the global mean atmospheric response to different large-scale modes of oceanic variability, (e.g., El Nino-Southern Oscillation (ENSO) and associated teleconnections), it may then be possible to predict how the flooding in Bangladesh will respond to certain oceanic situations from the knowledge of how the atmosphere (here Bangladesh flooding) has responded in the past to the variation of sea-surface temperatures (SST) in the tropical Pacific Ocean, with a variety of lag times. This predictability can be enhanced with the information achievable from monitoring the downstream stream-flows -- that are generated mainly from upstream rainfall conditions -- in advance of the flooding season.

The SOI-rainfall relation in the greater Ganges-Brahmaputra-Meghna (GBM) basin systems shows strong casual connection to SOI extremes indicating negative value to dry and positive value to wet. Therefore, when SOI is negative (i.e. strong El Nino years) the whole basin experiences less rainfall. The deficiency of rainfall causes Bangladesh rivers to be drying because of low-flow and, as a result, the country faces severe drought. Please note that the moderate El Nino years don't always cause a similar atmospheric response (drought).

On the other hand, when SOI is positive (both in strong and moderate La Nina years) 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.

ENSO-2005/06 and Bangladesh floods
Following the dissipation of the 2005-06 weak La Nina (cooler than average SSTs) in October-November-December 2005, sea surface temperatures across the central and eastern tropical Pacific continued to increase in January-February-March, and are currently running average. There is now a significant possibility that ENSO-neutral condition exists for next 3-months. Based on the behaviour of past ENSO-neutral conditions, Bangladesh is likely to experience normal flooding during the monsoon of 2006. This is a probabilistic type of picture that is based on monitoring of the ocean and knowledge of how the atmosphere has responded in the past to similar SSTs in Bangladesh, with a variety of lag times.

Dr. M. Rashed Chowdhury is Research Scientist of Pacific ENSO (El Nino-Southern Oscillation) Applications Centre (PEAC) at the University of Hawaii, USA.