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) (Fig. 2). 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 activity becomes very active in the region of western Pacific.

As of June 8, based on the patterns of anomalous ocean temperatures (SSTs), atmospheric circulation and precipitation are consistent in indicating La Niña conditions in the tropical Pacific.

ENSO-neutral conditions continued in the tropical Pacific during May 2007, with average to below-average sea surface temperatures (SSTs) extending from the date line to the west coast of South America (Fig. 1). The latest weekly SST departures are negative in the Niño 1+2 (-1.9ºC) and Niño 3 (-0.6 ºC) regions, and remain near zero in the Niño 3.4 (0.0 ºC) and Niño 4 (+0.4 ºC) regions (Fig. 2).

Nearly all of the model forecasts predict below-average SSTs in the Niño 3.4 region (5°N-5°S, 120-170°W) during the remainder of the year. While most statistical models show ENSO-neutral conditions persisting through August 2007, most dynamical models indicate La Niña will develop within the next three months.

Over the past several months most of the statistical and coupled model forecasts have trended towards cooler conditions in the tropical Pacific through late-2006. The spread of the some of the statistical and coupled model forecasts (ENSO-neutral to weak La Niña) 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 3 months. Some forecast models continue to predict a rapid transition to La Niña by July 2007.

What type of climatic impact does this La Nina can have in 2007 in Bangladesh?

Scientific views about this stronger-than-average easterly winds over the central equatorial Pacific and the decreasing SST anomalies throughout the central and eastern equatorial Pacific and teleconnections (Teleconnections is defined as atmospheric interactions between widely separated regions) with climate anomalies in Bangladesh had been explained by Walker during the 1920s.

According to his observations, lower than average atmospheric pressure prevails in the region from Australia to India when higher than average atmospheric pressure prevails in the eastern tropical Pacific Ocean.

The scientific reason is that air flows from high-pressure regions to low-pressure ones. As a result, a huge volume of moisture propagates from the Pacific Ocean to Bangladesh and India; due to the prevailing low atmospheric pressure in this area at that time, the moisture-laden air rises, causing heavy rainfall in Bangladesh and India.

Several scientific researches have identified that during La Niña years (either strong or moderate strength) Bangladesh is often a victim of flooding (e.g., 1984, 1988, and 1998).

Right now, it looks like a weak La Niña is developing, which may cause a slight enhanced rainfall during the monsoon season. So, based on the maturity stage of this year's La Niña, the possibility to experience a normal (or slightly higher than normal) flood is high.

But if the La Niña condition continues to gain strength then a different climate scenario may appear for which we need to wait and monitor very closely the on-going La Niña event.

Finally, it is important to note that the seasonal forecast prepared here is probabilistic type, which is based on monitoring of the ocean and knowledge of how the atmosphere has responded in the past to similar SSTs (and La Nina) in Bangladesh, with a variety of lag times.

Dr. Md Rashed Chowdhury is Research Scientist of the Pacific ENSO Applications Centre (PEAC) and a Faculty of the Department of Urban and Regional Planning, University of Hawaii, USA.