"The second rainy season starts now for the Horn of Africa and Eastern Africa - we expect the rains to be near normal over much of the Greater Horn of Africa," said Bwango Apuuli, deputy director of the Nairobi-based Climate Prediction and Application Centre (ICPAC) of the Inter-Governmental Authority on Development (IGAD), the regional grouping.

"There is a fear that La Niña could have a delayed impact on the rainy season, which starts in March 2008; the worst may be yet to come. National Meteorological and Hydrological Services of the region will be providing regular updates as the phenomenon develops," Apuuli said.

La Niña is characterised by unusually cold ocean temperatures in the eastern equatorial Pacific Ocean, recorded every three to four years, which cause a ripple effect felt across the globe, making wet regions wetter and dry ones drier.

El Niño is characterised by unusually warm ocean temperatures in the equatorial Pacific. The system oscillates between warm El Niño and neutral or cold La Niña conditions every three to four years on average.

Both phenomena result from interaction between the surface of the ocean and the atmosphere in the tropical Pacific Ocean, according to the US government's National Oceanic and Atmospheric Administration. "Changes in the ocean impact the atmosphere and climate patterns around the globe. In turn, changes in the atmosphere impact the ocean temperatures and currents."

La Niña has already prompted some of the most widespread rain recorded in Africa since the turn of the 1900s, according to Omar Baddour, head of the World Climate Data and Monitoring Programme at the UN's World Meteorological Organisation (WMO).

More than a million people have been affected by flooding in over 20 African countries stretching in an arc across sub-Saharan Africa from Mauritania to Kenya. La Niña's impact usually lasts for nine to 12 months and is expected to peak in December 2007/January 2008.

According to Baddour, La Niña usually brings above normal rains to the Sahel, but the rainbelt has been broadened this year by the combination of warmer conditions in the Indian Ocean. "In the East African countries, particularly Kenya, Somalia and parts of Tanzania and Uganda, La Niña could influence the short rainy season in October-November-December, in the way where less rain than average could occur."

Climate change

Apuuli pointed out that La Niña's impact has been more intense than the last event recorded in 2005/06 in Africa, and although it was difficult to attribute it all to global warming, "there are links".

WMO's Baddour, who has been studying the impact of the phenomenon on western Africa since the 1990s, said, "There is no statistically significant trend in the strength of the La Niña event itself as an ocean-related phenomenon.

"Scientists project that global warming would increase the occurrence and intensity of extreme weather events. In other terms, global warming could intensify/amplify the existing natural climate variability and its associated weather phenomena."

The floods have affected very large areas across the continent but, in terms of intensity, other "devastating flooding" in recent times was recorded in Mozambique in February 2000, in the Sahel summer flooding of 1998 and 1999, and again in 2005 in western part of Sahel, including Senegal, Baddour said.

"Put together, these events are the most devastating flooding recorded in sub-Saharan Africa since many decades."

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