| First Forum on Climate Prediction and its application in the Early
Warning System for Food Security in West Africa (PRESAO-1), Abidjan, Côte
d’Ivoire, 4-8 May 1998
|
There are enhanced probabilities of above-normal West African rainfall for the period July-September 1998 over the Gulf of Guinea coast region, especially west of central Nigeria, and over northwestern Senegal and southwestern Mauritania. Across the Sahel there are enhanced probabilities of near-normal rainfall in those months.
THE CLIMATE OUTLOOK FORUM
From 4-8 May 1998, a Climate Outlook Forum was convened to formulate predictive guidance for the July-September 1998 rainy season in sub-Saharan West Africa. The Forum reviewed the state of the global climate system and its implications for this region. Among the principal factors taken into account were the major El Niño event of 1997-98, which has weakened slowly in recent months, and warmer than normal sea-surface temperatures (SSTs) that currently extend across much of the tropical Atlantic. Considerable research has established the linkages between SST anomalies in the tropical Pacific and Atlantic oceans and rainfall variability in sub-Saharan West Africa. The Climate Outlook that follows assumes the present El Niño will weaken more rapidly during the next few months (following most model predictions), and that there will be no development of a tropical Atlantic SST pattern that is known to accompany extreme sub-Saharan rainfall conditions. Any change in these expected SSTs will necessitate a revision of the Outlook statement. Careful monitoring of tropical Atlantic and Pacific SSTs is therefore needed during the next few months.
METHODOLOGY
The development of the West African Climate Outlook was performed using coupled ocean-atmosphere models, physically-based statistical models, and expert interpretation. Most of the statistical models used were developed by participants at the Pre-Forum Capacity Building Workshop on Seasonal Prediction in West Africa (23 February to 30 April 1998), held at the African Centre for Meteorological Applications for Development (ACMAD). The current status of seasonal- to inter-annual forecasting allows prediction of spatial and temporal averages, and may not fully account for all factors that influence regional and national climate variability. This Outlook is relevant only to seasonal time scales and relatively large areas; local and month-to-month variations may occur. Users are strongly advised to contact their National Meteorological and Hydrological Services for interpretation and local adaptation of this Outlook, and for additional guidance.
The experts established probability distributions to indicate the likelihood of below-, near- or above normal rainfall for each sub-region (see Map). Above-normal rainfall is defined as within the wettest third of recorded rainfall totals in each region; below-normal rainfall is defined as within the driest third of rainfall totals; near-normal is the third centered around the climatological median.
OUTLOOK
July to September receives on average 80% of the annual rainfall total in the Sahel zone, between 12° and 18° N. Further south to the Gulf of Guinea coast, July-September includes the Little Dry Season and hence is a less important period for annual rainfall. However, July-September rainfall anomalies can significantly affect agricultural production in this coastal region.
The probability of above-normal West African rainfall for the period July-September 1998 is 50% for northwestern Senegal and southwestern Mauritania, and also for the Gulf of Guinea coast region as far east as central Nigeria. Further east along the Gulf of Guinea coast region, over south-eastern Nigeria and extending into Cameroon, that probability is reduced to 40% because tropical Atlantic SSTs have a weaker positive influence on rainfall there. Except in the extreme west, July-September 1998 rainfall across the Sahel is presently considered to be most likely in the near-normal (40%) or below-normal (35%) categories. The closeness of this probability of below-normal rainfall (35%) to that of the near-normal category (40%) stems from uncertainty about the longevity of the present El Niño. Since El Niño suppresses Sahelian rainfall, a slower weakening of El Niño than is currently predicted would increase the likelihood of below-normal Sahelian rainfall. For the extreme western Sahel, while the probability of near-normal rainfall remains the same as that for further east (40%) and higher than for the other two rainfall categories, above-normal rainfall is considered to be more likely (35%) there than below-normal rainfall (25%). This is the reverse of the situation for the rest of the Sahel, and instead reflects the influence of the tropical Atlantic SSTs.
PARTICIPANTS
Participants at the Forum included representatives of Meteorological
Services from eleven countries (Benin; Côte d’Ivoire; Burkina Faso;
Chad; Ghana; Guinea-Bissau; Guinea-Conakry; Mali; Niger; Nigeria; Senegal;
Togo) and climate scientists and other experts from national, regional,
and international institutes (African Center of Meteorological Applications
for Development; Climate Information and Prediction Service, World Meteorological
Organization; Cooperative Institute for Mesoscale Meteorological Studies,
University of Oklahoma; International Research Institute for climate prediction;
Laboratoire Météorologique Dynamique; Météo-France;
North Carolina State University; National Centers for Environmental Prediction,
National Oceanic and Atmospheric Administration; ORSTOM, Brest; United
Kingdom Meteorological Office; University of Zululand). Additional input
was supplied by the European Centre for Medium-Range Weather Forecasts.
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