Net Assessment Forecast Discussion for Africa

The IRI has prepared this experimental Climate Outlook for Africa for April 2001 - September 2001. Of relevance in the preparation of this Outlook is the predicted decay of the slightly cooler than normal sea-surface temperatures (SSTs) in the central equatorial Pacific Ocean and the forecast development of weak, warmer than normal conditions by the second forecast period. Near neutral equatorial Pacific SST conditions are in effect for the first season of the forecast, April-May-June 2001, while during the second season, July-August-September 2001, they are expected to be slightly above average. The SSTs of the tropical Indian Ocean were assumed to be near average throughout both forecast periods.

This Outlook was prepared using the following procedures and information:

A) Coupled ocean-atmosphere model predictions of tropical Pacific SST covering the forecast period. Particularly heavy weighting has been given to predictions from the coupled model operated by the NOAA National Centers for Environmental Prediction, Climate Modeling Branch. This model suggests the demise of slightly cool but near-average conditions during the first forecast season with the development of weak, slightly above-average temperatures during the second season. The forecast for slightly above average conditions is consistent with most other numerical and statistical forecasts of central and eastern Pacific SSTs.

B) Forecasts of the tropical Atlantic and the Indian ocean using statistical models developed by the CPTEC and IRI, respectively.

C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.

D) Other sources of information include Center for Ocean-Land-Atmosphere Studies; Goddard Space Flight Center .

The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in the region. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for April - September 2001 is dependent on the accuracy of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty concerning the evolution of SSTs. Spread (variation) in global SST predictions is a source of uncertainty in the Outlook provided here. In particular, the forecasts for the tropical Indian and Atlantic oceans have been an important influence on the forecasts over Africa. Note that even if perfectly accurate SST forecasts were possible, there would still be uncertainty in the climate forecast due to chaotic internal variability of the atmosphere. These uncertainties are reflected in the probabilities given in the forecast.

It is stressed that the current status of seasonal-to-interannual climate forecasting allows prediction of spatial and temporal averages, and does 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 variations should be expected, and variations within the 3-month period should also be expected. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.

This Outlook covers two seasons: April - June 2001 and July - September 2001. Maps are given showing tercile probabilities of precipitation and temperature. The maps for precipitation indicate the probabilities that the seasonal precipitation will fall into the wettest third of the years (top number), the middle third of the years (middle number), or the driest third of the years (bottom number). A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas that are marked by "D" represent regions experiencing their dry season (less than 15% of annual precipitation) and for which less than 5 cm of precipitation typically occurs over the season. Otherwise, for example, in the case of region V in April - June 2001 (Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 35% chance it will be in the near-normal third of the years, and a 40% chance that the precipitation will be in the driest third of the years.

Maps of temperature show expected probabilities that the seasonal temperatures will fall into the warmest third of the years, the middle third of the years, or the coldest third of the years (Map A). The numbers for each region on the temperature maps indicate the probabilities of temperatures to fall in each of the three categories, above-, near-, and below-normal.

An additional precipitation map is provided for the first season indicating probabilities for extreme precipitation anomalies. (Sometimes this map may be blank, due to no expected extremes.) Extremes are defined as anomalies that fall within the top and bottom 15th percentile of the observed records. A priori, there is a 15% probability of being within the extremely wet category, and a 15% probability of being within the extremely dry category, leaving a 70% probability that the precipitation will not be extreme. The maps indicate areas of increased risk of extreme precipitation totals. Three levels of increased risk are defined: slightly enhanced risk, enhanced risk, and greatly enhanced risk. For slightly enhanced risk, there is a 25-40% probability that precipitation will be within the indicated extreme, i.e. wet or dry. This represents an approximate doubling of the climatological risk. For enhanced risk, there is a 40-50% probability that precipitation will be within the indicated extreme. This represents an approximate tripling of the climatological risk. For greatly enhanced risk, the probability that precipitation will be within the indicated extreme exceeds 50%, i.e. the indicated extreme is the most likely outcome. A similar map is provided in the first season indicating probabilities of extreme temperature anomalies.

Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.

The following regions of enhanced probabilities of precipitation anomalies are indicated:

Region V) slightly enhanced probabilities of below-normal precipitation over most of central and western Angola.

The following regions of enhanced probabilities of temperature anomalies are indicated:

Region VII) slightly enhanced probabilities of above-normal temperatures across Morocco, most of Tunisia and northern Algeria;

Region VIII) slightly enhanced probabilities of near-normal to above-normal temperatures over a broad region of central Africa including all of the Dem. Rep. Congo and the Republic of the Congo, most of Gabon, the southern Central African Republic, northern Mozambique, most of Namibia, Botswana and Zimbabwe, northern and central Madagascar, and all of Angola, Zambia and Malawi;

Region IX) slightly enhanced probabilities of near-normal to below-normal temperatures across southern Uganda and Kenya, most of Tanzania and northern Madagascar;

Region X) slightly enhanced probabilities of above-normal temperatures across all of South Africa, southern Mozambique, and extreme southern sections of Namibia, Botswana and Zimbabwe;

Region XII) slightly enhanced probabilities of above-normal temperatures across all but southwestern Saudi Arabia, much of Oman, and all of the United Arab Emirates, all of Kuwait, and most of Iraq and Iran.

The following regions of enhanced probabilities of precipitation anomalies are indicated:

Region VIII) slightly enhanced probabilities of below-normal precipitation over southwestern Ethiopia, most of Uganda, and the southern border area of Sudan;

Region IX) slightly enhanced probabilities of below-normal precipitation over the southwestern corner of South Africa surrounding Cape Town.

The following regions of enhanced probabilities of temperature anomalies are indicated:

Region VI) slightly enhanced probabilities of above-normal temperatures over a large region of northwestern Africa including all North Atlantic coastal countries north and west of central Nigeria, as well as all of Algeria and Tunisia, western Libya, Mali, Niger, and Burkina Faso;

Region VII) slightly enhanced probabilities of below-normal temperatures over Somalia, southeastern Ethiopia, most of Kenya, and northeastern Tanzania.

Climate Outlook

AFRICA April 2001 - September 2001

Issued: March 2001