The IRI has prepared this experimental Climate Outlook for Africa for July - December 2003. Of relevance in the preparation of this outlook is the potential for a developing La Nina event. In early March 2003 equatorial Pacific sea surface temperatures (SSTs) returned from above-normal to near-normal in the eastern equatorial Pacific, and they continued cooling through early June reaching values on the order of -2C in the far eastern equatorial Pacific. Weak La Nina conditions are indicated in the SST predictions on which these climate forecasts are based. See the IRI's ENSO update for a discussion on the La Nina outlook (see IRI Probabilistic ENSO forecast). Warmer than average SSTs continue to dominate much of the tropical Western Pacific and Indian Ocean (SSTs). These are predicted to decrease through the forecast period. (July-September 2003, August-October 2003, September-November 2003, October-December 2003). Warmer than average SSTs currently exist in the northern and southern sub-tropical Atlantic Ocean. In May the equatorial Atlantic developed a small region of below-normal SSTs. The tropical Atlantic SST forecast slowly damps these anomalies. However, more recent observations show continuing development of cold anomalies on the equator and southward along the west coast of Africa. This discrepancy between the predicted and recently observed SSTs has been accounted for in the final forecasts.

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. The simple coupled model of the Lamont-Doherty Earth Observatory was also given attention, as well as the statistical Constructed Analogue model of the Climate Prediction Center of the National Centers for Environmental Prediction. These models suggest weak La Nina conditions during the four forecast seasons. The forecast for weak La Nina conditions is consistent with some, but not all, numerical and statistical forecasts of central and eastern Pacific SSTs.

B) Forecasts of the tropical Indian ocean using a statistical model developed by the IRI.

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 NASA's Seasonal to Interannual Prediction Project (GSFC-NASA) and also seasonal prediction research at COLA.

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 July - December 2003 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 four seasons: July-September 2003 August-October 2003 September-November 2003 and October-December 2003 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). The color shading indicates the probability of the most dominant tercile -- that is, the tercile having the highest forecast probability. The color bar alongside the map defines these dominant tercile probability levels. The upper side of the color bar shows the colors used for increasingly strong probabilities when the dominant tercile is the above-normal tercile, while the lower side shows likewise for the below-normal tercile. The gray color indicates an enhanced probability for the near-normal tercile (nearly always limited to 40%). As before, numbers and their associated histograms show the probabilities of the three terciles. In areas with lots of spatial detail, there may not be sufficient room on the map, to allow histograms for each region. In those cases, some idea of the probabilities may be gained from the color alone. 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 for which less than 3cm of precipitation typically occurs over the season. Otherwise, for example, in the case of Somalia in July-September 2003 (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. 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.

July-September 2003 through October-December 2003

The following discussion briefly describes the probability anomaly forecasts:

Precipitation

Enhanced probabilities for above normal precipitation are forecast for parts of the Sahel during mainly the first two forecast periods, and enhanced probabilities for below normal precipitation are forecast for parts of central or eastern equatorial Africa for all four forecast periods.

Temperature

OBSERVED CLIMATOLOGY DATA for Jul-Aug-Sep, Aug-Sep-Oct, Sep-Oct-Nov and Oct-Nov-Dec