The IRI Experimental Forecast Division has prepared this Climate Outlook for Africa for January - June 1999. Of relevance in the preparation of this Outlook have been the continuation of cooler than average sea-surface temperatures (SSTs) in the eastern and central equatorial Pacific Ocean (La Niņa), and the persistence of warmer than average conditions in the western Pacific (http://iri.columbia.edu/forecast/sst/images/nov98/atl_blanom_sst_Nov98NDJ.gif). The SSTs of the central tropical Indian Ocean have cooled from record high temperatures in 1998. The SSTs there are currently below average, and negative anomalies are expected to persist. It has been assumed that the northern and tropical Atlantic Ocean will remain warmer than normal, and that sea-surface temperatures in the South Atlantic will increase slightly during the forecast period.
METHODS -
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 continuation of moderate La Niņa conditions, slowly decaying by the end of the forecast period, with the persistence of positive sea-surface temperature anomalies in the region of Indonesia. This sea-surface temperature forecast is consistent with most other numerical and statistical forecasts.
B) Forecasts of the tropical Indian and Atlantic oceans using statistical models developed by the IRI and CPTEC respectively.
C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted sea-surface temperature patterns.
D) Statistical analyses of the observed climate over Africa.
E) The Southern Africa Regional Climate Outlook Forum http://iri.columbia.edu/forecast/sup/images/dec98/Ezulwini.html consensus guidance.
F) Other sources of information include (ACMAD http://www.acmad.ne; CPC; ECMWF http://www.ecmwf.int/products/forecasts/; South African Weather Service http://www.weathersa.co.za; UKMO).
The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by the national meteorological services in Africa. Thus, this product may differ from the official forecasts issued in those areas. The Climate Outlook for January - June 1999 is dependent on the quality of the SST predictions. For the tropical Pacific, these predictions can be expected to provide useful information, but there is some uncertainty between coupled model predictions concerning the detailed evolution of SSTs. This spread in global SST predictions is a source of uncertainty in the Outlook provided here.
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. For further information concerning this and other guidance products, users are strongly advised to contact their National Meteorological Services.
OUTLOOK -
This Outlook covers two seasons: January - March 1999 and April - June 1999. 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). 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. 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. A qualitative outlook of climatology ("C") indicates that there is no basis for favoring any particular category. Areas marked "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 Madagascar in January - March 1999 (Map A), there is a 40% 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 25% chance that the precipitation will be in the driest third of the years. 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. Boundaries between sub-regions should be considered as transition zones, and their location considered to be only qualitatively correct.
January - March 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.A) enhanced probabilities of below-normal precipitation in the northern parts of Morocco, Algeria and Tunisia;
P.B) slightly enhanced probabilities of above-normal precipitation over Libya and eastern Algeria;
P.C) enhanced probabilities of above-normal precipitation over the southern coast of West Africa;
P.D) slightly enhanced probabilities of below-normal precipitation over central Africa west of about 25E, between Central African Republic and Angola;
P.E) greatly enhanced probabilities below-normal precipitation over much of eastern Africa north of about 5S (a slightly enhanced risk of extremely dry conditions is indicated);
P.F) slightly enhanced probabilities of near-normal precipitation over southern Tanzania, northern Mozambique, northern Malawi, northern Zambia, and south-eastern Democratic Republic of Congo;
P.G) greatly enhanced probabilities of above-normal precipitation over southern Zambia, southern Malawi, central Mozambique, and most of Zimbabwe (a slightly enhanced risk of extremely wet conditions is indicated);
P.H) enhanced probabilities of above-normal precipitation over Botswana, most of Namibia and South Africa, Lesotho, and Swaziland;
P.I) slightly enhanced probabilities of near-normal precipitation over the south coast of South Africa;
P.J) enhanced probabilities of near-normal precipitation over the south-western tip of South Africa;
P.K) slightly enhanced probabilities of above-normal precipitation over Madagascar.
P.L) enhanced probabilities of near-normal precipitation over Mauritius.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.A) slightly enhanced probabilities of below-normal temperatures over Morocco, most of Angola, northern Mali and northern Mauritania;
T.B) greatly enhanced probabilities of above-normal temperatures between about 20N and 15S;
T.C) slightly enhanced probabilities of near-normal temperatures south of about 15S, except for the south coast of South Africa where there are enhanced probabilities of above-normal temperatures.
April - June 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.i) enhanced probabilities of below-normal precipitation over eastern Africa;
P.ii) slightly enhanced probabilities of above-normal precipitation over the south coast of West Africa;
P.iii) slightly enhanced probabilities of above-normal precipitation over South Africa, Swaziland and Lesotho.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.i) slightly enhanced probabilities of above-normal temperatures over much of West Africa (except inland), and along the Mediterranean coast;
T.ii) enhanced probabilities of above-normal temperatures over much of southern and eastern Africa;
T.iii) greatly enhanced probabilities of above-normal temperatures over East Africa between central Mozambique and central Kenya.
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