The IRI Experimental Forecast Division has prepared this Climate Outlook
for Africa
for October 1999 - March 2000. Of
relevance in the preparation of this Outlook has been the current weak
La Niño
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean) that are
expected to amplify slightly through the
end of 1999
and then decline slowly in early 2000.
The persistence of slightly
warmer than average conditions in the western Pacific has been assumed also. The SSTs
of the tropical Indian Ocean were assumed to return to near-normal
conditions with the period October-December 1999.
The forecast of tropical South Atlantic SSTs suggests a
warming trend through December 1999.
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 resurgence of weak to moderate La
Niņa conditions, with the persistence of slightly positive sea-surface temperature anomalies in the western Pacific.
The return of colder that average conditions in the central and eastern equatorial Pacific 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) The Greater Horn of Africa Regional Climate Outlook Forum and
the Southern African Regional Climate Outlook Forum consensus guidance.
E) Other sources of information include (
CPC ;
ECMWF ;
South African Weather Service
).
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 October 1999 - March 2000 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
concerning the detailed evolution of SSTs. Spread in global SST predictions is a source of uncertainty in the Outlook
provided here. In particular, the predicted warming over the southern tropical Atlantic has been a primary influence
on the forecasts over South America.
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: October - December 1999 and January - March 2000. 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 5cm of precipitation typically occurs
over the season. Otherwise, for example, in the case of Madagascar in October - December 1999
(Map B), there is a 30% probability that the precipitation will be in the wettest third of the years, a 50% chance it will be in the near-normal third
of the years, and a 20% chance that the precipitation will be in the driest third of the years.
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.
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.
Boundaries between sub-regions should be considered as transition zones, and their location considered to be only
qualitatively correct.
October - December 1999:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.A) slightly enhanced probabilities of below-normal precipitation along the south coast of West Africa between Ghana and central Nigeria;
P.B) greatly enhanced probabilities of above-normal precipitation in a zone immediately to the south of the Sahel, stretching from southern Senegal and Gambia to northern Somalia;
P.C) enhanced probabilities of above-normal precipitation over an area extending from north-eastern
Congo, through the northern half of the Democratic Republic of Congo, into southern Sudan, south-western
Ethiopia, Uganda, and western Kenya (a slightly enhanced risk of extremely wet conditions is indicated
over an area centered on Uganda);
P.D) slightly enhanced probabilities of below-normal precipitation over most of Somalia, south-eastern Ethipopia, eastern Kenya, and the north-easternmost part of Tanzania;
P.E) enhanced probabilities of below-normal precipitation over Rwanda, Burundi, most of Tanzania, northern Malawi, and the far northern part of Mozambique;
P.F) greatly enhanced probabilities of below-normal precipitation over Gabon, the south-western half of Congo, the far western part of the Democratic Republic of Congo, and the north-western part of Angola;
P.G) slightly enhanced probabilities of near-normal precipitation over south-western Angola, and northern Namibia;
P.H) slightly enhanced probabilities of below-normal precipitation over an area between about 15S and 20S, and east of about 20E;
P.H) slightly enhanced probabilities of below-normal precipitation over an area between about 15S and 20S, and east of about 20E;
P.I) enhanced probabilities of near- to below-normal precipitation over south-eastern Namibia, most of South Africa and Botswana, the southern parts of Zimbabwe and Mozambique, Swaziland and Lesotho;
P.J) slightly enhanced probabilities of near-normal precipitation over the south-western tip of South Africa;
P.K) slightly enhanced probabilities of near-normal precipitation over Madagascar;
P.L) slightly enhanced probabilities of near-normal precipitation over the Seychelles;
P.M) enhanced probabilities of near-normal precipitation over Mauritius and Reunion;
P.N) enhanced probabilities of near- to below-normal precipitation over most of the Saudi Arabian peninsula;
P.O) slightly enhanced probabilities of near-normal precipitation over Iraq and north-western Iran.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.A) greatly enhanced probabilities of above-normal temperatures over the north-western half of Lybia, Tunisia, and much of Algeria;
T.B) enhanced probabilities of above-normal temperatures over an area extending from the south-western half of Saudi Arabia, through the Sahara to the west coast of West Africa, and extending north to include Morocco;
T.C) slightly enhanced probabilities of above-normal temperatures over north-western Iran, northern Iraq, eastern Syria, and eastern Turkey;
T.D) greatly enhanced probabilities of above-normal temperatures over Djibouti, northern Somalia, Yemen, and the southernmost part of Oman;
T.E) greatly enhanced probabilities of near-normal temperatures over an area stretching from the Somali east coast across to Gabon and northern Nigeria;
T.F) enhanced probabilities of near-normal temperatures over Kenya, Uganda, Rwanda, and most of Tanzania;
T.G) greatly enhanced probabilities of above-normal temperatures over southern Democratic Republic of Congo, and much of Zambia;
T.H) enhanced probabilities of above-normal temperatures over south-eastern Africa, including Mozambique, Zimbabwe, Malawi, and the far western part of Tanzania;
T.I) greatly enhanced-probabilities of above-normal temperatures over Namibia, Botswana, Swaziland, Lesotho, and most of South Africa;
T.J) enhanced-probabilities of near-normal temperatures over the south coast of South Africa.
January - March 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
P.i) slightly enhanced probabilities of above-normal precipitation over Lybia;
P.ii) enhanced probabilities of above-normal precipitation over most of Saudi Arabia and Iran, United Arab Emirates, and the south-eastern half of Iraq;
P.iii) enhanced probabilities of near- to above-normal precipitation over much of Tanzania, Rwanda, Burundi, Kenya, eastern Uganda, southern Somalia, and the southernmost part of Ethiopia;
P.iv) slightly enhanced probabilities of near-normal precipitation over a large area of south-eastern Africa, extending north-westwards into Angola and southern Democratic Republic of Congo;
P.v) enhanced probabilities of above-normal precipitation over most of Namibia, and the western half of South Africa, except for the south-western tip;
P.vi) enhanced probabilities of near-normal precipitation over the Seychelles;
P.vii) enhanced probabilities of near-normal precipitation over Mauritius and Reunion.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
T.i) slightly enhanced probabilities of above-normal temperatures over the north coasts of Morocco, Algeria, and Tunisia;
T.ii) enhanced probabilities of above-normal temperatures over southern West Africa, between Sierra Leone and Nigeria;
T.iii) slightly enhanced probabilities of above-normal temperatures in a band extending from Burkina Faso in the west, and Cameroon in the south, to Sudan in the north and east.
![[key]](/climate/forecast/images/key.jpg)
