The IRI has prepared this experimental Climate Outlook for Africa
for October 2000 - March 2001. Of relevance in the
preparation of this Outlook has been the near-average sea-surface temperatures
(SSTs)
in the
central equatorial Pacific Ocean.
Near neutral equatorial Pacific SST conditions are in effect for both seasons of the forecast,
October-November-December 2000 and
January-February-March 2001.
The SSTs of the tropical Indian Ocean were assumed to remain normal.
Tropical Atlantic SSTs are above-normal during the first forecast season and are expected to return toward normal during
the second season.
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 a continuation of
near-average conditions during the first forecast season.
The forecast for near-neutral 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
CPC ;
Drought Monitoring Centre, Nairobi ;
ECMWF ;
South African Weather Service ;
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 October - March 2000-01 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.
OUTLOOK -
This Outlook covers two seasons: October - December 2000 and January - March 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 IX in October - December 2000
(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.
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 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region IX) slightly enhanced probabilities of below-normal precipitation over the Guinea coast of Africa, from Sierra
Leone westward to southwestern Nigeria;
Region X) enhanced probabilities of below-normal precipitation over southern Uganda, all except the coastal portions of
Tanzania, extreme western Kenya, and northern Mozambique;
Region XI) slightly enhanced probabilities of above-normal precipitation over eastern South Africa, southern Mozambique,
southeastern Botswana, and extreme southern Zimbabwe;
Region XII) slightly enhanced probabilities of above-normal precipitation over western Angola;
Region XIII) slightly enhanced probabilities of below-normal precipitation over a portion of southwestern South Africa;
Region XIV) enhanced probabilities of above-normal precipitation over Ethiopia, Somalia, southern Sudan and all except southwestern
Kenya;
Region XV) slightly enhanced probabilities of below-normal precipitation in Madagascar;
Region XVI) slightly enhanced probabilities of near-normal precipitation over the Mauritius and Reunion Islands.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region XIV) slightly enhanced probabilities of above-normal temperatures over most of Africa, including
nearly all of northern and western Africa except for the Guniea coast, much of the Arabian Peninsula,
and also more than half of the southern portion of Africa.
Region XV) greatly enhanced probabilities of above-normal temperatures over the Guinea coast from Sierra Leone westward through southern Nigeria to the Central African Republic and Democratic Republic of the Congo, as well as all of the Gulf of Guinea
countries extending southward to Congo;
Region XVI) greatly enhanced probabilities of above-normal temperatures over Namibia and western South Africa;
Region XVII) greatly enhanced probabilities of above-normal temperatures over the east coast of South Africa, the
southern and southeastern coasts of Mozambique, and most of Madagascar.
January - March 2001:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region VI) slightly enhanced probabilities of above-normal precipitation over Cameroon, Rio Muni, Gabon, and western Congo Republic;
Region VII) slightly enhanced probabilities of above-normal precipitation over Namibia and western South Africa;
Region VIII) slightly enhanced probabilities of below-normal precipitation over Tanzania, Kenya, of Rwanda and Uganda.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region V) slightly enhanced probabilities of above-normal temperatures over much of Africa, including the Guinea Coast,
parts of the Sahel, southeastern Sudan, the southwestern half of Madagascar, and most of the southern portion of the continent with the exception of southernmost South Africa. Note: This very large region also includes much of southern Asia, much of Indonesia, and part of Australia. A region of more greatly enhanced probabilities of above-normal temperature, Region VI, lies within this region.
Region VI) enhanced probabilities of above-normal temperatures over a central area within Region V, including much of
the Democratic Republic of the Congo, western Tanzania, southwestern Kenya, Rwanda, Uganda, southern Sudan, parts of
Chad and the Central African Republic, and parts of Niger, Nigeria, Mali, and Burkina Faso.
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