The IRI has prepared this experimental Climate Outlook for Africa
for July - December 2000. Of
relevance in the preparation of this Outlook has been the weakening remains of the 1998-2000
La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean).
Thus, near neutral but slightly below-average equatorial Pacific SST conditions are in effect for the first season of the forecast,
July-August-September 2000, and near-normal
conditions are expected for the second forecast season,
October-November-December 2000.
The SSTs of the tropical Indian Ocean were assumed to remain near-normal.
Tropical Atlantic SSTs were not expected to change significantly.
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 final demise of La
Niņa conditions and a return to near-average conditions during the first forecast season.
The forecast for 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 and Atlantic oceans using statistical models developed by the IRI and CPTEC,
respectively.
C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
D) The
Third Forum on Seasonal Climate Prediction and its Applications for the Benefit of Rural Communities in West Africa: Contribution of the New Information and Communication Technologies (RANET/CFAR) consensus guidance.
E) Other sources of information include
CPC ;
Drought Monitoring Centre, Nairobi ;
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 July - December 2000 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 in global SST predictions is a source of uncertainty in the Outlook
provided here. In particular, the forecasts for the tropical Atlantic have been an important influence
on the forecasts over West 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: July - September 2000 and October - December 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 region I in July - September 2000
(Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 40% chance it will be in the near-normal third
of the years, and a 35% 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.
April - June 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region I) slightly enhanced probabilities of normal precipitation over the Sahel of West Africa, from southern Mauritania and Sierra Leone to Chad;
Region II) slightly enhanced probabilities of above-normal precipitation over southern parts of West Africa between Liberia and south-western Nigeria;
Region III) slightly enhanced probabilities of below-normal precipitation over a large area of eastern Africa including southern and central Sudan, Uganda, western Kenya, Ethiopia, Eritrea, Djibouti, and north-western Somalia.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region I) greatly enhanced probabilities of above-normal temperatures over northern Africa including the northern halves of Morocco, Algeria and Libya, all of Tunisia, and north-west Egypt;
Region II) slightly enhanced probabilities of above-normal temperatures over Turkey, Syria, Jordan, Isreal, north-eastern Egypt, northern Saudi Arabia, most of Iraq, and north-western Iran;
Region III) slightly enhanced probabilities of above-normal temperatures over a large area extending eastward from the west coast of West Africa, across the Sahara into far western Saudia Arabia and most of Yemen, then southward into Ethiopia, and expanding to include most of central Africa, and extending further to include central Madagascar;
Region IV) greatly enhanced probabilities of above-normal temperatures over much of West Africa south of about 15S, extending into central Sudan and the far western part of Ethiopia;
Region V) slightly enhanced probabilities of normal temperatures over coastal parts of eastern Africa, and the far nothern tip of Madagascar;
Region VI) enhanced probabilities of above-normal temperatures over an area stretching from Angola to Mozambique and southern Madagascar, and extending westward again to include most of South Africa;
Region VII) slightly enhanced probabilities of above-normal temperatures over most of Namibia, Botswana, and western and central Zimbabwe;
Region VIII) slightly enhanced probabilities of normal temperatures over the southern part of South Africa.
October - December 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region IV) slightly enhanced probabilities of above-normal precipitation over the southern parts of West Africa;
Region V) slightly enhanced probabilities of normal precipitation over south-eastern Africa;
Region VI) slightly enhanced probabilities of above-normal precipitation over central Saudi Arabia.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region VII) slightly enhanced probabilities of above-normal temperatures over the Mediterranean coast of north Africa between Morocco and Libya;
Region VIII) slightly enhanced probabilities of above-normal temperatures over an area extending from entral Egypt and southern Libya in the north to the Demorcatic Republic of Congo and northern Zambia in the south;
Region IX) slightly enhanced probabilities of above-normal temperatures over the south coast of West Africa;
Region X) slightly enhanced probabilities of normal temperatures over central southern Africa.
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