The IRI has prepared this Climate Outlook for Africa for April -
September 2000. Of relevance in the preparation of this Outlook has been
the current
La
Niņa conditions (lower than average sea-surface temperatures (SSTs)
in the central equatorial Pacific Ocean) that had been amplifying
through the end of 1999. Thus, weakening La Niņa conditions are in
effect for the first season of the forecast,
April-May-June 2000, and a return to near-normal conditions is
expected for the second forecast season,
July-August-September 2000. The SSTs of the tropical Indian Ocean
were assumed to remain near-normal. The forecast of tropical North
Atlantic SSTs suggests a cooling trend through June 2000.
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 rapid weakening of La Niņa conditions and a return
to near-average conditions by the middle of 2000. The end of 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 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
Fifth Greater Horn of Africa 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 April -
September 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
evolution of SSTs. Spread in global SST predictions is a source of
uncertainty in the Outlook provided here. In particular, the predicted
cooling over the northern tropical Atlantic has been an important
influence on the forecasts over West Africa.
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: April - June 2000 and July - September
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, defined within the 1961-90 period (top number), the middle third
of the years (middle number), and 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 April - June 2000
(Map B), 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.
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.
Climatologically, 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 risk of extreme high or
low 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 for 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 above-normal precipitation
over Senegal, Gambia, Guinea, Guinea-Bissau, Sierra Leone, and
south-western Mali;
Region II) slightly enhanced probabilities of above-normal precipitation
over Liberia and Ghana;
Region III) slightly enhanced probabilities of below-normal
precipitation over Syria and north-western Iraq;
Region IV) enhanced probabilities of below-normal precipitation over
Jordan, south-eastern Iraq, and northern Saudia Arabia;
Region V) enhanced probabilities of below-normal precipitation over
southern Saudi Arabia, and Yemen;
Region VI) enhanced probabilities of below-normal precipitation over
Equatorial Guinea, Gabon, and the near-coastal areas of Congo,
Democratic Republic of Congo, and Angola;
Region VII) slightly enhanced probabilities of above-normal
precipitation over much of the Democratic Republic of Congo, eastern
Angola, Zambia, Malawi, and much of northern and central Mozambique;
Region VIII) slightly enhanced probabilities of normal precipitation
over Tanzania, Rwanda, Burundi, Uganda, and western Kenya;
Region IX) enhanced probabilities of below-normal precipitation over
south-eastern Ethiopia, eastern Kenya, and Somalia;
Region X) slightly enhanced probabilities of below-normal precipitation
over the Seychelles;
Region XI) slightly enhanced probabilities of below-normal precipitation
over Reunion and Mauritius;
Region XII) slightly enhanced probabilities of above-normal
precipitation over southern Namibia, the far south-eastern part of
Botswana, and much of the western half of South Africa, excluding the
south-west and southern coasts;
Region XIII) slightly enhanced probabilities of above-normal
precipitation over Lesotho, Swaziland, and much of the eastern half of
South Africa.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies
are indicated:
Region I) greatly enhanced probabilities of above-normal temperatures
over Tunisia, Algeria, and the northern half of Morocco;
Region II) slightly enhanced probabilities of above-normal temperatures
over a large area extending north-westwards from Liberia and Ghana to
the south-eastern half of the Arabian peninsula, and extending
south-eastwards to Malawi and northern Mozambique;
Region III) slightly enhanced probabilities of normal temperatures over
eastern Central African Republic, and cnetral and northern Democratic
Republic of Congo;
Region IV) enhanced probabilities of above-normal temperatures over a
large area extending from Equatorial Guinea and Gabon southwards and
eastwards to central and southern Mozambique, and including southern and
central Madagascar.
July - September 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation
anomalies are indicated:
Region V) slightly enhanced probabilities of below-normal precipitation
over the far northern parts of Algeria and Tunisia;
Region VI) slightly enhanced probabilities of below-normal precipitation
over the Sahelian belt from Senegal and Guinea to the easternmost part
of Sudan;
Region VII) slightly enhanced probabilities of below-normal
precipitation over part of east Africa, including Uganda, Kenya, and
Tanzania;
Region VIII) slightly enhanced probabilities of above-normal
precipitation over the far south-western part of South Africa;
Region IX) slightly enhanced probabilities of below-normal precipitation
over southern Mozambique, Swaziland, and south-eastern South Africa.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies
are indicated:
Region IX) slightly enhanced probabilities of normal temperatures over
Morocco, Algeria, Tunisia, and northern Mauritania;
Region X) greatly enhanced probabilities of above-normal temperatures
over a large area of West Africa between about 5 and 15N, extending
eastwards through central Sudan;
Region XI) slightly enhanced probabilities of above-normal temperatures
over much of Africa south of about 5N, extending into Eritrea, Ethiopia,
and Djibouti;
Region XII) enhanced probabilities of above-normal temperatures over the
south-eastern part of Africa, and over Madagascar.
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