AFRICA March - August 2002
Issued: February 2002
The IRI has prepared this experimental Climate Outlook for Africa for
March - August 2002. Of relevance in the preparation of this
outlook is the prediction of warmer than average conditions to
develop in the eastern equatorial Pacific during the next 3 to
6 months. Currently the sea surface temperatures (SSTs) in
the eastern equatorial Pacific are slightly below their long-term average
but have been increasing during the month of February.
Above average SSTs continue in the central Pacific near the
international date line, extending also across the western part of the basin.
During the course of the four forecast seasons
the SST anomalies in the eastern tropical Pacific are
expected to increase, becoming above normal by Apr-May-Jun and continuing to warm
through Jun-Jul-Aug. A developing weak El Nino is indicated in this scenario.
The somewhat warmer than average SSTs that have been present over much of the Indian
Ocean are expected to decrease slowly through the forecast period. The area
of above-average temperature in the tropical north Atlantic Ocean is
predicted to persist but slowly weaken through the period.
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
D) Other sources of information include
Seasonal to Interannual Prediction Project (GSFC-NASA)
and also seasonal prediction research at COLA.
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
March - August 2002
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
This Outlook covers four seasons: March-May 2002,
April-June 2002, May-July 2002 and June-August 2002.
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). The color
shading indicates the probability of the most dominant tercile -- that
is, the tercile having the highest forecast probability. The color bar
alongside the map defines these dominant tercile probability levels. The
upper side of the color bar shows the colors used for increasingly
strong probabilities when the dominant tercile is the above-normal
tercile, while the lower side shows likewise for the below-normal
tercile. The gray color indicates an enhanced probability for the
near-normal tercile (nearly always limited to 40%). As before, numbers
and their associated histograms show the probabilities of the three
terciles. In areas with lots of spatial detail, there may not be
sufficient room on the map, to allow histograms for each region. In
those cases, some idea of the probabilities may be gained from the color
alone. 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 for which less than 3cm of
precipitation typically occurs over the season.
Otherwise, for example, in the case of
Sierra Leone in March-May 2002
there is a 50% probability that the precipitation will be in the
wettest third of the years, a 30% 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.
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
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.
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. A similar map is provided
in the first season indicating probabilities of
Boundaries between sub-regions should be considered as transition zones,
and their location considered to be only qualitatively correct.
March-May 2002 through June-August 2002:
The following discussion briefly describes the probability anomaly forecasts.
In this disussion, the first, second, third and fourth forecast periods
refer, respectively, to March-April-May, April-May-June, May-June-July,
Enhanced probabilities for above normal precipitation are forecast
for portions of the southern part of western Africa, sometimes including
the Guinea Coast, for all four predicted periods. For the first period
the Morocco region has this forecast also. In three of the
four periods, a part of the south coast of South Africa has enhanced
probabilities for above normal precipitation. Enhanced probabilities for
below normal precipitation are forecast for parts of western Angola for
the second and the third forecast periods.
Enhanced probabilities for above normal temperature dominate the forecast
for Africa for all four forecast periods, with the most strongly enhanced
probabilities found along the coast of western Africa and in much of the
south-central portion of the continent. A relatively small region of enhanced
probabilities for below normal temperature is forecast in part of the
Guinea coast for the first two forecast periods.
OBSERVED CLIMATOLOGY DATA for Mar-Apr-May,
Apr-May-Jun, May-Jun-Jul and Jun-Jul-Aug