AFRICA July - December 2003
Issued: June 2003
The IRI has prepared this experimental Climate Outlook for Africa for July - December 2003.
Of relevance in the preparation of this outlook is the potential for a developing La Nina event.
In early March 2003 equatorial Pacific sea surface temperatures (SSTs) returned from above-normal to near-normal in the eastern equatorial Pacific, and they continued cooling through early June reaching values on the order of -2C in the far eastern equatorial Pacific. Weak La Nina conditions
are indicated in the SST predictions on which these climate forecasts are based. See the IRI's ENSO update for a discussion on the La Nina outlook (see IRI Probabilistic ENSO forecast). Warmer than average SSTs continue to dominate much of the tropical Western Pacific and Indian Ocean (SSTs). These are predicted to decrease through the forecast period. (July-September 2003, August-October 2003, September-November 2003, October-December 2003).
Warmer than average SSTs currently exist in the northern and southern sub-tropical Atlantic Ocean. In May the equatorial Atlantic developed a small region of below-normal SSTs. The tropical Atlantic SST forecast slowly damps these anomalies. However, more recent observations show continuing development of cold anomalies on the equator and southward along the west coast of Africa. This discrepancy between the predicted and recently observed SSTs has been accounted for in the final forecasts.
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.
The simple coupled model of the Lamont-Doherty Earth Observatory was
also given attention, as well as the statistical Constructed Analogue
model of the Climate Prediction Center of the National Centers for
These models suggest weak La Nina conditions during the four forecast seasons.
The forecast for weak 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 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
July - December 2003
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: July-September 2003
August-October 2003 September-November 2003 and October-December 2003
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
Somalia in July-September 2003
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.
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.
July-September 2003 through October-December 2003
The following discussion briefly describes the probability anomaly forecasts:
Enhanced probabilities for above normal precipitation are forecast for parts
of the Sahel during mainly the first two forecast periods, and
enhanced probabilities for below normal precipitation are forecast for parts
of central or eastern equatorial Africa for all four forecast periods.
An enhanced probability for above normal temperature is forecast for much
of Africa for all four forecast periods, with strongest and most extensive
enhancement for warmth during the first period.
OBSERVED CLIMATOLOGY DATA for Jul-Aug-Sep,
Aug-Sep-Oct, Sep-Oct-Nov and Oct-Nov-Dec
TERCILE THRESHOLDS (33%-ile & 67%-ile):
EXTREME THRESHOLDS (15%-ile & 85 %-ile):