RAINFALL SEASONAL FORECAST FOR THE PERIOD JULY-AUGUST-SEPTEMBER 2003 IN WEST AFRICA, CHAD AND CAMEROUN
At
interannual timescale, the variability of the sea surface temperature in the
tropical Pacific (El Nino/La Nina) and in the
tropical Atlantic influences the seasonal climate on West Africa and the
Sahelian-Soudanese region.
La Nina conditions (or El Nino) often coincide with a
rainy season July-September above normal (or below normal), relative to the
usual amounts (average of the
period) for the major part of the
Sahelian-Sudanese region.
Over this same season, when sea surface temperature in the Gulf of Guinea
is warmer (or colder) than usual (average conditions) during July-September,
drier condition is often observed (or wetter) on the Sahel and wetter (or drier)
conditions on the southern part of the region south of about 10 degrees North.
Lastly, when the Atlantic dipole is in a configuration of anomalies cold
in the North of the equator and warm in the south (or warm in north and cold in
the south), along with similar contrasts on a more global scale, then this
usually coincides with periods of drier (or wetter) years in the
Sahelian/Soudanese regions.
B. STATE OF THE GLOBAL OCEAN IN SPRING 2003:
The rainfall seasonal forecast was elaborated in Niamey (Niger) at the African Centre for Meteorological Application to the Development (ACMAD) on May 12 2003 for the period from July to September 2003 on the West Africa region, Chad and Cameroun. The elaboration of this forecast took into account the current state of the Global Ocean such as its evolution and its impacts on the regional climate.
Over the Pacific (NINO3):
For the period El Nino 2002/2003, the cooling of the sea surface
temperature started since the beginning of January persisted in a significant
way to La Nina conditions from the beginning of the second part of may. These La
Nina conditions have been maintained until the end of the first part of June.
But has stated to move up from the beginning of the second part of June and
according the estimations of the global models of World Climate Prediction
Centres, this will continue during the coming months. So there is the
possibility that the negative anomalies will maintain only slightly below zero.
This situation usually corresponds to a rainfall close to an average value of
the season in the Sahel.
But, it necessary to be very careful about the state of Pacific Ocean,
because the La Nina conditions could appear again.
Over the tropical Atlantic:
A warming of the sea surface temperature since the
beginning of the year on the coasts of the Gulf of Guinea and on the Southern
tropical Atlantic was maintained until may. From this month, a slight cooling
has started and significant negative anomalies were established in June. The
greater negative anomalies observed from 8 to 14 June over an important area.
From the 15 to 21 June, this area was reduced and some small areas of positive
anomalies had appeared on the coasts of Gulf of Guinea. From 22 to 28 June,
these positives anomalies had intensified and interestingly over all the Gulf of
Guinea but couldn’t persist for a long period. The significant negatives
anomalies will take place during July. This situation corresponds
statistically to below normal precipitations on the areas of the Gulf of Guinea
(south of about 10 degrees North) and above normal on the Sahelian part.
Concerning the North-eastern part of the tropical
Atlantic, the warming which was observed since April was maintained and
intensified until the end of June. This warming will persist during the season.
So it would bring a configuration of the Atlantic dipole, positive in North and
negative in the South, which generally leads to an above normal precipitation in
the Sahelian regions and important precipitations over Mauritania and Senegal.
It is worth to point out that a strong variability characterizes the
temperatures on the tropical Atlantic and their prediction is still under
research. Therefore it is strongly advised to pay attention to the evolution of
the temperature anomalies of the tropical Atlantic during the next coming months.
Remarks: The forecasts in the figure
below represent, the current stage, a consensual forecast of the impacts
of the various behaviours of the oceans on the regional precipitations. A second
update will be done at the end of July, using new observations of sea surface
temperature.
C-
METHODOLOGY.
The
development of this forecast was carried out from the results of the coupled
Ocean – Atmosphere dynamic models and the synthesis of the national and
international models physically plausible and based on a statistical approach.
It is strongly advised to contact the National
Meteorological and Hydrological Services for more detailed interpretation of the
results of the forecast and eventually to obtain additional directives.
For
the forecast presentation, the experts use information of the models to estimate
the probability that the seasonal total amount of precipitations of this year
belongs to the one of the three categories defined below.
Definition
of the categories
The « normal » is
defined here as the average of 30 years seasonal rainfall, over the period
1961-1990.
The category « above normal »
corresponds to the third of observations whose seasonal cumulative total
amount of precipitations were highest (33%).
The category « below normal »
corresponds to the third of the observations whose seasonal cumulative total
amount of precipitations were weakest (33%).
The category « near normal »
corresponds to the group of the remaining years (33%).
The
forecast domain was divided into two zones after re-examination of the forecast
information.
Zone
I:
Zone to the west of Sahel including the south of Mauritania, the south-west of
Mali, the Senegal, the
Gambia and Guinea Bissau.
Zone II: Limited to the north by latitude 17°N, to the south
by 8°N, to the north-west by zone I, to the south-west by
the Guinea Conakry coasts of Atlantic ocean and to the east by longitude
11°E and constituted by the countries of the Gulf of Guinea, that is from the
Southern half of Guinea along 8°N up to southern Nigeria and the South-west of
Cameroon.
Zone
III : this zone constitutes the eastern part of Niger, the north-east of
Nigeria, Chad and the north of Cameroon.
Zone VI : It’s situated to the south of zone II and from the southern half of Sierra Leone through latitude 8°N to the East of Nigeria cutting through south-west of Cameroon.
D. SEASONAL RAINFALL FORECAST FOR THE PERIOPD
JULY-AUGUST SEPTEMBER 2003 OVER WEST AFRICA CHAD AND CAMEROUN (see
chart)
Following
consideration of consensus forecasts of those done at national level and those
issued by the global Climatic Centres, the chart here below was established to
illustrate the seasonal forecast of rains on the sub-region. The second update
will be done at the National Services during the next coming weeks, during the
forum in Banjul at he beginning of August.
For the Zone I, there are slightly dominant probabilities for « above
normal with tendency to a near normal » conditions.
For the Zone II, « near normal to a above normal » conditions
are the most likely.
For the zone III, « below
normal to a near normal » conditions are the most likely.
For the zone IV, « below normal to a near normal » conditions
are the most likely
Although
we mentioned above the categories of stronger probability this year, the users
are reminded that by planning the coming season, there is possibilities of
observing precipitations in the other categories.
Remarks:
Following the capacity building of previous workshops, each National
Meteorological Service participant has a specialist trained for the
interpretation of these seasonal forecasts. The users are advised to contact
their national meteorological service for any help in interpretation, additional
necessary information and the update of the forecast as soon as new information
becomes available.
Legend of the map:
For
each zone defined by regional forecasting experts, the numbers within the 3
cases represent the probability that the seasonal rainfalls are in the category
" above normal " (upper box), " near normal "
(middle box) and " below normal " (lower box). Hence in zone I,
there is a 45% of chance that the seasonal cumulative rainfalls will be in the
category " above normal ", 35% of chance that it will be in the
category " near normal ", and 20% of chance that it will fall in the
category " below normal " (cf section C for the definition of
categories). The boundaries of the map should be considered as areas of
transition for the forecast.
HYDROLOGICAL
SEASONAL FORECAST
Catchment
of the Gambia river
For
the hydrologic year 2003/2004, normal flows to slightly higher than the normal
with probabilities of 40% for normal and 35% for above normal conditions are
predicted.
Catchment
of the Senegal river
It
is estimated that within this catchment for the hydrologic year 2003/2004, flows
will be normal to slightly lower than the normal with probabilities of 40% for
normal and 35% for lower than the normal conditions.
Catchment
of the Niger river
For
the upper part of the catchment to the entry of the interior delta in Mopti, for
the hydrologic year 2003/2004, normal flows to above normal with probabilities
of 40% for normal and 35% for above normal conditions are predicted.
In
the part Niger average until in Malanville, normal flows for a probability of
40% and probabilities of 30% for flows above and below the normal conditions are
expected.
Catchment
of the lake Chad
For
the whole rivers in Chari-logone, it is predicted for the hydrologic year
2003/2004 normal to above normal flows with probabilities of 40% above normal
and 35% normal conditions.
In
the same way as regards the lake Chad, it is envisaged a maximum
level of normal filling to superior with the normal with respective
probabilities of 35% and 40%.
Catchment
of Volta
Over the high catchment of Volta, it is expected during the period of high waters going from August to October 2003 a normal to below normal flows 40% of probability for normal and below normal condition.
Seasonal
forecast of the flows of the principal rivers in
West Africa for the
hydrological season 2003/2004
Caption chart
For
each zone, the numbers appearing in the 3 boxes represent the
probability that the monthly hydraulicity of high waters is in the
category " strong hydraulicity " (limps higher), "
hydraulicity close to the normal
" (limps of the medium), and " low hydraulicity " (limps
lower). For example for the
catchment of the lake Chad, there is 40% of
chance that the monthly flows of high waters are higher than the
normal, 35% of chance that these flows are close to the normal and 25%
to chance that the monthly flows of high waters are lower than the
normal (cf section D for the definition of the 3 categories).
Definition
of the categories
the hydrological normal is defined here as being the average of the flows of the historical series available to the level of the station.
strong
the hydraulicity category corresponds to the third of the years whose flows have
tee most raised (33%)
the
weak hydraulicity category corresponds to the third of the years whose flows were
lowest (33%)
normal
the hydraulicity category corresponds to the group of the remaining years (33%)
FORUM OF CLIMATIC SEASONAL FORECAST, FOOD SAFETY AND MANAGEMENT OF WATER RESOURCES
CENTRAL AFRICA (PRESA-AC/02): Libreville, GABON from the 13 to October 18, 2003
PRESA-AC/01 Archive
