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CLIMATE WATCH AFRICA
BULLETIN
N° 04
APRIL 2012
SUMMARY
1. Month’s Synoptic Situation
2. Month’s climatological
Situation / Impacts
3. Outlook
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HIGHLIGHTS FOR APRIL 2012: The Azores high weakened while the St. Helena and Mascarene strengthened
slightly. The northern part of the Sahel, most of the northern Africa countries, and northern part of GHA countries
were generally dry while the rest of the continent had significant amounts of rainfall. Increase in temperature was
observed over most of the countries north of the equator.
1. SYNOPTIC SITUATION DURING THE MONTH OF APRIL 2012
This section provides the strengths of the surface pressure systems; the 850hPa general circulation anomalies;
upper troposphere thermal regimes; relative humidity; sea surface temperature (SST) and El Nino/Southern
Oscillation (ENSO).
1.1 Centres of Surface Pressure Systems
The Figure 1 shows surface pressure systems compared to the past month as described below:
The Azores high of 1026hPa weakened slightly by
1hPa and shifted south-west. Its centre was located
over north Atlantic Ocean at about 38°N/33°W
extending a ridge over the western part of Northern
Africa.
The St. Helena high of 1022hPa strengthened
slightly by 1hPa and shifted south-east. Its centre
was located at about 34°S/00° over south Atlantic
Ocean.
The Saharan Thermal Lows of 1006hPa deepened
by 2hPa. It had two centres located at about
13°N/18°E over southern Chad.
The Mascarene high of 1024hPa strengthened by
3hPa and shifted north-east. The centre was located
at about 35°S/75°E over southern Indian Ocean.
Figure 1 : Mean Surface Pressure during April 2012
(Source: NOAA/NCEP)
1.2 The 850hPa wind anomaly
The Figure 2 shows wind anomalies at 850hPa derived from reference period 1971-2000.
Strong north-westerly/westerly wind anomalies
from north Atlantic ocean were observed over
northern Algeria and Morocco.
Over Democratic Republic of Congo strong
easterly wind anomalies prevailed.
In Eritrea, Djibouti and northern Ethiopia,
Somalia north-western to westerly wind
anomalies were observed.
The average wind anomaly speed (shaded)
was observed at about 03m/s and above.
Figure 2 : April 2012, Wind Anomalies at 850hPa
(Source : IRI/NOAA/NCEP)
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1.3 Thermal index
In the month of April 2012, the Thermal Index
(TI) regime at 300hPa, Figure 3, had an
isotherm value of 242°K covering southern part
of the Gulf of Guinea, most of the Central
Africa, of GHA and northern part of southern
Africa countries. The high TI (≥242°K) triggered
heavy rainfall with floods over the areas
characterized by high relative humidity (>60%)
as shown in Figure 4 while the low TI regime
≤241°K
values
were
associated
with
suppressed convection over the rest of Africa.
Figure 3: Thermal Regime at 300hpa
(Source: NOAA/NCEP)
1.4 Relative Humidity at 850hPa
The 850hPa (Figure 4) shows high RH (>60%)
in April 2012, over extreme southern part of the
Gulf of Guinea countries, most part of the
Central Africa, most part of GHA countries, and
eastern part of southern Africa and
Madagascar. However, most of the northern
part of the continent located north of 10°N and
the western part of southern Africa experienced
dry conditions characterized by the lowest RH
(≤ 40%).
Figure 4:RH at 850 hPa (Source : NOAA/NCEP)
1.5 Sea Surface Temperature (SST) and El Nino/Southern Oscillation (ENSO)
Warming conditions persisted in most of equatorial eastern and western parts of the Pacific Ocean while in
the north-eastern, north-western and south-eastern parts, cooling conditions continued. Neutral to warming
conditions continued over most of northern and southern parts of the Atlantic Ocean while in most of the
central part cooling conditions continued with high development over coastal zones of West Africa. Neutral
to warming conditions persisted in most of Indian Ocean and Mozambique Channel.
Figure 5: Sea Surface Temperature (Source IRI).
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2. CLIMATOLOGICAL SITUATION AND IMPACTS DURING APRIL
The section provides the general climatological situation covering two major parameters, the rainfall and
temperature.
2.1 Rainfall
The estimated rainfall for April, 2012 in Figure 6, shows some increase in rainfall pattern over the Sahel,
the Gulf of Guinea and GHA countries. The remaining part of the continent had no significant change. In
detail:
North Africa: Most parts remained generally dry. However, some localized slight increase of rainfall
ranging from 10mm to 80mm were observed over northern parts increasing from 80mm to 150mm with
maximum reaching 800mm over north-western Morocco.
The Sahel: northern part remained dry under the influence of the Harmattan characterised by cool, dry
and dusty conditions with localized dust episodes observed over the region. The southern part had
rainfall amounts between 10mm to 200mm over Burkina Faso, southern Mali and Chad.
Gulf of Guinea countries: had slight increase in rainfall distribution and amounts, ranging from 10mm
to 200mm intensifying to a maximum of 300mm over south-eastern and south-western Nigeria.
Central Africa: had rainfall amounts ranging from 10mm to 250mm over most part increasing to the
maximum ranging from 250 to 400mm over Democratic Republic of Congo, Gabon and Cameroon.
GHA countries: had increase in rainfall distribution and amounts ranging from 10mm to 300mm
intensifying to amounts between 300mm to 500mm over the highlands of Ethiopia, western Kenya and
Tanzania.
Southern Africa had localized light amounts of rainfall. However, the extreme north-western part and
Madagascar had amounts ranging from 10mm to 400mm.
In April, 2012 the rainfall anomalies compared to the reference period 1971-2000, Figure 7 showed
rainfall deficits over most of parts of Central Africa, parts of the Gulf of Guinea countries, extreme
northern part of Southern Africa, extreme southern and eastern part of GHA countries. However,
excessive rainfall was observed over the extreme northern part of northern Africa, Great Lakes countries
and extreme north-eastern part of Southern Africa countries and Madagascar.
Figure 6: Monthly cumulative rainfall
(Data Source: NOAA/NCEP)
Figure 7: Monthly Precipitations Anomalies
(Data Source: NOAA/NCEP)
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2.2 Surface Temperature Anomalies
In April 2012, the temperature anomalies (Figure 8)
compared to 1971-2000 base period, were warmer
(above 1°C) over most of the countries north of the
equator and over northern Madagascar with the
highest temperature anomalies between 2°C and
2.5°C over north-eastern Nigeria, most of Niger,
extreme western Chad, extreme south-eastern Algeria
and central Tunisia. However, low temperature
anomalies less than –1°C were observed over north of
Southern Africa and southern Botswana.
Figure 8 : Monthly Temperatures Anomalies
(Data Source: NOAA/NCEP)
3. OUTLOOK
The subsections provide the expected SSTs and ENSO characteristics and evolution of events based on
Figures 9 and 10 respectively and expected rainfall outlook.
3.1 Forecast Sea Surface Temperature (SST)
The figure 9 shows the forecast SST anomalies from May for the period May-June-July 2012.
Pacific Ocean: cooling conditions will prevail
over southern and north-eastern parts while
over the rest of the basin warming will persist.
Atlantic Ocean: Neutral to warming condition
will persist over the northern and south-central
parts while over most of the equatorial and
extreme south-western parts cooling is
expected.
Indian Ocean and Mozambique Channel:
Neutral to warming conditions are expected to
persist in most of the Ocean and the
Mozambique Channel.
Figure 9 : Forecast Sea Surface Temperatures Anomalies
(source IRI)
3.2 El Ni Niño/La Niña
The set of dynamical and statistical model
forecasts of ENSO over Nino 3.4 domain
(5°N – 5°S, 120°W – 170°W) are shown in
Figure 10.
Most of the dynamical and statistical model
predictions issued during late March and
early April 2012 predict neutral ENSO
conditions for the April to June 2012 season
currently in progress currently in progress.
The probabilities for Neutral conditions (using
-0.45°C and 0.45°C thresholds) are 88%,
71% and 58% for April-May-June, May-JuneJuly and July-August-September 2012,
respectively
Figure 10 : Multi-model ENSO Forecast
(source IRI)
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3.3. Rainfall
The prevailing high relative humidity coupled with high conditional instability manifested by TI regimes at
300hPa will continue to maintain heavy rainfall with highest probability of flooding over the Gulf of Guinea
countries, the northern part of central Africa, southern part of GHA countries and extreme northern part of
Southern Africa countries.
The ITD will continue its northwards migration, strengthening the monsoon winds over the Gulf of Guinea
countries and southern part of the Sahel. Over northern part of the Sahel countries, the influence of the
Harmattan will continue. In detail:
North Africa countries: will have no significant change in rainfall distribution and amounts. However, some
localised light amounts ranging from 10mm to 100mm will be observed over the northern parts.
The Sahel: the northern part will be mostly under the influence of the Harmattan, characterized by cold, dry
and dusty conditions. However, the southern part could have rainfall with amounts between 10mm to
200mm intensifying up to 400mm over south Mali and Chad.
Gulf of Guinea countries: will experience rainfall amounts ranging from 10mm to 250 mm increasing to
about 500mm over the coastal areas.
Central Africa countries: will continue to experience rainfall amounts ranging from 10mm to 200mm,
intensifying to maxima of about 400mm over Gabon and Cameroon.
GHA countries: will have rainfall increase in distribution and amounts ranging from 10mm to 200mm over
most parts, intensifying to about 400mm over Great Lakes countries and highlands of Ethiopia.
Southern Africa countries: will observe some rainfall amounts ranging from 10mm to 100mm over the
northern, eastern and southern parts and eastern part of Madagascar.
3.4. IRI seasonal Rainfall outlook for Africa issued in April 2012 for MJJ
The IRI seasonal rainfall forecast issued in April
for the period of May-June-July 2012 shows:
High probability of below normal to normal
rainfall western part of West Africa.
Elsewhere, the climatology will prevail.
Figure 11: IRI forecast
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3.5 ACMAD seasonal Rainfall outlook for Africa issued in April 2012 for MJJ and JJA
In May-June-July 2012 (figure12 ):
- Slightly below average precipitation is very likely
along the West African coast from Liberia to Benin;
- Above average precipitation is very likely during
MJJ 2012 over southern Nigeria, southern Chad and
northern Cameroon and the highlands of East Africa
from Ethiopia to Tanzania;
Figure 12: ACMAD rainfall outlook for MJJ
In June-July-August 2012 (figure:13) ,
- slightly below average precipitation is very likely
over the west African coast from southern
Senegal to southern Benin;
- above average precipitation is very likely over
southwestern Sudan and the East African
highlands from Ethiopia to Tanzania and around
the lake Chad area in Niger, Nigeria, Chad and
Cameroon
Figure 13 : ACMAD rainfall oulook for JJA
3.6 Temperature and Precipitation Outlook for Northern Africa issued on January 2012 for
FMA
3.6.1
Precipitations
The precipitation probability forecast (see map on figure 14) expected over north Africa during
February-March-April 2012 indicates a slight shift of the chances toward below normal or near normal
rainfall during the season in the western one-third of the region, with a somewhat smaller than
average chance for above normal rainfall. There is a greater than average probability for near normal
rainfall in the central one-third of the region, while in the eastern portion there is no shift in the
likelihood away from the climatologically probabilities of one-third for each of the precipitation
categories.
 Below normal to normal precipitation
most likely over eastern western part
of North Africa covering Morocco,
Algeria and Tunisia.
 Normal
to
above
normal
precipitation most likely over central
part of North Africa including Eastern
part of Tunisia and Libya.
 Normal precipitation most likely over
Egypt.
Figure14: Precipitation probability forecast for northern Africa region for
Feb-Mar-Apr 2012.
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3.6.2 Temperature
With only slight differences within the region, the temperature probability forecast (Fig. 15) is for
approximately equal chances for below, near, or above normal seasonal conditions across the northern
Africa region. The probability for above normal temperature is very slightly reduced in the western onethird of the region.
 Normal to below normal temperatures
most likely over coastal parts of Morocco,
Algeria and Tunisia.
 Normal temperatures most likely over
coastal parts of Libya and Egypt.
 Normal to above normal temperatures
most likely over continental North Africa.
Figure 15: Temperature probability forecast for northern Africa region
for Feb-MarA
pr 2012.
3.7 Greater Horn of Africa Consensus Climate Outlook for March to May 2012 issued by
ICPAC
 Zone I:
Near normal to below normal rainfall is
indicated over southern, eastern and northern
Tanzania, Burundi; Rwanda; Uganda as well as
western and southern Kenya.
 Zone II:
Increased likelihood of normal to above
normal rainfall over much western Tanzania.
 Zone III:
Increased likelihood of near normal to
above normal rainfall over southern coast of Somalia
and northern coast of Kenya.
 Zone IV: Increased likelihood of below to near normal
rainfall over much of eastern and northern Kenya;
southern, eastern and northeastern Ethiopia; Somalia
and Djibouti.
 Zone V: Increased likelihood of normal to above
normal rainfall over South Sudan; southwestern Sudan
and southwestern Ethiopia.
 Zone VI: Near normal to below normal rainfall is
indicated over southeastern Sudan as well as northwestern and central Ethiopia.
 Zone VII: Climatology is indicated over much of
northern Sudan.
Figure 16: Greater Horn of Africa Consensus Climate
Outlook for the March to May 2012
ADVICES:


The high rainfall variability in the region may cause risks with adverse effects throughout the season, particularly on goods
and persons (flooding) on plants (locust invasion) and Public Health (malaria epidemics and other waterborne diseases
such as cholera).
Users of this product are encouraged to contact NMHSs for more detailed advices at country level.
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