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IGAD CLIMATE PREDICTION AND APPLICATIONS CENTRE (ICPAC)
CLIMATE WATCH FOR THE PERIOD MARCH-MAY 2011
(REF: ICPAC/CW/NO.24, MAY 2011)
SUMMARY
This climate watch provides a review of the observed climate conditions during March to May
2011 period and the associated impacts over the Greater Horn of Africa. It further gives the
climate outlook for the month of June 2011. Severe drought has persisted over most parts of the
eastern sector of equatorial GHA since the last quarter of 2010 with far reaching socio-
economic implications that include lack of water, pasture, energy and food; famine; loss of
livestock, life and property; mass migration and environmental refugees, among others. The
persistence of drought over some of these areas has been associated with La Niña conditions.
According to the World Meteorological Organization (WMO) May El Niño/La Niña Update, La
Niña conditions have continued across the tropical Pacific through early May 2011, and near-
neutral conditions are currently being considered as the most likely outcome for the second half
of 2011.
La Niña events are often associated with above average summer rainfall over most parts of the
northern sectors of the GHA, and parts of the western equatorial areas. Drought conditions are
however expected to continue over equatorial areas that have been under drought for several
months. Details of the climate outlook for June-September 2011 based on the on-going La Niña
evaluations and the observed regional climate circulation anomalies will be provided by ICPAC
and partners at the Twenty eight Regional Climate outlook forum that will be held in Nairobi
Kenya between 16-17 June 2011 after a regional workshop supported by UK Met office, WMO
among many other partners. July-August Months are also the coldest months over many parts
of the equatorial GHA region.
1. INTRODUCTION
Most of the Greater Horn of Africa (GHA) may be classified to have Arid and semi-arid climate
that is characterized by high variability in rainfall, and recurrences of extreme climate events
such as drought and floods. Some of these climate extremes have been associated with El Niño
/ La Niña events, modulated by regional climate systems and circulation anomalies over the
Indian and Atlantic Oceans sub regions. According to the World Meteorological Organization
(WMO) May El Niño/La Niña Update, La Niña conditions have continued across the tropical
Pacific through early May 2011. Although the central Pacific sea surface temperatures currently
reflect only weak La Niña conditions, the atmospheric indicators continue at moderately strong
La Niña levels, suggesting a likelihood that some climatic aspects of La Niña will continue into
June, and the risk of near-neutral conditions are currently considered the most likely outcome
for the second half of 2011.
As a WMO Regional Climate Centre (RCC) for the GHA, ICPAC provides regular GHA 10 days;
monthly and seasonal climate updates as well as timely information on major regional climate
stress and associated impacts. This climate watch bulletin updates are based on the recent La
For more information contact 1
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Niña updates by WMO, together with other regional climate anomalies, including Indian Ocean
Dipole circulation and circulation anomalies over the African continent and Atlantic Ocean. The
regional climate information included here are cumulative climate impacts of up to the end of
May 2011; analogue year based on past observed rainfall record as well as the expected
climate conditions up to August 2011.
Severe drought has persisted over most parts of the eastern sector of equatorial GHA since the
last quarter of 2010 with far reaching socio-economic implications that include lack of water,
pasture, energy and food; famine; loss of livestock, life and property; mass migration and
environmental refugees, among others. This was well predicted by the Twenty seventh
consensus Regional Climate outlook forum (RCOF 27) as shown in figure 1a. The amount of
rainfall received so far up to end of April 2011 is shown in figure 1b. During June-August most of
the rains in GHA are concentrated north of the equator; over parts of the western and Coastal
sectors. Classical La Niña conditions have been associated with enhanced rainfall over these
areas. There are however significant variations from one La Niña event to another. Details of
the climate outlook for June-September 2011 based on the ongoing La Niña evaluations will be
provided by ICPAC and partners at the Twenty eighth Regional Climate outlook forum that will
be held in Nairobi Kenya between 16 and17 June 2011 at the end of a regional workshop
supported by UK Met office, WMO among many other partners. The amount of observed rainfall
for 1999 that is close to the year 2011, however, is shown in figure 8.
(a) Consensus Outlook - March to May 2011 (b) Observed March-April 2011 rainfall
Figure 1: Greater Horn of Africa Consensus Climate Outlook for the March to May 2011
(GHACOF27 product of ICPAC RCC, 28 February 2011)
2. Recent La Niña and Global Sea Surface Temperaturas
The term La Niña is used to describe the periodic building up of unusually cold waters in the eastern and
central equatorial Pacific Ocean (see Figure 2a). On the other hand, El Niño, (Spanish word for "the
Christ-child"), refers to the periodic building up of a large pool of unusually warm waters in the same
ocean basin (see Figure 2b). Thus, La Niña and El Niño are mutual exclusive phenomena that
periodically occur in the equatorial Pacific Ocean region. In general, most of El Niño events are
For more information contact 2
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
immediately followed by La Niña events that have often resulted into floods following droughts or vice
versa in many parts of the world.
When large pools of warm/cold waters are observed in any parts of the global oceans, the atmosphere
and the neighbouring oceans respond to cooling and warming in various ways. The atmosphere, for
example, may respond to strong El Niño and La Niña events by shifting the east-west air circulation cells
commonly referred to as the Walker cell (Figure 2). The close linkages between La Niña / El Niño events
and the atmospheric circulation response have made many scientists to often refer to the two systems
simply as El Niño / Southern Oscillation (ENSO). In such cases, El Niño and La Niña phenomena are
simply referred to as the warm and cold ENSO phases respectively. The warm and cold ENSO phases
(El Niño / La Niña events) are known to trigger worldwide anomalies in ocean currents and atmospheric
air circulation that consequently have various impacts on rainfall and temperature in specific areas
around the world as summarized in Figure 3.
It has been observed that during El Niño and La Niña events, world-wide weather and climate extremes
such as droughts, floods, cold/hot spells, tropical cyclones, etc are common, even in some regions that
are very far away from the Pacific Ocean basin. Such weather and climate extremes are often
associated with far reaching socio-economic impacts including loss of life and property; mass migration
of people and animals; lack of water, energy, food and other basic needs of human kind.
(a) La Niña (b) El Niño
Figure 2: Typical Sea Surface Temperature anomalies and zonal Walker circulation over the
eastern and central equatorial Pacific Ocean during (a) cold and (b) warm ENSO events
(courtesy of Australian Bureau of Meteorology).
Table 1: Onset of past La Niña events during January- May season based on the available
records
Past La Niña Events centered on January- May months
1963; 1965;1971; 1974; 1989; 1999, 2008
For more information contact 3
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Figure 3: Regional Impacts of La Nina event (Source: National Oceanic and Atmospheric Administration)
Climate scientists are currently able to use mathematical models to extrapolate, with some reasonable
skill, some aspects of La Niña expectations with lead times of several months to over one year in
advance. Evolution indicators of the current La Niña and global sea surface temperatures show that the
ENSO status up to April 2011 is similar to 1963, 1965, 1971, 1974, 1989 and 1999 but very close to
1999 patterns than other years. Figure 4 shows comparison of current versus similar past conditions of
the global Sea Surface Temperatures and observed rainfall during 1999.
For more information contact 4
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Figure 4: Current La Nina condition and global SST anomalies most similar to April 2011.
(a) Observed May 1999 (b) Observed March – May 1999 season
Figure 5: Amount of rainfall observed in 1999 which is analogue year Current La Nina conditions.
Attempts are also being made in a number of regions with strong La Niña signals to develop seasonal
climate prediction and regional downscaling of weather and climate expectations using the projected La
For more information contact 5
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Niña signals and associated sea surface temperature anomalies. Parts of GHA have strong ENSO
signals during some seasons, as presented in the next section.
3. Typical World-wide Consequences of El-Niño and La Niña phenomena
ENSO events are now known to have severe global climatic implications, especially in the tropics. During
a strong El Niño / La Niña event, it has been observed that there are some displacements of the warm
areas of the tropical oceans and circulation cells with corresponding shift in convergence/divergence
patterns of the air. The observed impacts have large degree of variability both in time and space.
Variations in the impacts of El Niño / La Niña including the onset, peak and withdrawal periods have also
been observed during the various evolution phases (Figure 4a).
3.1 Typical impacts of El-Niño and La Niña on the climate of the Greater Horn of Africa
(GHA)
In general, it has been observed that the occurrence of strong El Niño (warm episode) and La Niña (cold
episode) events have devastating impacts in the GHA sub-region. There are however large space-time
variations in the observed impacts due to the complex nature of the regional topography and the
existence of large water bodies, which play significant roles in modifying the general circulation, and
regional El Niño and La Niña–related anomalies in the sub-region. Thus, El Niño and La Niña impacts
not only vary from location to location and from season to season, but also from one El Niño / La Niña
event to another, in terms of the intensity, onset, peak and withdrawal of the events.
In the GHA, rainfall is the most important climate element due to heavy socio-economic dependence on
it. It has high degree of variability both in space and time and very strong seasonality. In terms of rainfall
regimes, the GHA sub-region can be divided into three sectors namely the northern sector (Sudan, Djibouti,
Ethiopia, Eritrea and northern Somalia), equatorial sector (Kenya, Uganda, Burundi, Rwanda, southern
Somalia and northern Tanzania) and Southern sector (central and southern Tanzania).
The northern sector of the GHA sub-region has its peak rainfall concentrated largely during the northern
summer months of June to September. The southern sector has its peak rainfall concentrated mainly during
the southern hemisphere summer months of December to February. The rainfall patterns over the
equatorial parts of the GHA are quite complex. Close to the large water bodies, substantial rainfall is
received throughout the year. However, much of the equatorial sector of the GHA has two major
seasonal rainfall peaks during March-May and October-December. The western and coastal parts also
receive significant rainfall during the months of July-August.
It has been mentioned in the previous paragraphs that the rainfall anomalies that are associated with
warm and cold episodes in the GHA vary significantly from season to season and from location to
location. These anomalies are highlighted below for the specific sectors of the sub-region.
3.1.1 Northern and Southern sectors
The onsets of strong El Niño/La Niña events are often associated with below/above average summer
rainfall over most parts of the southern and northern sectors of the GHA. Thus enhanced June-September
2011 rainfall would be expected over several parts of the northern GHA sector, if classical La Niña
conditions and associate regional circulations are dominant over the region. Details of the climate
outlook for June-September 2011 based on the on-going La Niña evaluations and the observed
regional climate circulation anomalies will be provided by ICPAC and partners at the Twenty eight
Regional Climate outlook forum that will be held in Nairobi Kenya between 16-17 June 2011 after a
regional workshop supported by UK Met office, WMO among many other partners
3.1.2 Equatorial sector
For more information contact 6
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
The cold ENSO episodes (La Niña) are largely associated with suppressed October to December rainfall
especially over much of the eastern parts of the equatorial sector. The October – December droughts
often persists into the March–May of the following year rainfall season, as has been observed in 2010.-
2011 over many eastern parts of the GHA.
3.2 . Are El Niño and La Niña events the only causes of climate extremes such as
floods and droughts in GHA?
It should however be noted that no two El-Niño or La Niña events are exactly alike, thus their impacts on
climate patterns will also be dissimilar. In addition, extremes in climate over the region are also
influenced by monsoon systems, the Inter Tropical convergence zone (ITCZ), tropical cyclones, Madden
Julian Oscillation (MJO), Indian Ocean Dipole (IOD), among other regional and global systems. The
major moisture sources for rainfall generation over the GHA are the Mediterranean Sea, the Indian and
Atlantic Oceans. Large SST anomalies in these regions have significant modifications in the regional
patterns of moisture and energy transports into GHA. The overall impacts associated with the El Niño
and La Niña events in the GHA often depend on how individual events interact with the regional
systems.
3.3 One key question is how the available El Niño/La Niña information can be used in the
monitoring, prediction and early warning of climate extremes?
It was noted in the previous sections that extreme climate anomalies tend to occur during El Niño and La
Niña years. WMO and partner global climate centers including ICPAC have released some early warning
information on the evolving La Niña event. Such information and updates from the climate community
provides early warning and climate risk management planning tools for addressing some disaster
management challenges in regions like the GHA where the impacts of climate extremes are often
devastating.
4. Observed rainfall conditions during March-April 2011
Figure 5 shows the percentage of the March-April-May (MAM) long-term mean rainfall that was received
during the March-April 2011. The northern, eastern and western parts of the northern sector of the GHA
have received less than 25% of the MAM long-term mean rainfall by the end of April 2011. The central
and southern parts of the northern sector received between 25% and 75% of the MAM long-term mean.
This extended into the much of the western and central parts of the equatorial sector of the GHA and
northern coast of Tanzania. The eastern parts of the equatorial sector recorded less than 25% of the
MAM long-term mean during the March-April 2011 period. The southwestern parts of equatorial sector
extending to most parts of the southern sector received between 75% and 125% of the MAM long-term
mean. None of the locations within the GHA had received more than 125% of the MAM long-term mean
by the end of April 2011. Figure 6 provides more detailed information of extended climate anomalies
over the region through March – April 2011.
For more information contact 7
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
(a) Observed March-April 2011 rainfall (b) Observed March-April 1999 rainfall
Figure 5: Comparison of the observed March-April rainfall so far received and the analogue year over the
Greater Horn of Africa
Figure 6a: Comparison of the rainfall received during March and April 2011 and the long-term
mean for the March-April-May seasonal rainfall over the northern sector of the GHA
For more information contact 8
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Figure 6b: Comparison of the rainfall received during March and April 2011 and the long-term
mean for the March-April-May seasonal rainfall over the equatorial sector of the GHA
Figure 6c: Comparison of the rainfall received during March and April 2011 and the long-term
mean for the March-April-May seasonal rainfall over the southern sector of the GHA
8. Climate outlook for May – August 2011
Enhanced June-September 2011 rainfall would be expected over several parts of the northern GHA
sector, if classical La Niña conditions and associate regional circulations are observed over the region.
For more information contact 9
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
Details of the climate outlook for June-September 2011 based on the on-going La Niña evaluations and
the observed regional climate circulation anomalies will be provided by ICPAC and partners at the
Twenty eight Regional Climate outlook forum that will be held in Nairobi Kenya between 16-17 June
2011 after a regional workshop supported by UK Met office, WMO among many other partners. Figure 7
however provides an attempt to prove down scaled regional May 2011 rainfall based products from
GPCs and simple SST driven empirical models. Figure 7 shows that during May 2011, enhanced
rainfall being concentrated to the Northern and western areas of GHA. Drought conditions are dominant
over most of the areas south of the equator and to the east of the equatorial sectors.
Figure 7: Updated regional forecast of GHA rainfall for May - June 2011
Figure 8: Observed June-September 1999 rainfall
For more information contact 10
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
ANNEX I: WMO El Niño/La Niña Update May 2011
http://www.wmo.int/
For more information contact 11
Director, ICPAC P.O. Box 10304, 00100 Nairobi, KENYA; Tel: 254-020-3514426; Fax: 254-020-3878343
E-mail: director@icpac.net; Website: www.icpac.net
