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ELNINO- LANINA updates( create an update page) What is El-Niño and La Niña The term El Niño, (Spanish word for "the Christ-child"), refers to the periodic building up of a large pool of unusually warm waters in large parts of the eastern and central equatorial Pacific Ocean (see figure 1). On the other hand, the opposite, which is known as La Niña, is used to describe the periodic building up of unusually cold waters in large parts of the same ocean basin. Thus, El Niño and La Niña are phenomena that periodically occur in the equatorial eastern Pacific Ocean region. The atmosphere and the neighbouring oceans respond to warming and cooling of the equatorial eastern Pacific Ocean region (El Niño and La Niña events) 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 that are commonly referred to by the climate scientists as the Southern Oscillation (SO), as indicated in Figure 2. The close linkages between El Niño / La Niña events and the closely matched atmospheric circulation response (SO) have made many scientists to often refer to the two systems simply as El Niño / Southern Oscillation (ENSO) (WMO 1984). In such cases, El Niño and La Niña phenomena are simply referred to as the warm and cold ENSO phases respectively. The warming / cooling of the eastern and central equatorial Pacific Ocean during El Niño / La Niña events is also known to trigger world-wide anomalies in sea surface temperatures (SST) and the circulation of the ocean currents. It has been observed that during periods of El-Niño and La Niña phenomena, world-wide extremes in weather and climate 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 society and animals; lack of water, energy, food and other basic needs of human kind. Known
El-Niño events based on available records are given in the box
below. Local South American mariners and fishermen however have documentary
evidences of El Niño phenom-enon dating back several centuries.
El-Niño events recur with a period between 2-7 years, and some of the El-Niño events are usually followed by La Niña. Climate scientists are currently able to use mathematical models to extrapolate, with some reasonable skill, some aspects of El-Niño and La Niña expectations with lead times of months to over one year in advance. Attempts are now being made in a number of regions of the tropics to develop seasonal climate prediction and regional downscaling of weather and climate expectations using the projected El-Niño and La Niña signals and associated sea surface temperature anomalies. It is therefore not surprising that El-Niño and La Niña phenomena have received a lot of attention at global, regional and local levels over the last few years. 3. Typical Word-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 and
cold air masses together with the patterns of air convergence / divergence
(Figure 2-3). Such displace-ments cause anomalous and enhanced convective
activities over the central eastern Pacific, central western equatorial
Indian Ocean, along the coast of eastern Africa and off/near the Atlantic
equatorial coast of Africa, north-western South America, northern part
of the Greater Horn of Africa (GHA) sub-region among many other parts
of the tropics during a strong El Niño event. As a result, abun-dant
rainfall tends to occur in these areas. 4. Typical impacts of El-Niño and La Niña on the climate of the Greater Horn of Africa (GHA) One of the major questions that are being asked by many people world wide is why anomalies in the sea surface temperature over eastern and central equatorial Pacific Ocean should trigger weather and climate anomalies in many parts of the world, especially within the tropics, including GHA. This issue was partially addressed in section 1 by highlighting the impacts of El Niño and La Niña events on atmospheric and ocean circulations. For the GHA, a simplified system that could provide some linkage framework with the Pacific Ocean is evident in Figures 1 and 2. These include time lagged atmospheric and ocean responses to the Pacific Ocean warming or cooling. 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 the large in land water bodies. The regional physical features and the neighbouring oceans play significant roles in modifying the general circulation, and thus also modify 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 phase to another, including the intensity, onset, peak and withdrawal of the events. In GHA, rainfall is the most important climate element because of the heavy socio-economic dependence of many activities on rainfall. It is equally important due to its high degree of variability both in space and time and very strong seasonality. On the basis of the rainfall regimes observed in the GHA, the 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 (southern Tanzania). The northern sector of the GHA sub-region has peak rainfall concentrated largely during the northern summer months of June to September. The southern parts of the sub region have peak rainfall concentrated mainly within the southern 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. Over much of the sector, however, the major rainfall periods are concentrated within two peak seasons of March-May and October-December. Parts of the western and coastal regions 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. 4.1 Northern sector Strong El-Niño events are often, associated with suppressed (below normal) rainfall over most parts of the northern sector of the GHA region during the peak rainfall season of July to September. On the other hand, cold events (La Niña) tend to enhance (above normal) rainfall over most parts of the sector during the same period. 4.2 Equatorial sector Over the equatorial sector, enhanced rainfall is common during October to December of the year of onset of an El Niño. The significant rainfall in October to December tends to extend into the usually dry months of January and February of the following year. On the other hand, rainfall tends to be suppressed during the June to September period in the western parts of the sector. It should be noted that suppressed instead of enhanced rainfall dominated the sub region during the onset of El-Niño in some years such as 1986, 1991, and 1994 when the Sea surface temperature (SST) anomalies in the Indian and Atlantic oceans were observed to be significantly different from the known El Niño related patterns. On the other hand, cold ENSO episodes, (La Niña), are largely associated with suppressed rainfall especially in September to December period over much of the equatorial sector as was evident by the prolonged drought of 1999- 2001 in some parts of the subregion. 4.3 Southern sector Strong El-Niño and La Niña events are often associated with below/above average rainfall amounts over most parts of the southern sector of the GHA. It should however be noted that not all El-Niño or La Niña events give the same impacts in the GHA. In other words, no two events are exactly alike, thus their impacts on climate patterns will also be dissimilar. In addition, the overall influence on the rainfall patterns is dictated by a number of other factors such as the time of onset, month of peak intensity, seasonal persistence and withdrawal phases as well as strength of the event and the characteristics of the surrounding ocean basins. The Mediterranean Sea, the Indian and Atlantic Ocean basins are the major moisture sources for rainfall generation in GHA. The overall impacts associated with the El Niño and La Niña events in GHA often depend on how these large surrounding water bodies and the general atmospheric circulation respond to individual events. Within
GHA region the devastating impacts of the 1997/98 El Niño related
floods in the equatorial parts of the sub-region are still very fresh
in the memories of all residents. The effects of the 1997/98 El Niño
associated floods, were immediately followed by those of the 1999-2001
La Niña related drought. The 1997/98 El Niño related floods
led to loss of life and property, destruction of infrastructure and large
losses to the economy. The El Niño floods were immediately followed
by one of the longest and severest droughts in the history of the region.
The 1999-2001 drought had harsh negative impacts on agriculture, livestock,
wildlife, Tourism, Water resources and hydroelectric power generation.
The low water levels in the dams led to severe power rationing in some
countries, which resulted into large losses in their economies. Water
supply for industrial and domestic consumption was not spared by the drought.
There were serious water shortages both in urban and the rural areas.
Lack of water and pasture led to severe conflicts between wildlife and
pastoral communities. 5. Are El Niño and La Niña events the only causes of climate extremes such as floods and droughts in GHA? Climate anomalies do occur all over the world each year. Many of them have very little to do with El Niño and La Niña. However, during some years and in some specific regions and seasons some consistent extreme climate anomalies tend to occur during some specific El Niño and La Niña phases. Thus, El-Niño and La Niña are not the only causes of climate extremes in the GHA. In general, weather and climate extremes are caused by regional/ large-scale anomalies in the atmospheric wind motions and the global ocean currents that are collectively referred to as the General Circulation by the climate scientists. El Niño and La Niña are therefore just two of the many systems that can induce such anomalies. El Niño and La Niña phenomena have however been gaining more prominence in the recent years due to the recent ability of the climate scientists to provide some skilful lead time projections, sometimes extending from seasonal to inter annual time scales. For example, the major climate centres have been indicating since last year that there was some high potential for El Niño to occur this year. The only details that such models could not provide accurately were the time of onset of the current El Niño expected intensity and details of the expected seasonal characteristics. One key question is how the available El Niño / La Niña information could be used in the monitoring, prediction and early warning of climate extremes? It was noted in the previous sections that during some years and seasons some consistent extreme climate anomalies tend to occur during some specific El Niño and La Niña phases in GHA. Details of some of these are highlighted in Section 3. DMCN, in collaboration with some partner global climate centres used the projected El Niño evolution and the associated global SSTs to provide the September - December climate outlook for GHA in August. Such information now provides new planning tools in addressing some disaster management challenges and new applications of climate information for sustainable development. 6. Status of the evolving El-Niño At the moment, a weak to moderate El-Niño event has developed in the Pacific Ocean. Forecast models have indicated that this event will persist throughout November to December 2002 period that are the peak rainfall months in the equatorial parts of the GHA. There is however some time lags before the impacts of the weak El Niño can spread over most of GHA. It is anticipated that any major impacts of the evolving El Niño may only be discernible in the GHA towards the end year and/or early months of next year. In other words, the overall climate patterns that are likely to be observed in the GHA during the November to December period will be driven largely by the unique evolutions of the Sea Surface Temperature (SST anomalies over the surrounding ocean basins of the Indian and Atlantic Oceans, together with other factors such as changes in the wind patterns of the upper levels of the atmosphere in the GHA and the environs. Some significant SST anomalies are currently evident over the Indian and Atlantic Oceans that could play a significant role in driving the space-time variability of rainfall in the GHA during the month of November. These anomalies include significant warmer than average SSTs over the northwestern Indian Ocean (around the Red Sea area) that have been spreading southwards and have also merged with the positive anomalies over the central Indian Ocean. In addition, the SSTs over the Atlantic Ocean have also been generally warmer than average (Figure 7). By the end of the month of October 2002, the Indian and Atlantic oceans also seemed to have some unique SST anomalies that favour the 1986, 1991, 1994 and 1996 type of scenarios. Unless drastic changes occur in the SST anomaly patterns over the two GHA neighbouring oceans during the month of November, traditional SST anomaly patterns that are common in the two ocean basins during El Niño may still be insignificant during the month. The DMCN has carried out a variety of analyses to update the outlook that was developed during the tenth climate outlook forum for the Greater Horn of Africa at the end of August 2002. The results have not indicated any significant deviations from the outlook that was developed at the forum. The update climate outlook (refer to cover page), for the remainder of the season however indicate some enhancement of rainfall activities over southern Sudan, extreme southwestern Ethiopia, western and coastal parts of Kenya, Uganda, Rwanda, Burundi, much of eastern Somalia as well as northwestern and northeastern Tanzania especially during November as depicted in figure 6 below. This was based on conditional probabilities generated by DMCN using the October rainfall scenario. It should be noted that sporadic extreme rainfall events are common during the peak rainfall months even during drought years. Such events often give rise to heavy rainfall episodic events concentrated within short time scales. DMCN provides regular 10 day, monthly and seasonal climate up dates as well as the evolution status of the current El Niño. A synthesis publication known as the DMCN Climate Watch is also released every month by DMCN to provide updated information regarding the status of the evolving El Niño event and its potential impacts in the GHA. This publication is the GHA Climate Watch for the month of November 2002. DMCN further provides regional products in collaboration with the NMHSs in the GHA. National details and downscaled national climate information for specific sector users are provided regularly by the NMHSs. All users are therefore encouraged to contact the NMHSs for detailed national information, products and updates. 7. Observed Climate Conditions in GHA during October 2002 Figure 4 depicts the major rainfall anomalies that were experienced in the sub region during October 2002. Dry conditions persisted over some parts of central and southwestern Ethiopia, eastern Sudan and western Kenya with some locations experiencing driest 10 days conditions on record since 1961. The dry conditions have persisted in some of these areas since 1999 (see, for example, figure 5), with devastating socio-economic implication. Dry conditions were also dominant over western Uganda and extreme western Sudan in September. During the month of October wet conditions, in some cases, accompanied with lightning and strong winds were mostly observed over localised areas of the sub region including Djibouti, Rwanda, Sudan, Ethiopia and Burundi; western, central and some coastal parts of Kenya ; and some coastal parts of Tanzania. In most cases, the wet conditions were mainly associated with short-lived heavy episodic rainfall events lasting a day or two within specific ten-day periods as highlighted in figure 4.
Coghlan, C. (2002): El-Niño - causes, consequences and solutions. Weather, 57 pp 209-215 Ogallo, L.J. (1988): Relationship between seasonal rainfall in East Africa and the Southern Oscillation Index. Journal of Climatology, 8 pp 31-43 Ropelewski, C.F. and M.S. Halpert (1987): Global and Regional Precepitation patterns associated with the El-Niño / Southern Oscillation. Monthly Weather Review, 115 pp 1606-1626 WMO (1984): The Global Climate System: A Critical Review of the Climate System During 1982 - 1984. World Meteorological Organization
Figure
3: Sketch diagram showing Teleconnections between East-West through
atmospheric Circulation over the tropics during a strong El Niņo event |
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