By Dr. Daniel Werede (Advisory team) - E-Green Life-Addis Ababa
The term El Niño is Spanish for “the boy”/ “Christ Child”. This is because the El Niño episode usually happens around the Christmas season. It was during this time of the year that fishermen first discovered how the ocean currents would become much warmer near the coasts of Peru and Ecuador.
We now use the term El Niño to refer to the large scale warming of sea surface temperatures that occur in the Pacific Ocean every three to six years. On average, an El-Nino would last for nine to twelve months, but may extend for as long as 18 months. These El Niño episodes can greatly affect the weather system across the globe.
what causes El Niño?
El Niño results from interaction between the surface layers of the ocean and the overlying atmosphere in tropical Pacific. It is the internal dynamics of the coupled ocean-atmosphere system that determine the onset and termination of El Niño events. The physical processes are complicated, but they involve unstable air-sea interaction and planetary scale oceanic waves. The system oscillates between warm (El Niño) to neutral (or cold- La Nina) conditions with a natural periodicity of roughly 3-4 years.
However, the El Nino phenomenon does not occur on a regular basis and as mentioned above, it has a dramatic impact on the global weather system. Hence, it is important to understand the causes of EL Niño and accurately predict its appearance. As a result, numerous climatic models, forecasting models and statistical models have been designed to try to predict its causes.
NASA Climate model forecast shows 2015
Regional Agro-ecological impacts of El Niño
El Niño impacts are typically quite severe over Southeast Asia, including Malaysia, Indonesia, Papua New Guinea, Borneo and northern Australia. Throughout much of that region drought is common, agriculture is affected and tropical forest fires become a problem. Although western Pacific typhoons are no more frequent than normal, they tend to affect areas farther to the east, as far as Tahiti, that is otherwise less affected. To the west there is typically a failure of the summer (wet) monsoon over the Indian subcontinent, whose populations depend critically on the monsoon rains. Droughts are also known to occur in southern, eastern and northeastern Africa.
In 1876, the extreme weather caused by El Nino led to the most deadly global famine of the 19th Century. "The combination of extreme droughts and monsoons led to millions of deaths under hellish conditions," wrote historian Cormac O Grada in his 2010 book Famine: A Short History.
However, the presence of an El Niño doesn’t always guarantee or represent a disaster but it will make disasters more likely to happen. For example, in the case of Ethiopia, the FAO Agricultural Stress Index System (ASIS) assesses crop-growing conditions in El Niño episodes from 1986 to 2013 and in particular, detects agricultural areas with a high likelihood of water stress or drought. During the period for which ASIS data is available, a total of nine El Niño events occurred, with varying levels of intensity. In Ethiopia, four out of the nine El Niño – 1987, 1991, 2002 and 2009 – coincided with intense and/or extended drought conditions from April to November, which encompassed the main Meher cropping season. However, the relationship between El Niño intensity and impacts is clearly not linear or straightforward – five out of the nine El Niño were not associated with drought during Meher. Also, the El Niño between May 1997 and April 1998 was reported to be one of the strongest ever recorded, and the most severe and prolonged of the latest 25 years. Although serious impacts were reported in eastern Africa, the same FAO report noted that for Ethiopia there were, “… satisfactory crop growing conditions with only localized mild agricultural stress, and in 1997 total cereal production was similar to the output gathered in the previous year (which was ENSO (El Niño Southern Oscillation) -neutral) and 42 per cent higher than the average of the previous five years”. “(AKLDP (Agriculture Knowledge, Learning Documentation and Policy) project, Ethiopia). Another example is the 1992-93 El Nino that resulted in an end to severe long-term drought conditions that had persisted since 1986/87, and to a much-needed replenishment of water reserves. Similarly, the above-normal rainfall occurrences due to an El Niño episode would indicate reduced chances of wildfires especially in dry area like Florida.
Therefore, when interpreting the climate information linked to El Niño, it is important to note that while abnormal temperature and rainfall patterns can and sometimes do result in severe climate conditions, they do not imply catastrophic conditions in many cases.
El Niño impact on the Ethiopian local weather system
Ethiopian economy and food security of its population is highly dependent on rain fed agriculture. According to the Ethiopian National Metrological Agency, an emerging El Niño episode was associated with the delayed onset of the 2015 main summer kiremt rains, normally falling in June to September.
Rainfall in Ethiopia is influenced by weather systems which evolve thousands of miles away in the Pacific Ocean, especially the central and east central parts of the Pacific around the Equator. One of the main factors driving these systems is the temperature of the Pacific’s surface, because this affects the air pressure above the sea, and related wind and rainfall patterns across large areas of the tropics and sub-tropics. A warmer sea leads to high air pressure, and a cooler sea leads to low air pressure. Relatively large changes in the sea’s temperature create unusual weather patterns, such as drought or excessive rainfall and storms. (AKLDP project Ethiopia)
While forecasting the intensity of an El Niño episode is difficult, in Ethiopia there is general agreement that an El Niño episode has a high probability to cause:
- Above-normal rainfall from October to March in the south and south east areas; typically associated with supporting pastoralist areas due to good pasture but can also cause flooding and outbreaks of human and livestock diseases.
- Above-normal Belg rains with an early onset as early as January or February. At the same time, harvesting the main season cereal crops between October and November may be disrupted due to off-season rains.
- Below-normal kiremt rains, which are also typically late, erratic and shorter than usual. (Read more at: Agriculture Knowledge, Learning Documentation and Policy (AKLDP) Project, Ethiopia (Sep, 2015))
2015-16 El Niño calendar (El Niño implications and scenario for 2015, WFP.org)
·NOAA. 1994. El Niño and Climate Prediction—Reports to the Nation on Our Changing Planet. A publication of the University Corporation for Atmospheric Research pursuant to National Oceanic and Atmospheric Administration Award No. NA27GP0232–01. UCAR, Boulder, Colorado.