Friday, June 27

El Niño Southern Oscillation


ENSO stands for El Niño/ Southern Oscillation. The ENSO cycle refers to the year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño – Spanish for boy child because was first noticed by Peruvian fishermen in December and is identified with the Christ Child - and La Niña represents opposite extremes in the ENSO cycle.
El Nino Phenomena

El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle. La Niña refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. It represents the cold phase of the ENSO cycle.

The fluctuations in ocean temperatures during El Niño and La Niña are accompanied by even larger-scale fluctuations in air pressure between the western and eastern tropical Pacific known as the Southern Oscillation.
El Nino Phases

During El Niño events, the eastward shift of thunderstorm activity from Indonesia into the central Pacific can result in abnormally dry conditions over northern Australia, Indonesia and the Philippines. Drier than normal conditions are also often observed over southeastern Africa and northern Brazil, during the northern winter season. During the northern summer season, Indian monsoon rainfall tends to be less than normal, especially in northwest India where crops are adversely affected. Wetter than normal conditions are observed along the west coast of tropical South America, and at subtropical latitudes of North America (Gulf Coast) and South America (southern Brazil to central Argentina).
El Nino and La Nina Cases
During an El Niño event in winter, mid-latitude low pressure systems tend to be more vigorous than normal in the region of the eastern North Pacific. These systems pump abnormally warm air into western Canada, Alaska and the extreme northern portion of the contiguous United States. Storms also tend to be more vigorous in the Gulf of Mexico and along the southeast coast of the United States resulting in wetter than normal conditions in that region.

ENSO Update 2014

There is a 60% likelihood of an El Niño being fully established between June and August, increasing to 75-80% for the October to December period, according to an El Niño Update issued by the World Meteorological Organization (WMO). Based on advice from National Meteorological and Hydrological Services, many governments have already started preparing for the arrival of El Niño, which is associated with regional-scale drought and flood situations in different parts of the world and has a warming influence on global average surface temperatures. Above graph shows El Niño impacts on temperatures.

El Niño is characterized by unusually warm ocean surface temperatures in the central and eastern tropical Pacific, coupled with typical atmospheric circulation patterns. It is a natural phenomenon with a recurring interval of 2-7 years and has a major impact on the climate around the world. The last El Niño was in 2009/2010.

Tropical Pacific Ocean temperatures have recently warmed to weak El Niño thresholds but atmospheric conditions (such as sea level pressure, cloudiness and trade winds) have remained neutral. This indicates that El Niño has not yet become fully established, as it essentially depends on the interaction between the ocean and the atmosphere. However, atmospheric patterns that are typical of a fully developed El Niño event on the basin-wide scale are still likely to appear, according to the WMO Update, which is based on consensus from experts around the world.

The tropical Pacific Ocean is expected to continue to warm during the coming months, peaking during the last quarter of 2014. Its potential intensity remains uncertain, but a moderate strength event currently appears more likely than a weak or strong one.
“Our understanding of El Niño and La Niña has increased dramatically in recent years and this knowledge has enabled us to develop very successful climate services for society. Advance warning has given governments around the world time to make contingency plans for the impact of this year’s expected El Niño on the agriculture, water management, health and other climate-sensitive sectors,” said WMO Secretary-General Michel Jarraud. “We remain vulnerable to this force of nature but we can protect ourselves by being better prepared.”

The US National Oceanic and Atmospheric Administration and the Japan Meteorological Agency’s Tokyo Climate Centre, which is one of WMO’s regional climate centres, both reported that average global temperatures in the month of May were the highest on record, even without an El Niño event.

One explanation for the lack of atmospheric response so far may be that the sea surface temperatures are above average across virtually the entire tropical Pacific, not just in the eastern and central portions. This may be maintaining west-to-east temperature differences more typical of neutral conditions. The far eastern tropical Pacific has already had higher than normal sea surface temperatures since May, causing above average rainfall along parts of the coast of equatorial South America
The latest outlooks suggest that central tropical Pacific Ocean surface temperatures are likely to warm further into the third quarter of 2014, while the atmospheric patterns associated with El Niño are also expected to form and strengthen.

Consensus from models and expert opinion is that the event will reachpeak strength during the fourth quarter and endure into the first few months of 2015 before dissipating. The substantially above-average oceanic heat content beneath the sea surface of the central and eastern tropical Pacific Ocean, triggered by strong westerly wind events earlier this year, suggested an event of significant strength. However, the delayed atmospheric response, and a potential lack of subsequent westerly wind events in the coming months, may limit the peak strength of the El Niño.

It is important to stress that no two El Niño events are the same, and that other drivers also influence climate patterns. At the regional level, seasonal outlooks are needed to assess the relative impacts of both the El Niño/La Niña state and other locally relevant climate drivers. For example, the state of the Indian Ocean Dipole, or the Tropical Atlantic SST Dipole, may impact the climate in adjacent land areas.

Locally applicable information will be available via regional/national seasonal climate outlooks, such as those produced by WMO Regional Climate Centres (RCCs), Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).

An international conference on El Niño/Southern Oscillation (ENSO), co-sponsored by WMO, will be held in Guayaquil, Ecuador, in November 2014, which will seek to improve operational ENSO predictions and bridge the gaps between global ENSO science and regional processes, extremes and impacts.

based on email circulated by Clare Nullis, Media Officer, World Meteorological Organization