El Niño-Southern Oscillation

El Niño and the Southern Oscillation ( ENSO ) describe a complex circulation system of coupled atmosphere and ocean in the Pacific. El Niño stands rather for the oceanic contexts, while the Southern Oscillation and the Southern Oscillation is the atmospheric interrelations.

ENSO has three phases: El Niño is the best known. There is also the "normal" state of the ocean and atmosphere, as well as La Niña, which is the counterpart of El Niño, and is therefore often referred to as anti -El Niño.

During ENSO acts in the area of the tropical Pacific, there are many teleconnections that seem to be associated with ENSO. The variability of the Indian monsoon or hurricane frequency in Central and North America are closely associated with ENSO.

Continuous measurements in the mesopause show that the variability of ENSO reach great heights, far to the south and north can be transported.

• 2.1 South and Central America
• 2.2 Rest of the World - teleconnections

The three phases of ENSO

Normal phase

In normal years there is a zonal temperature gradient along the tropical Pacific; that is, to the west off the coast of Indonesia is the sea surface temperature of about 28 ° C higher than in the east off the coast of South America. Here, the water temperature is around 24 ° C. The reason for the lower water temperature before South American coast is the upwelling region, which is formed there by Ekman transport due to the equatorial easterly winds - a transport of cool deep water to the surface (see also Upwelling ). This upwelling areas are to be found in front of the coast of Peru and along the equator in the central Pacific to. They form a part of the cool ocean currents, ie of the Humboldt current and the Südpazifikstroms. Thanks to the many nutrients in deep water, there is in the upwelling areas a rich food supply for high fish stocks off the coast. As a result of the strong equatorial easterly winds the sea on the west coast of South America is half a meter lower than on the east coast of Australia or Indonesia where off the coast of Peru "missing" water is pushed toward. This difference in sea level triggers a rear deep current in the east. This fact is the starting point of a possible solution approach, which assumes that the sea level difference generated eventually becomes so great that he can no longer received by the winds and there is a wave to the east.

The high water temperature in the western Pacific provides a large-scale convergence of moist air. The air thus rises over Indonesia in an area of ​​low pressure to the top and forms clouds which bring almost daily rain. The Ascended air is both meridional, ie north and south, as well as zonal, that is, transported along the equator. In the meridional transport spoken of Hadley circulation and in which transport of the zonal Walker circulation. In the high pressure area of the subtropical high pressure belt and in the area of ​​high pressure over the eastern Pacific air masses sink down. Since such Absinkprozesse associated with clouds resolution, very little rain falls in these areas. The air then flows back towards the Indonesian lows. These winds are also called trade winds.

However, the trade winds cause not only the swelling of cool deep water, but also an elevation of the thermocline, a thermocline the water temperature. The processes that lead to this elevation are caused by the Ekman spiral.

The circulation over the Pacific is subject to seasonal fluctuation. In March and April, the Walker circulation is poorly developed, while the Hadley circulation is strong. In September, the situation is reversed. Just as the circulation are also the areas with a strong convergence, ie with strong cloud formation and frequent rainfall, a seasonal migration. In summer, the South Pacific Convergence Zone ( SPCZ ) is weak, but the Intertropical Convergence Zone ( ITCZ ) is strongly developed along the entire length of the Pacific. Until the beginning of the winter rainfall areas of strong migrate southeastward. The SPCZ is stronger while weakening the ITCZ. From April these areas migrate back north-west, while the ITCZ gaining strength and weakens the SPCZ.

These variations may be due to the outgoing longwave radiation - s: - are documented with the help of weather satellite outgoing longwave radiation ( OLR ). It provides a measure of the temperature of the cloud top and thus for the amount dar.

Seasonal changes in the trade winds can be closely linked to the movement of the ITCZ. In September, when the ITCZ is very far north, the south-east trade very strong and the northeast trade wind is weak. In March and April, it is the other way around.

El Niño

In El Niño years, there is an interruption of the seasonal cycle. Kelvin waves that propagate eastward between the water surface and the thermocline, the thermocline in the eastern Pacific are lowered such that the upper water layer does not undergo mixing with the cool, nutrient-rich deep water. Therefore, there is a warming of the water off the coast of Peru and to the death of plankton, which caused the migration of schools of fish.

The Intertropical Convergence Zone ( ITCZ ) is shifted further by the warm water in the eastern Pacific Ocean to the south and the South Pacific Convergence Zone ( SPCZ ), from which the low-pressure area over Indonesia is a part further west. There is therefore a disruptive potential circulation and thus connected to strong anomalies in precipitation, air pressure, wind direction and water temperature.

The rain over Indonesia remains off while the dry land is washed away in Peru by large amounts of rain. While people are threatened in Indonesia through many forest fires, there are many floods in Peru. Agriculture also suffers from these extreme weather conditions.

La Niña

La Niña years have resulted in an increase in the normal phase of ENSO. The low pressure area over Indonesia is particularly strongly developed. The trade winds are also strong, thereby strengthening the Ekman spiral, which leads to a particularly strong cooling of the eastern Pacific. In Indonesia, there are especially a lot of rain, while it is particularly dry in Peru.

Effects

South and Central America

Due to the drought and the heat is the area which is covered by rain forests and field crops, often affected by extreme harvest dips and forest fires.

Rest of the World - teleconnections

• The rain forest in the Amazon region is suffering from drought.
• The number and strength of hurricanes occurring before Mexico is increasing.
• In Southeast Asia and Australia, it comes by the lack of rain to bush fires and huge forest fires.
• In East Africa, in countries such as Kenya and Tanzania, there is more rain, while it is much drier in Zambia, Zimbabwe, Mozambique and Botswana.

There are a few years where droughts in Africa, but also climate variability in Europe ( negative NAO by colder winters and hotter summers ) stemmed from an El Niño phase forth, an influence on these regions, however, is very limited.

Indices

In order to determine the phase of ENSO better, different indices have been developed. The most popular is the Southern Oscillation Index ( SOI). This is determined by variations in air pressure in the bottom level. Others are of the Oceanic Niño Index ( ONI ) of NOAA and the index of the Japan Meteorological Agency ( JMA ), which both relate in certain regions of the tropical Pacific to the water temperature of the near-surface layer. There is also the Multivariate ENSO Index, which combines several factors, such as air pressure, water temperature, wind, air temperature and cloud cover into account.

Modeling

In order to limit the damages that result from an El Niño, as low as possible, we shall endeavor, as long as possible to predict the phase of ENSO. Several different climate models have been programmed, etc when entering water temperature, wind direction, surface pressure, ocean currents, rainfall the phase of ENSO to predict up to one year. Seasonal variations are captured with the help of long time series and statistical considerations. The complicated coupling between ocean and atmosphere are calculated using dynamic models. (see also: Control Theory )

Periodicity

It is believed that the periodicity composed of two superimposed systems, the atmosphere probably has the short-term about 7 years periodicity, while the ocean currents have a 30 -year period, one speaks of warm and cold phases of the Pacific Ocean or the Pacific Decadal Oscillation.