El Niño-Southern Oscillation
El Niño and the Southern Oscillation ( ENSO ) describe a complex coupled circulation system of Earth's atmosphere and ocean currents in the equatorial ialen Pacific . El Niño stands for the oceanic connections, while the Southern Oscillation or Southern Oscillation stands for the atmospheric connections.
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 to El Niño, and is therefore often referred to as "Anti-El-Niño".
While ENSO operates in the tropical Pacific area, there are many teleconnections that appear to be related to ENSO. The variabilities of the Indian monsoons or the hurricane frequency in Central and North America are closely associated with ENSO.
Continuous measurements in the area of the mesopause show that the variabilities of the ENSO extend to great heights and can thus be transported far to the south and north.
Origin of name
The term ENSO (El Niño - Southern Oscillation) consists on the one hand of the term El Niño ( Spanish for "the boy, the child", here specifically: "the Christ child "), the occurrence of unusual, non-cyclical, changed currents and the warm sea surface temperatures associated with it gives it a memorable name. The name is derived from the time of occurrence, usually around Christmas time . It comes from Peruvian fishermen, who get to feel the effect economically because of the lack of schools of fish. The second part of the designation refers to the oceanographic - meteorological conditions described by the Southern Oscillation (these indices can use both sea surface temperature and air pressure as a basis for measurement) in the entire equatorial Pacific, which are divided into positive (El Niño) and negative phases (La Niña) let distinguish.
The three main phases of ENSO
In normal years there is a zonal temperature gradient along the tropical Pacific; d. H. in the west off the coast of Indonesia , the temperature of the sea surface is around 28 ° C, significantly 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 America's coast is the upwelling , which there by Ekman transport occurs due to the equatorial easterly winds - a transport of cool deep water to the surface (see also upwelling ). These upwelling areas can be found off the coast of Peru and along the equator as far as the central Pacific. They form part of the cool ocean currents, i.e. H. the Humboldt Current and the South Pacific Current . Thanks to the many nutrients in the deep water, there is a rich food supply for high fish stocks off the coast in the upwelling areas. As a result of the strong equatorial east winds, the sea level on the west coast of South America is half a meter lower than on the east coast of Australia or Indonesia, where the “missing” water off the coast of Peru is being pushed. This difference in sea level triggers a backward deep current to the east. This fact is the starting point for the idea that the sea level difference produced could become so great that it cannot be sustained by the winds and that a near-surface mass balance to the east would occur.
The high water temperature in the western Pacific ensures a large-scale convergence of humid air. The air rises above Indonesia in a low pressure area and forms clouds that bring rain almost every day. The ascended air becomes both meridional, i.e. H. to the north and south, as well as zonal, d. H. along the equator, transported away. Meridional transport is referred to as Hadley's circulation and zonal transport is referred to as Walker circulation . In the high pressure areas of the subtropical high pressure belt and in the high pressure area over the eastern Pacific, the air masses sink. Since such subsidence processes are associated with cloud dissolution, very little rain falls in these areas. The air then flows back towards the Indonesian low. These winches are also known as the Passate .
But the trade winds not only cause the swelling of cool deep water, but also a raising of the thermocline , a thermocline because of discontinuum the water temperature. The processes that lead to this elevation are brought about by the Ekman spiral .
The circulation over the Pacific is subject to seasonal fluctuations. In March and April the Walker circulation is poorly developed, while the Hadley circulation is strong. In September it is the other way round. Just like the circulation, the areas with pronounced convergence, i. H. with heavy cloud formation and frequent precipitation , a seasonal migration. In summer the South Pacific Convergence Zone (SPCZ) is only weakly developed, but the Innertropical Convergence Zone (ITCZ) is strongly developed over the entire length of the Pacific. Until the beginning of winter, the areas of heavy precipitation migrate southeast. The SPCZ becomes stronger, while the ITCZ weakens. From April these areas migrate back northwest, while the ITCZ gains strength and the SPCZ weakens.
These variations may be by the outgoing longwave radiation - English outgoing longwave radiation (OLR) - with the help of weather satellites are documented. It represents a measure of the temperature on the cloud surface and thus of its height.
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 southeast trade wind is very strong and the northeast trade wind is weak. In March and April it is the other way round.
In El Niño years there is a break in the seasonal cycle. Kelvin waves , which propagate eastward between the water surface and the thermocline, lower the thermocline in the eastern Pacific to such an extent that the upper water layer does not experience any mixing with the cool and nutrient-rich deep water. As a result, the water off the coast of Peru warms up and the plankton die off , causing the schools of fish to migrate.
The Innertropical Convergence Zone (ITCZ) is shifted further south by the warm water in the Eastern Pacific and the South Pacific Convergence Zone (SPCZ), of which the low pressure area over Indonesia is part, further to the west. This leads to a complete change in the circulation and the associated severe anomalies in precipitation, air pressure , wind direction and water temperature.
The rain over Indonesia does not occur, while the dry land in Peru is washed away by large amounts of rain, for example at the transition between 1997 and 1998. While the people of Indonesia are threatened by many forest fires, Peru has experienced a lot of flooding . Agriculture also suffers from these extreme weather conditions.
La Niña years mean a reinforcement of the normal phase of the ENSO. The low pressure area over Indonesia is particularly well developed. The trade winds are also strong and thus reinforce the Ekman spiral , which leads to a particularly strong cooling of the eastern Pacific. There is a lot of rain in Indonesia, while Peru is particularly dry.
More recent observations describe phenomena that cause the hot water masses to stop halfway in the middle of the Pacific as El Nino Modoki , and the hot water masses that appeared unusually late on the Peruvian coast in March 2016 as "coastal El-Nino".
Further descriptive indices
In order to better determine the phase of the ENSO, in addition to the Southern Oscillation Index (which is determined by variations in air pressure at ground level), other indices such as the Oceanic Niño Index (ONI) of the NOAA and the index of the Japan Meteorological Agency (JMA) have been developed . Both refer to the water temperature of the near-surface layer ( SeaSurfaceTemperature ) in certain regions of the tropical Pacific. There is also the multivariate ENSO index, which takes into account several factors such as air pressure, water temperature, wind, air temperature and degree of cloudiness .
South and Central America
Due to the drought and the heat, the area covered by rainforests and field plants is often affected by extreme crop collapses and forest fires.
Rest of the world - teleconnections
- The rainforest in the Amazon region suffers from drought.
- The number and strength of cyclones occurring off Mexico is increasing.
- In Southeast Asia and Australia, the lack of rain leads to bushfires and huge forest fires.
- In East Africa in countries like Kenya and Tanzania there is more rain, while in Zambia, Zimbabwe, Mozambique and Botswana it is significantly drier.
There are a few years in which droughts in Africa, but also climatic fluctuations in Europe (negative NAO, resulting in colder winters and hotter summers), resulted from an El Niño phase, but there is only evidence of an influence on these regions very limited possible.
Effects on tropical cyclones
Above all, the effects on the hurricanes in the Atlantic Ocean can be clearly recorded statistically. In moderate to strong El Niño years there are around 60% fewer hurricane days, overall fewer cyclones (in the Atlantic) and less energy in the Atlantic weather system. The effect is likely caused by the increased westerly winds. The effects on tropical cyclones in the Pacific are significantly lower.
It is assumed that the periodicity is mainly composed of two superimposed systems, with the atmosphere probably having a short-term periodicity of approx. 7 years, the ocean currents a 30-year period; one speaks here of warm and cold phases of the Pacific or the Pacific Decadal oscillation .
Effects of climate change
Based on the climate models available today, researchers assume that if the Walker circulation is weakened by climate change, the likelihood of positive and more positive ENSO events (El Niño phase) increases. The faster warming of the equatorial Pacific could lead to heavy precipitation in the area of the equatorial east Pacific and to a shift of the Pacific innertropical convergence zone into equatorial areas. This corresponds to characteristics of an El Niño phase. A higher frequency of negative ENSO events (La Niña phases) is also expected. The reasons for this are seen as a continent that is warming more rapidly and a greater warming of the ocean surface temperature . Other researchers see El Niño phases as a higher probability of a higher number of tropical cyclones in the Pacific in the future.
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