Low impact of El Niño on autumn rainfall in Iran

September 15, 2018 - 23:10

TEHRAN — The El Niño, will have a little impact on Iran's autumn rainfall this year, head of climatological research institute affiliated to the Iran’s Meteorological Organization has said.

Iman Babaeian made the remarks regarding the World Meteorological Organization (WMO) latest update announced on September 10, demonstrating a 70 percent chance of El Niño developing by the end of this year, which intensity is currently uncertain, but a strong event appears unlikely.

“When El Nino occurs, the probability of increase in rainfall gets high, which can affect Iran, provided that other phenomena such as the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO) and Madden Julian Oscillation (MJO) pattern coincide with it,” he said.

The aforesaid phenomenon occurs only on a part of the planet but affects the entire earth, he added.

He went on to say that this year, due to El Niño phenomena, the Tropical Eastern Pacific will experience at most 1 Celsius degree increase in temperature, so, El Niño might have a positive impact on precipitation amounts but it is weak and not very significant.  

However, it will increase Iran's precipitation rate during autumn, as long as the other aforesaid phenomenon are in line with it, he highlighted.

Iran is one of the Middle East regions that El Nino indirectly affects, in fact, its direct impact is mainly on the countries including the western coasts of South America and the eastern shores of Asia, such as Japan, Korea, Australia and Indonesia, he said, adding that the greatest impact will be on Australia and Indonesia and the rest of the regions farther away of El Nino, such as Iran, will be indirectly affected.

Babaeian further explained that most of the climatic prediction models predict a weak El Nino formation at the beginning autumn, the impact of El Nino will be in late amount and then it will gradually weaken by the beginning of winter.

If the other phenomena are not accompanying El Nino, even a strong one does not necessarily lead to the rise of precipitation in Iran, since the frequency of the ENSO is between 3 and 7 years and the frequency of regional phenomena such as the MJO (about 45 days) are different, so that their occurrence at the same time only can increase the precipitation.

The three phases of ENSO

The three phases of the ENSO cycle are called the neutral phase, El Niño and La Niña. The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific (approximately between the International Date Line and 120 degrees West).

In the neutral state (neither El Niño nor La Niña) trade winds blow east to west across the surface of the tropical Pacific Ocean, bringing warm moist air and warmer surface waters towards the western Pacific and keeping the central Pacific Ocean relatively cool. The thermocline is deeper in the west than the east.

The term El Niño refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific.

La Niña is mainly referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO. These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate.

El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events may last for years. While their frequency can be quite irregular, El Niño and La Niña events occur on average every two to seven years. Typically, El Niño occurs more frequently than La Niña.

The regional phenomena 

The NAO is a large-scale seesaw in atmospheric mass between the subtropical high-pressure system over the Azores Islands and the subpolar low-pressure system over Iceland. It is one of the most important modes of atmospheric variability in the northern hemisphere, which has a larger amplitude in winter than in summer.

The Arctic Oscillation has a global scale, more zonally symmetric, also called the Northern Annular Mode (NAM), which has a connection to stratosphere, the NAO can be regarded as a local representation of the AO in the North Atlantic.

The MJO is the largest element of the intraseasonal (30-90 day) variability in the tropical atmosphere. It is a large-scale coupling between atmospheric circulation and tropical deep convection. Unlike a standing pattern like the ENSO, the Madden–Julian oscillation is a traveling pattern that propagates eastward, through the atmosphere above the warm parts of the Indian and Pacific oceans, which manifests itself most clearly as anomalous rainfall.


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