Will Iran experience above-normal rainfall during autumn?

August 5, 2019

TEHRAN – According to the forecasts, El Nino–Southern Oscillation (ENSO), is in a neutral phase during autumn, not contributing to a rise in precipitations in the country, especially during autumn and winter, Iman Babaeian, head of the climatological research institute affiliated to Iran’s Meteorological Organization has said.

The ENSO cycle has three phases called the neutral phase, El Niño and La Niña. It 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.

How ENSO affects autumnal rainfall in Iran?

Referring to the impact of ENSO on Iran's autumnal rainfall, Babaeian said that forecasting centers and models in Australia, the U.S. and Europe that have been specially envisioning ENSO, have consistently predicted in the past few months that ENSO will continue in weak El Niño phase until this winter. 

El Niño led to an increase in precipitation in the Middle East and Iran in particular, but the latest forecast of these centers has emphasized that during autumn and winter this year, ENSO will be in neutral phase, he added.

According to the forecasts, ENSO is in a neutral phase during autumn, thus not contributing to a precipitation rise in the country, especially during autumn and winter, he said, ISNA reported on Monday.

During the second month of autumn, precipitation is expected to reach above normal averages in the northwest of the country, while other areas will experience below normal rainfall, he added.

Iran meets normal rainfall averages 

This month, due to the eastern waves in the southern half of the country and the positive phase of the Indian Ocean Dipole (IOD), the average rainfall in the southern and southeastern coasts of the country fluctuates from normal to above normal, he explained.

The IOD is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean. It involves an aperiodic oscillation of sea-surface temperatures, between "positive", "neutral" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean, which tends to cause droughts in adjacent land areas of Indonesia and Australia.

Precipitation in western part of the Caspian Sea is expected to meet normal or lower than normal averages over the next month (August 23- September 22), while other parts of the country are forecasted to experience normal rainfall, he explained.

Also, during the next two months all provinces across the country will receive normal rainfall except for Gilan province which is anticipated to experience below normal precipitation, he noted.

Temperature to rise nationwide 

Elsewhere in his remarks, he stated that temperature across Iran rises on average by 1.5 degrees Celsius during this month and next month, while increases by 2 degrees Celsius during the first month of autumn compared to the long-term averages.

Sadeq Ziaeian, director of the national center for drought and crisis management, affiliated to the Meteorological Organization has said that Iran experienced an average 1.7 degrees Celsius temperature rise during the past Iranian calendar month of Tir (June 22-July 22) compared to the long-term mean.

FB/MG

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