Autumn rainfall anomalies and regional atmospheric circulation along establishment of weak la Nina after strong El Nino in Iran

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)


1 دانشگاه آزاد اسلامی واحد علوم و تحقیقات تهران

2 رئیس، پژوهشکده هواشناسی


To study of the Iran precipitation anomaly in (September to November (SON) of 2016) and its probable communication with El Niño-Southern Oscillation (ENSO), this period with the similar cases in the 55 past years (1964, 1983, and 1995) according to NOAA data was investigated. In the all cases, ENSO large-scale frequency changing from strong El-Nino to weak La-Nina after a very brief neutral period.. The data set includes mean sea level pressure and wind, humidity and geopotential heights fields at 850, 500 and 300 hPa, from NCEP-NCAR and precipitation of 44 synoptic weather stations of Iran for three months (SON) of every year in the period 1981-2017.
The results show that, along the establishment of weak La Niña after strong El Niño, Iran autumn precipitation decreases strongly, so that severe negative precipitation anomaly (up to 100%) happens in more than 70% of the weather stations. Also atmospheric anomaly patterns in these conditions, suggest the development and deployment of a high pressure systems over Iran, so that when Siberian high pressure system strengthened, prevent the precipitation systems to pass from Iran and it causes cold and dry air masses deployment over the country. Also during these period, extensions of subtropical ridges over Iran and eastward expansion of Azores high pressure system and also weakening of Iceland low pressure, prevents of eastward movement of baroclinic waves. At this time transfusion of moisture flux has not happened and also it has decreased according to long term mean.



عنوان مقاله [English]

Autumn rainfall anomalies and regional atmospheric circulation along establishment of weak la Nina after strong El Nino in Iran

نویسندگان [English]

  • faranak bahrami 1
  • abbas ranjbarsaadatabadi 2
  • amirhosein meshkati 1
  • gholamali kamali 1
1 Department of meteorology, science and research branch, Islamic Azad University, Tehran, Iran
2 Head of Atmospheric Science and Meteorological Research Center, Tehran, Iran
چکیده [English]

Results of the analysis shows that, when La Nina is weak in autumn and accrued after changing strong El Nino to the very brief neutral phase of ENSO, drought occurs in the fall season in Iran. In order to get this, 4 cases, is identified with these conditions, and rainfall anomalies were calculated in these cases. The result show that in this condition, precipitation reduces in the center, east, southeast, west and also northwest of Iran and a very dry fall dominates in these parts. In all analyzed years, most of Iran experiences a negative abnormality more than 60%. Also Synoptical studies for selected cases was conducted. It showed that, fall droughts caused by weak and short La Nina dominance after a severe El Nino are controlled by six major pressure patterns. These major are: Arabian and North African subtropical high pressure, Siberian high pressure, Azores high pressure, Mediterranean low pressure and Iceland low. When ridges of Arabian high pressure develop towards the north, prevent eastward movement of Sudanese low pressure. Sometimes the ridges of this pressure center develop to higher altitudes and prevent entrance of Mediterranean current from west of Iran, so Mediterranean current pass in higher altitudes. The development of African high pressure towards south and east of the Mediterranean Sea and development of the Azores high pressure ridges towards north and east, and covering its ridges on the center and west of the Mediterranean Sea, causes weakness or lack of formation of the Mediterranean trough. The most common pattern among effecting patterns on drought of studding years is increase of central pressure of Iceland low pressure center in the north of Europe, which its reinforcement or weakness, effects Mediterranean low pressure. Weakness of Iceland low pressure center causes weakness or lack of formation of Mediterranean trough. So the existence of Siberian high pressure in the north, which causes development of dry and cold air on Iran and also the existence of Arabian, north African and Azores high pressures, and of course weakening of the Iceland low center, that's respectively prevent incoming of Sudanese low pressure and formation of Mediterranean trough, beside, lack of sufficient moisture feeding on Iran, have all caused severe low precipitation in the fall season in analyzed years. As a result, in the occurrence of a weak La Nina after a severe El Nino, should expect dry autumns in Iran, especially in the center, east, southeast, west and also northwest regions. And this result can be considered as a seasonal forecast.

کلیدواژه‌ها [English]

  • weak La Nina
  • strong El Nino
  • Precipitation anomaly
  • Atmospheric circulation
  • Iran