Autumn Rainfall Anomalies and Regional Atmospheric Circulation along Establishment of Weak La Nina after Strong El Nino in Iran

To study the Iran precipitation anomaly in September to November of 2016 and its probable connection with ENSO (El Nino-Southern Oscillation). This period with similar cases in the previous 55 years (1964, 1983, and 1995 according to forecasting center of NOAA) was investigated. In all cases, ENSO changed from strong El-Nino to weak La-Nina after a very brief neutral period. In the following, observational data from 44 synoptic stations of Iran, which have long term statistics about 30 years duration, have been used; and also NCEP-NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, which include mean sea level pressure, geopotential height at 850, 500 and 300 hPa, wind and humidity at 850 hPa and also 300 hPa zonal and meridional wind have been used. Results show that, along the establishment of weak La Nina after strong El Nino, autumn rainfall decreases strongly so that severe negative precipitation anomaly (up to 100%) happens in more than 70% of synoptic stations. Also, atmospheric anomaly patterns in these conditions suggest the development and establishment of a high-pressure system over Iran so that when Siberian high pressure system strengthens, it blocks the passage of precipitation systems reaching Iran and causes cold and dry air mass establishment in the country. Besides, during these period, extensions of subtropical high over Saudi Arabian, North African high and eastward expansion of Azores high as well as weakening of Iceland low pressure, stop the activity of Mediterranean low-pressure and Sudanese systems. At this time, the transport of moisture has also decreased relative to long term mean.