Synopsis of the dramatic climate change in Iran: a seasonal synoptic analysis

Abstract

This study examines the NCEP (National Centers for Environmental Prediction)/NCAR (National Center for Atmospheric Research) reanalysis model datasets to display the existence of the anomaly periodic time with the significant atmospheric changes in Iran. Datasets such as surface air temperatures, relative humidity, surface soil moisture, precipitation rate, mean sea level pressure, 500 hPa geopotential height, near-surface vorticity and surface scale winds have been used. The anomalies’ departures from climate normal for the meteorological datasets are investigated, and synoptically patterns have been analysed. The yearly seasonal mean surface air temperature time series during 1948–2018 displays a dramatic warming shift with an abnormal pattern in Iran from 1994 onwards. The analysis of the other datasets resulted from ERA-Interim and MERRA-2 database also confirm a warming shift nearly from 1994 over Iran. The statistical analysis of t-tests shows the existence of a significant difference in seasonal mean surface air temperatures during the anomaly period (1994–2018) and the mean values during climatology normal periodic time (1981–2010). This suggests remarkable warming over all parts of Iran with the highest changes in surface temperatures ranging ~0.1–1 °C in spring and the lowest change in fall ranging ~0.1–0.7 °C. Significant changes have resulted in precipitation rate, atmospheric humidity, soil moistures and surface wind speeds. Also, due to the strengthened northern high-pressure system along with the intensification of the 500 hPa ridge and less atmospheric humidity, a rather stable, warmer and drier atmosphere with less precipitation (respect to climate normal) over the most part of Iran has been indicated. The low-level vorticity anomalies have displayed the intensified flows from the regional desert areas to the central and north-east of Iran and from Iraq to the west and south-west of Iran for all seasons (with the higher gradient in summer rather than other seasons). Also, mean sea level pressure anomalies have exposed the increase in pressure gradient mostly over the western and central parts in all seasons except winter (with decreased wind speeds leads to haze or accumulated pollutants). This may cause instability in these parts with the possibility of frequent dust storm because of wind acceleration and warmer and dry atmospheric conditions rather normal mean. However, this study shows that during the fall seasons, humidity and precipitation values in the south-west and south-east area have been increased in comparison with the normal mean values. The increase in relative humidity during fall in the south-east areas can be associated with the impact of the intensified south-easterly flows (southern low-pressure tongue). This is accompanying by rather warmer sea surface temperatures in the southern marine areas, especially in the Oman sea and the northern Indian Ocean. Overall, this study presents some synoptic abnormal patterns associated with the recent Iran significant atmospheric climate changes.