The Relationship between Severe and Pervasive Droughts and Wet Years in Half East of Iran with Synoptic Troposphere Condition

Document Type : مقاله پژوهشی


1 kharazmi university

2 shahid beheshti university



Drought is a reptile climatic phenomenon whose occurrence is unavoidable in all climates. Because of human and ecosystem dependence on the existence of water and lack of water resources in an arid and semi-arid climate, drought can exacerbate the limitation of water resources even with low intensity and cause countless harmful damages. Drought is different from other natural disasters such as floods, storms and earthquakes. Most of these differences are in the effect of gradual droughts during a relatively long period, the impossibility of accurately determining the time of start and end, and the geographical extent of its effect. On the other hand, the absence of a precise and global accepted definition of drought has added to the complexity and confusion of this phenomenon (Zare et al., 2004).  Generally drought is the unexpected decrease in precipitation over a period of time in an area that is not necessarily dry (Alijani & Kaviani, 2011), and or precipitation reduction relative to the long-term mean (Garajzadeh, 2007). Iran is one of the countries that is located in the desert belt of the earth and in some years will suffer from drought and other years from wetness.

Materials and Methods

Our research region includes a large part of Iran including desert and dry areas in the east, center and southeast of the country which includes Sistan and Baluchestan, Hormozgan, Kerman, Yazd, South Khorasan, Qom, southern half of Khorasan Razavi province, Isfahan and Semnan provinces. The data of 35 ground stations used in the research include daily rainfall data for the period 1985-2014 located in 9 provinces of Khorasan Razavi, South Khorasan, Semnan, Qom, Sistan and Baluchestan, Yazd, Kerman, Isfahan and Hormozgan, which are obtained from the Machinery Service Department of the Meteorological Organization Upper surface data, include retrieved data of geopotential heights, sea level pressure, humidity, meridian and orbital wind at 1000 to 500 HPA which are used by the European Center for Middle Eastern Weather Forecast (ECMWF) with a spatial resolution of 0.75 arcs. Then, using the Grades software, the combined sea level pressure maps and geo potential height of 700 and 500 hp, and the wind vector and moisture at 1000 and 700 hp in the range of 0 to 75 north degrees and 0 to 90 east longitude degrees east were plotted. Using RAI (Anomaly of Annual Rainfall Index), the severe and pervasive drought-wet years of the studied area were extracted and its relationship with the standard situation of the troposphere was investigated.

Results and Discussion

The peak of drought severity during the entire statistical period belonged to Saravan station with an intensity of 6.44 in 2001, also the highest duration was at Zabul station from 2006 to 2014. The maximum frequency was also observed at Anar, Bam, Kerman, Yazd, Mashhad and Torbat Heydariyeh stations with 17 occurrences during 30 years of statistical period. According to the year of the minimum rainfall, the most pervasive year that was coincident with the most severe drought in the research area was seen in the years 2001, 2010, and 2008, respectively. The most severe wet year occurred in 1991, 1986, 1992, and 1997, respectively. The peak of wet year intensity with 88/11 belonged to Chabahar Station in 1997, and the longest duration occurred in Sirjan and Shahr-e-Babak stations from 1991 to 1997. Maximum frequency was observed in east of Isfahan, Garmsar and Qom stations with 17. According to the maximum rainfall year, it was determined that the most severe and pervasive wet year was in 1991, 1986, 1992, and 1997. In the mentioned years, most stations recorded positive values of the RAI profile. The mentioned years are the most severe wet and drought years during the statistical period in 9 under study provinces in the east and south-east of Iran. The analysing of zoning maps showed that drought and wet years with different intensities occur in each point of the research area. Also, at the time of drought’s occurrence, showed more pervasiveness and less temporal and spatial fluctuations than wet years. this feature indicates more drought’s relationship and effectiveness from synoptic troposphere condition as pervasive and zoning in the research area. But in the wet year’s analysis, even some nearby stations (especially the stations located in Khorasan Razavi and Semnan) have shown a great difference in terms of wet year intensity during the peak years. Synoptic analysis in the peak years of drought showed that the interaction between Siberian high pressure in sea level and sub-tropical anti cyclone in the middle level of troposphere create dry, without turbulence, stable and quiet in the research area. So that the penetration of the Siberian high-pressure with a north eastern direction to the southwest and the extent of the ridge from the near regions equator to high geographical latitudes in northern Europe and Russia and settlement the weak orbital and meridian contours which created from the ridge on the research area prevents the penetration of the southern thermal low pressure (Sudan) and western (Mediterranean) and the deep north troughs and consequently create upward and unstable flows and provide the basis for the most severe and pervasive droughts. In the years which the severity of the drought has increased the axis of the ridge on Iran in the middle level will be southeast- more northwest direction. Also the synoptic analysis conditions of the peak days showed that with the retreat of the Siberian high pressure and European migrant to higher latitudes, the east-ward of anti- cyclone from Saudi Arabia to the Red Sea are faced with two changes. One on the sea level with penetration of thermal Sudan low pressure and Mediterranean, and also the second one on the middle levels of the troposphere, north European cyclone and Russian systems stretched on Iran and the research area.  Also, when the severity of wet year was increasing, the troughs are deeper and its direction become northeast to the southwest. Locating the half east of trough on the Red Sea, the Gulf of Aden, the Persian Gulf and the Eastern Mediterranean And centre of the anti-cyclone with high ridge of north east to the southwest in west of the Arabian sea was the best position in order to strengthen divergent and upward flows, and consequently, the convection of the most volumes of moisture  to the research area.


Analysis of changes in the RAI index values in the research area indicates that the entire statistical period is divided into two parts: one wet year period (1985-1999) and one large and durable droughts period. By the beginning of the 21st century, the severity, magnitude, frequency and pervasive of droughts have been significantly increased. The peak of drought and wet years severity during 1985-2014 were observed at stations in south-east of Iran. In fact, it can be said that the highest fluctuation of the RAI drought index was observed in stations located in lower latitudes with lower height in south and south-east of Iran and stations located in higher latitudes in the north and west of the study area have experienced more normal conditions. Analyzing the effects of synoptic scale systems in years with decreasing in precipitation for Iran which are experiencing a severe decline in water resources over recent years is important. Therefore, using dynamic and synoptic indicators and the development of a warning system can well predict the droughts affected by synoptic systems in the area. Also, with managing water resources, evaluating the risks of climate change, and modifying and developing predictions, we can minimize the hazards of drought in the future.


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