Introduction

Sand and dust storms are the most important atmospheric phenomena in arid and semi-arid regions that have been recognized as having a wide range of environmental and climate impacts including distractive effects upon air quality and human health (Prospero et al. 2008; Bennion et al. 2007; Meng and Lu 2007), agricultural activities, land use and soil formation (Fung and Tegen 1995), and they are also recognized as the factor of desertification (Han et al. 2008). Dust particles are important components of the earth’s climate system (Huang et al. 2014) as they affect the balance of solar radiation by scattering and absorption (Tegen and Lacis 1996). Dust storm detection and tracking as well as specifying their source(s) may help planners and decision makers to control and plan the dust impacts.

Many methods for dust tracking and analyzing in the world climate system are documented. These methods include synoptic (Mao et al. 2011), remote sensing (Ackerman 1997; Samadi et al. 2014), and dust modeling (Solmon et al. 2012) approaches. By analyzing the atmospheric circulation pattern of dust events, one can understand the main controlling factors for dust emission. These atmospheric factors include strong surface heating which causes convective disturbances (Duce 1995; Alpert et al. 2004), the air instability and uplift, and wind velocity. Moreover, erosion, soil type, soil moisture, vegetation cover, grain size of the dust particles (Ginoux et al. 2001), and the amount of precipitation are among the surface factors that influence dust generation.

The Middle East is one of the largest dust-producing regions in the world (Goudie and Middleton 2006). Regarding to the geographical location of Iran in this region, it is frequently exposed to local and regional dust systems. Over the last decade, dust storm in the Middle East especially in the Arabian region (including Saudi Arabia, Iraq, Syria, and Jordan) has significantly been increased (Khoshakhllagh et al. 2012). The main sources of dust event in this region are Saudi Arabian deserts, Iraq, and east of Syrian arid areas (Najafi et al. 2014; Boloorani et al. 2014) (Fig. 1).

Fig. 1
figure 1

Main sources of Arabian dust which affect west of Iran (Boloorani et al. 2014)

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Increasing dust events in this region is due to the changes of regional and global climate patterns, the biogeochemical cycle, and human environments (Awad and Mashat 2014). The Arabian dust affects west of Iran in many cases. During the recent years, there has been an increase in the trend of dust activities in Iran (Alizadeh-Choobaril and Najafi 2017), especially in spring and summer (Azizi et al. 2012). The aim of this study is to explore the main atmospheric circulation patterns in times of dust storms events in west of Iran.

The study area

Located in the west of Asia, Iran stands at the arid and semi-arid belts of the world (Hojati et al. 2011). The study area is western part of Iran, a region bounded between 27° 17′ N to 40° N and 45°.24′ E to 52° E (Fig. 2). The study area is geographically bounded by the high northwestern Iran’s high mountains on the northwest, and coasts of Persian Gulf on the south. Depending on the topographical features of the landscape, the climate of the region varies from north to south, such that there is a drier and cooler climate on the northern area compared to the middle parts of the study area (Fig. 2b).

Fig. 2
figure 2

The study area

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Overall, northern part of study area has a semi-arid climate, with an annual precipitation of 294.18 mm. The annual mean air temperature in the highlands of the northern part is 11.9 °C, nearly 1.5 °C more than its middle highlands of the study area with the annual mean air temperature of 13.4 °C. The observed annual precipitation of the middle region of study area is 530 mm. The southern part have a hot, humid, and semi-arid climate, with an annual precipitation of 277 mm and mean air temperature of 25 °C. The annual number of dusty days in the north, middle, and southern parts of the study area are 11, 30, and 89 days, respectively.

Data and methods

In this study, the structure of atmospheric circulation is investigated to explain the mechanisms of dust event in the Middle East and its emission over western Iran. The hourly visibility and dust dataset of 34 synoptic stations (Fig. 2) located in the western part of Iran were obtained from the Iran Meteorological Organization (in 2000–2013 period) to extract dust events in the area of study.

The NCEP/NCAR 6-hourly reanalysis dataset with 2.5° × 2.5° horizontal resolution is used for mentioned period. We used air temperature, sea level pressure, geopotential height, zonal (u) and meridional (v) wind components, and relative and specific humidity from 1000 to 100 hPa to identify atmospheric circulation patterns.

As one of important atmospheric circulation patterns of dust storm events in western Iran is frontal dust storm, the frontogenesis function has been calculated. In this study, the 2-dimentional version of frontogenesis function has been used because the function is only considered in lower level of troposphere (850 hPa). The frontogenesis function can be expressed by the following equation (Martin 2006):

$$ {\Im}_{2D}=\frac{d}{dt}\left|\nabla \theta \right|=\frac{d}{dt}{\left(\frac{\partial {\theta}^2}{\partial x}+\frac{\partial {\theta}^2}{\partial y}\right)}^{\frac{1}{2}}=\frac{1}{\mid \nabla \theta \mid}\left[\left(-\frac{\partial \theta }{\partial x}\right)\left(\frac{\partial u}{\partial x}\frac{\partial \theta }{\partial x}+\frac{\partial \nu }{\partial x}\frac{\partial \theta }{\partial y}\right)-\left(\frac{\partial \theta }{\partial y}\right)\left(\frac{\partial u}{\partial y}\frac{\partial \theta }{\partial x}+\frac{\partial \nu }{\partial y}\frac{\partial \theta }{\partial y}\right)\right]. $$
(1)

Frontogenesis function calculated for values ≥1 × 10−4 k m−1 s−1.

Another pattern of dust event is blocking pattern. The modified version of the procedure of Tibaldi and Molteni (1990) has been used for identifying blocking patterns. In this procedure, two geopotential height gradients including mid-latitude geopotential height gradient (GHGS) and high-latitude geopotential height gradient (GHGN) are calculated at each longitude at 500 hPa level on a particular time:

$$ {\displaystyle \begin{array}{l}\mathrm{GHGN}=\frac{\mathrm{Z}\left({\varphi}_n\right)-Z\left({\varphi}_{\mathrm{o}}\right)}{\left({\varphi}_n\right)-\left({\varphi}_{\mathrm{S}}\right)},<10\hfill \\ {}\mathrm{GHGS}=\frac{\mathrm{Z}\left({\varphi}_o\right)-Z\left({\varphi}_{\mathrm{s}}\right)}{\left({\varphi}_o\right)-\left({\varphi}_{\mathrm{S}}\right)},\ge 0\hfill \\ {}{\varphi}_n={80}^{\circ }N+\varDelta, \cdot {\varphi}_o={60}^{\circ }N+\varDelta, .{\varphi}_s={40}^{\circ }N+\varDelta, \cdot \mathrm{and}\cdot \varDelta =-{4}^{\circ },{0}^{\circ }, or{4}^{\circ}\hfill \end{array}} $$
(2)

Where GHGN is northern branch; GHGS is southern branch; Z is geopotential height at 500 hPa level; λ is longitude; Ø is latitude, and Δ is the measure of latitude displacement (that can be 4° lower or higher than base latitude). Based on GHGS and GHGN, Blocking is defined to occur where and when these conditions are met at 20 consecutive degrees of longitude for at least five consecutive days (Waston and Colucci 2002).

One of the most important components in formation of precipitation in the Middle East is the condition of convergence and divergence of moisture flux. Many atmospheric circulation patterns in times of Arabian dust events are similar to those in times of precipitation events, so the condition of moisture flux is an important component to differentiate the atmospheric pattern that cause dust or precipitation.

Next, the convergence and divergence of moisture flux from 1 day prior to representative days is calculated and for each pattern the average map of humidity flux is used. The convergence and divergence of moisture flux is calculated at 1000 to 700 hPa levels to analyze the condition of moisture flux in dust storm events. For this purpose, 4-daily NCEP/NCAR reanalysis data including specific humidity and zonal and meridional wind components (u, v components) have been used. Divergence moisture flux is computed according to Eq. (3).

$$ {\mathrm{HFD}}_{ij}=-\left[{u}_{ij}\frac{q_{i+ lj}-{q}_{i\kern0.5em lj}}{x_{i+ lj}-{x}_{i\kern0.5em lj}}+{v}_{ij}\frac{q_{ij+l}-{q}_{ij\kern0.5em l}}{y_{ij+l}-{y}_{ij\kern0.5em l}}+{q}_{ij}\frac{u_{i+ lj}-{u}_{i\kern0.5em lj}}{x_{i+ lj}-{x}_{i\kern0.5em lj}}+{q}_{ij}\frac{v_{ij+l}-{v}_{ij\kern0.5em l}}{y_{ij+l}-{y}_{ij\kern0.5em l}}\right] $$
(3)

Where HFD is horizontal divergence of moisture flux (negative and positive signs mean convergence and divergence, respectively). q is specific humidity, u and v are zonal and meridional wind components, and x and y are distances in directions of longitude and latitude (Farajzadeh et al. 2007).

The total vertically integrated of horizontal divergence of moisture flux is computed according:

$$ {Q}_{\mathrm{vi}}=1/g.\underset{p_1}{\overset{p^2}{\int }} vq. dp $$
(4)

Q vi is total moisture flux in kilogram per square meter per second, (vq) are HDF and p is pressure level. In this study, total moisture flux calculated for 700 to 1000 hPa pressure levels.

Results and discussion

The atmospheric circulation patterns which lead to generation of dust events in the Arabian region classifying in two categories of cold and warm period patterns. In warm period of the year, the Shamal winds and summer systems of westerly winds, and in cold period, two frontal and non-frontal patterns lead to formation of Arabian dust and transport to western Iran.

Cold period patterns

Frontal

In the frontal events of Arabian dust storms that occur in the cold period of the year, dust is created under the influence of emigrate systems of westerly winds setting in the West Asia region. So that, formation of a divergence system in mid-level of troposphere (500 hPa) leads to formation of a surface convergence center as well as air uplift and atmospheric instability condition in the source areas of Arabian dust storms. Generally, Arabian dust events which occur due to the frontal pattern are similar to the precipitation systems except the lack of humidity in case of dust generation in arid lands of the Arabian region. Frontal patterns are divided into two patterns including trough and blocking. These two patterns are the dominant patterns of dust generation in November to May in this region in cold period. In this study, we have used the average maps of each pattern in selected dust events period to analyze the atmospheric circulation patterns.

Trough pattern

This pattern which contains 32.7% of the Arabian dust events in the west of Iran, forms when at 500 hPa level there is a trough and its associated cut of low over the west Asia in the mid-level of atmosphere (Fig. 3a, b). The position of cut of low varies from the east of Mediterranean to the mid-western Iraq. On the earth surface, there is a low pressure depending on divergence intensity in mid-levels of atmosphere. Such conditions cause frontogenesis, instability, and air uplift in the Middle East. One of the factors which affect the intensity of this pattern, especially in winter, is the setting position of polar front jet stream (Fig. 3c). The jet stream position as one of the enhancing factors of instabilities and air uplift in the region can be affective in frontogenesis enhancement and generation of dust. It is worth to mention that polar front jet stream only affects the Middle East at cold period of year and leads to instability intensification in this region. The trough and cut of low lead to form an intensive convergence in the northern part of southwest of Asia at mid-levels of atmosphere. This condition is lead to create a low pressure center and intensive convergence in Iraq, Syria, and Northeastern part of the Arabian Peninsula at sea level. Setting of such pattern leads to intensive air uplift in the surface and frontogenesis over Iraq, the north of Arabian Peninsula and Syria as well as generating instability in abovementioned regions (Fig. 3d). Moreover, the present instabilities in this region is enhanced by setting polar front jet stream over the west Asia in which its tongues are advanced to 600 hPa level.

Fig. 3
figure 3

Frontal pattern (trough). a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Cross section of vorticity and c position of polar front jet stream at 32.5 latitude. d Frontogenesis function at 850 hPa and e moisture flux at mean of 1000 to 700 hPa

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This pattern is one of the common patterns of precipitation forming in the Arabian region. In this pattern, the mean of humidity flux condition indicated that there is a moisture flux divergence in central to western areas of Iraq and north of Arabian Peninsula at 1000 to 700 hPa levels. Discharging the humidity of this area leads to lack of precipitation in central to western areas of Iraq, north of Arabian Peninsula, and even east of Syria (Fig. 3e). Based on Samadi’s (2011) results, aforementioned areas are the main sources of Arabian dust storms. Frontogenesis function also showed that frontogenesis condition is extended from internal areas of Saudi Arabia to east and south of Iraq. Considering sufficient moisture neither over Iraq nor in the north of Arabian Peninsula, the instability and gusty winds create dust storms in the region. Sometimes the dust events that result from the setting of this pattern in the west Asia create extremely intensive dust, and sometimes the dust storm intensity is quite limited and affects few parts in the west and southwest of Iran.

Next, the 20 cases of dust events with different intensity and distribution patterns in west and mid-west of Iran have been selected to explain the different variables which are affecting dust generation in the Arabian region (Table 1). The intensity of each events is classified according to the amount of horizontal visibility. Dust haze, blowing dust, dust storm, and dust devil are the main classifications with corresponding visibilities of about 10 km, 1–10 km, 0.5–1 km, and < 0.5 km, respectively (Goudie and Middleton 2006).

Table 1 The atmospheric parameters and intensity of each Arabian dust event which generated due to trough pattern
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Based on Table 1 in the trough pattern, the factor which leads to dust generation in different distribution pattern is the intensity of atmospheric system. The more intensive divergence in mid-levels of atmosphere leads to more instability and uplift in these levels. It also causes the more intensive surface convergence. The aforementioned conditions would directly affect dust intensity in the study area. The other circumstance which affects dust formation in the Arabian is the lack of precipitation in the areas that are well-known as dust sources. In some of the dust events that moisture flux convergence happens in the dust source regions, this condition does not lead to precipitation in the sources. Therefore, lack of precipitation and existence of instability in surface and mid-levels of atmosphere, leads to generation of dust in the Arabian region arid areas. Persistent duration of atmospheric system over dust storm sources is not as important as other factors in dust storm intensity in this region.

Blocking pattern

Blocking is a large scale and quasistatic phenomenon which blocks the westerly wave path and its emigrate systems such as cyclones and fronts, and make them more permanent (Habibi 2008).

Blocking pattern in Iran often causes heavy and permanent precipitation and intensive cold weather. However, regarding to the system position it may cause less precipitation or even higher temperature (Dargahian et al. 2014). Khoshakhllagh et al. (2012) showed that blocking system based on their setting location, sometimes causes dust event in the Iraq and Syria and consequently west of Iran. During period of 2000–2013, only 5% of dust events in the west of Iran were the result of blocking pattern which its intensity was in the class of dust storm or blowing dust and its impact extension was limited to west and southwest of Iran.

In this pattern, in mid-levels of atmosphere, at least for a 6-day period, setting of a quasistatic system influences a large area from the Atlantic Ocean to the East of Europe. The anticyclone center setting in the west of Europe and two cut of lows in its right and left sides, affect Atlantic Ocean and south East of Europe. The blocking systems were recognized based on Eq. 2 (Fig. 4a). Setting a cut of low in south eastern Europe, the western areas of the Middle East have been affected by emigrate systems of westerly winds and frontogenesis (Fig. 4b) and create divergence in mid-levels of atmosphere as well as a strong low pressure center in the lower levels over the East of Mediterranean to East of Syria. This condition leads to instability over desert areas of western Iraq and east of Syria which considering the moisture divergence and lack of precipitation in these areas, dust storm in Arabian region is generated.

Fig. 4
figure 4

Frontal pattern (blocking). a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Frontogenesis function at 850 hPa. c Moisture flux at mean of 1000 to 700 hPa

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None-frontal

This pattern has not generated strong dust in west of Iran, and dust events which are resulted from this pattern rank in blowing dust class. In this pattern, neither frontogenesis happens (Fig. 5c) nor there is a polar front jet stream which causes instabilities in the Arabian region dust storm sources. In this pattern, at 500-hPa level, a vast area from the west of Iran to central Arabian Peninsula is affected by a ridge, and such condition makes stable condition in mid-level of troposphere in the Arabian region (Fig. 5a). However, dust generation is due to the regional circulation system in the lower level of troposphere. At 850 hPa, the existence of an anticyclone tongue in the northwest to the east of Arabian Peninsula leads to formation of an anticyclonic circulation in this region. On the other side, there is a cyclonic circulation over the northwest of Iran. This condition makes a vorticity gradient and wind speed increasing in the central and southeast of Iraq (Fig. 5b). As the result, this mechanism makes limited dust in the aforementioned areas. This pattern generates 5% of the Arabian dust events in the west of Iran.

Fig. 5
figure 5

Non-frontal pattern. a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Geopotential height (contour) and vorticity (shaded) at 850 hPa. c Frontogenesis function at 850 hPa

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Warm period patterns

In the summer, entire regions in southwest of Asia in upper troposphere are dominated by an anticyclonic circulation pattern resulted from a subtropical high pressure (STHP). This quasistatic anticyclone affects a vast area over the Middle East and Arabian region. Additionally, subtropical jet stream which locates over the northern parts of Iran, affects extensive parts of southwest region of Asia (Mofidi 2007). The existence of such condition leads to a clear and stable atmosphere in this region.

In summer, the earth surface thermal condition plays an essential role in instability formation in desert areas of southwest of Asia (Notaro et al. 2013; Hamidi et al. 2013). In some cases, when the surface thermal systems lead to formation of Shamal winds and/or a thermal-dynamic synthetic system in the Middle Eastern dust sources, they may cause dust generation over the region (Ranjbar and Azizi 2012). Khoshakhllagh et al. (2012) indicated that in dust events periods, the surface temperature in the southwest of Asia, especially in dust sources, was increased. Such condition leads to increase the earth surface thermal patterns, and to decrease the relative humidity in comparison to long-term average (1981–2010).

Shamal wind pattern

Summer pattern

According to Mofidi and Jafari (2011) findings, three regional circulation systems in lower levels of troposphere, play important roles in formation of Shamal winds. These systems are Saudi Arabian anticyclone, Turkmenistan anticyclone, and Zagros thermal low. Depending on the intensity of each pressure system, the intensity of the dust event varies.

In this pattern, the existence of STHP over the southwest Asia at mid-levels, leads to air mass subsidence in this region (Fig. 6a). In the lower level (850 hPa), the Arabian anticyclone is in its summer position over central and eastern areas of the Arabian Peninsula. There is also an anticyclone in the east of Caspian Sea which is called Turkmenistan anticyclone. Based on Mofidi and Jafari (2011) findings, this anticyclone is important in formation of dust in the Arabian region by enhancing the Zagros thermal low intensity. The southward movement of Turkmenistan anticyclone enhances the Zagros low which settles in the west and southwest of Iran parallel to the Zagros Mountains orientation. Therefore, the Zagros low is the other atmospheric system which is influential in the generation of dust in this pattern (Fig. 6b).

Fig. 6
figure 6

Shamal wind pattern—summer. a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Geopotential height (contour) and vorticity (shaded) at 850 hPa. c Wind speed and direction at 850 hPa

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By enhancing the Zagros low associated with Arabian anticyclone, the vorticity gradient is enhanced between the western Iran and northeastern of Arabian Peninsula and south of Iraq. This enhancement increases the wind speed and makes the northwesterly Shamal winds in this region (Fig. 6c). The events which are created by this pattern are ranked in dust devil, dust storm, and blowing dust classes. Totally, 24.6% of the Arabian dust events belong to this pattern.

Transitional pattern

This pattern related to the transitional period of year from cold period to warm period. Also, in this pattern at the high and mid-levels, an anticyclone system affects the middle and southern parts of the Middle East and makes a stable atmosphere in these regions (Fig. 7a). Mofidi and Jafari (2011) mentioned that in transitional period of the year, three summertime atmospheric systems including the Saudi Arabian anticyclone, Turkmenistan anticyclone, and Zagros low did not settle in their summer positions yet. At this time, Zagros low is settled in southern parts of Zagros Mountains and shows less strength. While the Turkmenistan anticyclone does not reached to its maximum strength, it may enhance Zagros low by its southward movements. The Arabian anticyclone does not reach to its summer position over east of Arabian Peninsula either. This pattern generates dust when Zagros low strengthen by southward movement of Turkmenistan anticyclone (Fig. 7b). So that a vorticity gradient generates between the Zagros low and the Arabian anticyclone over the northeastern of Arabian Peninsula and south of Iraq. Such condition leads to rise of wind speed and makes northwest–southeast Shamal wind (Fig. 7c) and dust event in the desert lands of the Arabian region. Resulted dust events from this pattern are ranked as dust haze and blowing dust classes, and they mainly affect southwest of Iran. This pattern generates 6.5% of the Arabian dust events.

Fig. 7
figure 7

Shamal wind pattern—transitional. a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Geopotential height (contour) and vorticity (shaded) at 850 hPa. c Wind speed and direction at 850 hPa

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Western trough pattern

In this pattern, existence of a mid-troposphere trough is the most important system in generating of dust in the Arabian region. The existence of trough in the south of Black Sea leads to enhance the mid-level divergence over Iraq and Syria, and also strengthen the surface low pressure and increases the atmospheric uplift over the region. Therefore in this pattern, formation of the surface systems in lower levels of the atmosphere is affected by extension of atmospheric systems at mid-levels. In warm period of year, at the sea level, south of Iran and Persian Gulf are affected by a thermal low pressure system and tongue of it progresses to the northern parts of Iraq as well. By extension of a divergence system to mid-levels, this thermal low pressure system is developed and enhanced. Thus, its tongues progress to the north of Iraq, and in some cases to the south of Turkey and initiate a thermal-dynamic synoptic system (Fig. 8a). At 850 hPa level, for the reason of mid-level trough extension, a low center, low tongue, as well as a cyclone are formed in the northern part of Iraq and east of Syria. In this pattern, emigrate systems of westerly winds are important factors in the formation of high speed winds in the region. At this time, two other summertime circulation systems in the southwest of Asia, including Saudi Arabian anticyclone and Turkmenistan anticyclone, play an important role in formation of Arabian dust. Turkmenistan anticyclone with its southward movement enhances the extended cyclone over Iraq and Arabian anticyclone by setting in northeast of Arabian Peninsula makes a vorticity gradient and increase of wind speed in the northern parts of Arabian Peninsula and south of Iraq (Fig. 8b). Therefore, the dynamic instabilities and surface high-speed winds cause the lifting of loose surface particles from the desert areas of Iraq, east of Syria, and north of Arabian peninsula, and finally transfer them to the west of Iran (Fig. 8c). The dust events which are created by this pattern, rank as dust storm and dust devil classes, and affect a vast area in western part of Iran: 14.6% of the Arabian dust events belongs to this pattern.

Fig. 8
figure 8

Summer trough. a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Geopotential height (contour) and vorticity (shaded) at 850 hPa. c Wind speed and direction at 850 hPa

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Synthetic pattern of Shamal wind and summer trough

In this pattern, dust is created by the impact of a trough which is extended in mid-levels of atmosphere as well as existing the three previously mentioned regional systems including Zagros low, Saudi Arabian anticyclone, and Turkmenistan anticyclone which are contributed in formation of Shamal wind pattern. By extension a trough at mid-level that its axis is mainly located over Iraq to the northeast of Arabian Peninsula, a divergence at mid-levels strengthens and affects lower level circulation systems as well. So that surface convergence strengthens, and a strong cyclone forms over Iraq and Syria. Also, at sea level, a tongue of low pressure over the Persian Gulf extending to the northern areas of Iraq and this condition generally forms instability and uplift at lower to the mid-levels of the atmosphere. This condition which associated with existence of pressure gradient at surface causes increase of wind speed in Arabian dust sources. Therefore, a pattern similar to trough pattern takes place in the Middle East (Fig. 9a).

Fig. 9
figure 9

Synthetic pattern of Shamal wind and summer trough. a Sea level pressure (shaded) and geopotential height (contour) at 500 hPa. b Geopotential height (contour) and vorticity (shaded) at 850 hPa. c Wind speed and direction at 850 hPa

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On the other hand, in 850 hPa level, there is a strong synthetic low which settles over Iraq and extended to the south of Iran which along with the Arabian anticyclone that settles in the eastern and northeastern parts of Arabian Peninsula provides a condition for an intensive vorticity gradient in the Northern parts of Arabian Peninsula, south of Syria, and southern half of Iraq to the southwest of Iran. This condition leads to the increase of wind speed and generates dust events in these regions (Fig. 9b, c). Generated dust storms by this pattern mainly rank in dust devil and dust storm classes, and 11.8% of the Arabian dust events in 2000–2013 periods belong to this pattern.

Conclusion

This study identified the atmospheric circulation patterns which lead to generation of dust events in the Arabian region and west of Iran. These patterns are classified into frontal and non-frontal patterns, as cold period patterns and Shamal wind and also summer systems of westerly winds as warm period patterns.

Frontal pattern as a cold period patter is divided into two sub-patterns including Trough and Blocking. These two patterns are the main patterns of dust generation in November to May in the Arabian region. This pattern is one of the common patterns of precipitation forming in the west of Asia, but discharging and lack of the humidity in this area lead to lack of precipitation in dust storm source areas and cause dust generation in these areas. In non-frontal pattern, dust generation is due to the regional circulation system at the lower level of troposphere.

In summer, Shamal wind plays an important role in dust events in the west and southwest of Iran. The Saudi Arabian anticyclone, Turkmenistan anticyclone, and Zagros thermal low play the key role in the formation of regional scale Shamal wind and consequently dust events in Iraq and the west of Iran.

Western summer trough is another pattern of dust generation in the Arabian region, and this thermal–dynamic system causes instability and increase of wind speed in Iraq, Syria, and north of Saudi Arabia and initiates dust generation in deserts of this region. Finally in synthetic pattern of Shamal wind and summer trough, dust is created by the impact of a trough which is extended in mid-levels of troposphere as well as existing the summer regional circulation systems at lower levels of troposphere which are contributed in formation of Shamal wind pattern.

Results of this study indicated that the atmospheric circulation patterns that lead to dust events in the Arabian region and western Iran are classified into two systems in synoptic and regional scales. Synoptic circulation systems mainly lead to dust generation in the cold period of the year. The frontal pattern is classify in circulation systems in synoptic scale. Another scale is regional circulation systems which mainly lead to formation of dust in this region during the warm period of the year. However, sometimes regional and synoptic circulation systems are combined and lead to dust formation of dust in the southwest of Asia and consequently west of Iran.