# Effects of Monsoon, Shamal and Levar winds on dust accumulation over the Arabian Sea during summer - The July 2016 case

Alireza Rashki, Dimitris G. Kaskaoutis, Abbas Mofidi, Fanny Minvielle, Isabelle Chiapello, Matthieu Legrand, Umesh Chandra Dumka, Philippe François

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## To cite this version:

Alireza Rashki, Dimitris G. Kaskaoutis, Abbas Mofidi, Fanny Minvielle, Isabelle Chiapello, et al. Effects of Monsoon, Shamal and Levar winds on dust accumulation over the Arabian Sea during summer - The July 2016 case. Aeolian Research, 2019, 36, pp.27-44. 10.1016/j.aeolia.2018.11.002. insu-03686282

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https://insu.hal.science/insu-03686282v1

Submitted on 11 Aug 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Effects of Monsoon, Shamal and Levar winds on dust accumulation over the
Arabian Sea during summer – The July 2016 case

A. Rashki¹, D.G. Kaskaoutis²*, A. Mofidi³, F. Minvielle⁴, I. Chiapello⁴, M. Legrand⁴, U.C., Dumka⁵, P. Francois⁴

¹Department of Desert and Arid zones management, Ferdowsi University of Mashhad, Mashhad - 91735, Iran

²Institute for Environmental Research and Sustainable Development, National Observatory of Athens, 11810 Athens, Greece

³Department of Geography, Ferdowsi University of Mashhad, Mashhad - 91735, Iran

⁴LOA, University of Lille-1, 59655 Villeneuve d'Ascq, France

⁵Aryabhatta Research Institute of Observational Sciences, Nainital – 263 001, India

*Corresponding Authors: Dimitris G. Kaskaoutis Email: dimitriskask@hotmail.com, dkask@meteo.noa.gr
# 1 Abstract

This study examines the atmospheric circulation patterns that are associated with contrasting wind regimes, desert dust outflows and accumulation of dust over the Arabian Sea in the summer season. The examined period is 11 - 16 July 2016, when several dust plumes of various intensity and from different sources (Arabian Peninsula, Somalia and Iran), affected the Arabian Sea. During summer, south Asia is under the influence of the southwest monsoon, with main characteristics being the thermal low-pressure system centered over northwest India/Pakistan with secondary lows over south Iran and southeast Arabian Peninsula, and the Arabian ridge combined with the Indian trough in the mid troposphere. Apart from the dominant southwest monsoon flow over the Arabian Sea, strong local winds are developed over the desert areas, such as the northwest Shamal along the east Arabian Peninsula and northern Levar in southeast Iran. These wind regimes initiate several dust outbreaks, while their convergence over the Arabian Sea results in accumulation of large amounts of dust aerosols ($AOD > 2.0$), as observed by satellite (MODIS, MISR, CALIPSO) observations and reanalysis (MERRA-2). The dust-accumulation area (central/north part of the Arabian Sea) is characterized by weak winds, while the position of the Inter-Tropical Discontinuity (ITD) plays an important role in merging the contrasting dust plumes. The vertical profiles of the monsoon, Shamal and Levar winds are especially important for the dust accumulation, since the low-altitude dust plumes come from the southeastern Arabian Peninsula and Somalia, while those above 3000 m from northern directions (Iran, Pakistan).

23

24 **Keywords:** Dust; monsoon; Shamal; Levar; ITD; Arabian Sea

25

# 26 1. Introduction

The southwestern part of the Asian continent (Middle East, Arabian Peninsula and Iran-Pakistan-Afghanistan, IPA) contains several deserts (Syrian-Iraqi desert, Rub-Al-Khali, An-Nafud, Al-Dahna, Karakum, Margo, Registan) and semi-desert areas (Iranian Plateau, Sistan) responsible for large emissions of dust aerosols that are usually accumulated over the Arabian Sea (e.g., Prijith et al., 2013; Hamidi et al., 2014; Kaskaoutis et al., 2014; Jin et al., 2016). This area is characterized by a complex topography with mountains up to ~5 km, extended plateau, deep valleys and drainage basins. The dust activity exhibits a distinct seasonality, maximizing during late winter and spring over the Israeli deserts, Syrian-Iraqi plains and northwest Saudi Arabia (Kutiel and Furman 2003; Yu et al., 2013; Notaro et al., 2015), and in June - August over the southeastern Arabian Peninsula and IPA (Rezazadeh et al., 2013; Mashat et al., 2017). Long-term satellite observations revealed large inter-annual and intra-seasonal heterogeneities in aerosol distribution over the Arabian Sea during the summer season, due to remarkable changes in dust lifecycle (i.e.,
emission, transport, accumulation and deposition), which are strongly related to certain meteorological phenomena and yearly changes in the atmospheric circulation patterns (Kaskaoutis et al., 2012; Jin et al., 2018). Several studies have shown that the dust radiative heating may favor the low-level cyclogenesis over the south Arabian Peninsula, leading to further intensification in dust emissions, changes in atmospheric stability and modulation of the monsoon circulation and rainfall over India (Das et al., 2015; Jin et al., 2015, 2016; Solmon et al., 2015; Kaskaoutis et al., 2018a).

The synoptic and large-scale meteorology strongly affect the dust emissions from the Arabian Peninsula and IPA (Barkan and Alpert, 2010; Najafi et al., 2014; Yu et al., 2015; Awad and Mashat, 2016; Hermida et al., 2018). Recent research revealed that changes in the climatic Caspian Sea – Hindu Kush Index (CasHKI) modulate the weather conditions (temperature, wind speed) and dust activity over the Sistan Basin in southeast Iran and surrounding areas (Kaskaoutis et al., 2016, 2017). Furthermore, large-scale atmospheric circulation seems to affect the climatic conditions (precipitation distribution, drought) over the Middle East and southwest Asia (Mariotti, 2007; Zoljoodi and Didevarasl, 2013; Notaro et al., 2015). The La-Nina phase, combined with negative Pacific Decadal Oscillation (PDO), is usually associated with anomalous anticyclonic circulation, prolonged droughts and intensification of the Shamal wind over the Middle East and the Arabian Peninsula, favoring the dust activity (Yu et al., 2015, 2016). Beyond the synoptic and large-scale dynamics, local dust storms in the form of Haboobs, are strongly impacted by short-scale phenomena, like frontal systems, low-level jets (LLJs) and development of local thermal lows (Miller et al., 2008; Alizadeh Choobari et al., 2013; Awad and Mashat, 2013; Najafi et al., 2017).

Numerous studies have examined the dust lifecycle, emission sources, optical characteristics and transport pathways along with the associated meteorological patterns over the Middle East, Arabian Peninsula and surrounding oceanic regions via the synergy of ground-based measurements, satellite observations, model (NMMB/BSC, WRF-Chem, CAM5, CHIMERE, SKIRON, RegCM4, SPRINTARS, Meso-NH) simulations and re-analysis (Huang et al., 2007; Badarinath et al., 2010; Aloysius et al., 2011; Maghrabi et al., 2011; Alharbi et al., 2013; Das et al., 2013Kalenderski et al., 2013; Saeed et al., 2014; Sannazzaro et al., 2014; Alobaidi et al., 2016; Basart et al., 2016; Farahat et al., 2016; Jose et al., 2016; Kalenderski and Stenchikov, 2016; Bou Karam et al., 2017; Sijikumar et al., 2017). In addition, Fattahi et al. (2012) classified the synoptic patterns associated with dust storms in southwest Iran via principal component analysis, while Al-Jumaily and Ibrahim (2013) and Beegum et al. (2018) examined the synoptic meteorology favoring the formation of various dust storms in Iraq and Saudi Arabia, respectively. In a similar way, Hamidi et al. (2013) analyzed the atmospheric circulation patterns associated with dust events in the Middle East between 2003 and 2011, Awad and Mashat (2014) and Mashat et
al. (2017) examined the synoptic characteristics favoring widespread dust plumes in spring over central-east and southwest Saudi Arabia, respectively. Kaskaoutis et al. (2015a) studied the dynamic and synoptic meteorology linked with summer dust storms over Sistan from 2001 to 2012, highlighting the important role of the Levar wind.

The summer season in the southwestern part of Asia is characterized by the development of three distinct wind regimes, i.e. (i) the predominant southwest monsoon over the Arabian Sea, (ii) the northwest Shamal wind along the eastern Arabian Peninsula and, (iii) the north Levar blowing with high intensity along east Iran (Alizadeh Choobari et al., 2013, 2014; Yu et al., 2016; Bou Karam et al., 2017).

Although the numerous studies cited above have examined these wind regimes and the associated dust storms, the interrelation between them, the mechanisms and characteristics of the dust accumulation over the Arabian Sea have not been well documented. This study examines the atmospheric circulation patterns and the driving forces related with these wind regimes, the sources and transport pathways of the dust plumes, the dust accumulation over the Arabian Sea and the specific role of the Inter-Tropical Discontinuity (ITD). The analysis emphasizes on July 2016 (~11 - 16 July) using meteorological and aerosol data from ECMWF ERA-Interim and MERRA-2 reanalysis, along with satellite retrievals (MODIS, MISR, CALIPSO, Meteosat).

## 2. Background knowledge

During summertime, the dust storms from the desert areas (Syrian-Iraqi desert, Rub-Al-Khali, An-Nafud, Al-Dahna, Karakum, Margo, Registan, Sistan) in the southwestern part of Asia (see Fig. 1) are driven by strong local wind regimes that are differentiated by the position of the ITD, which lies north from equator during the boreal summer, and more specifically, along the southeast Arabian Peninsula and over the northern coast of the Arabian Sea and the Indus valley. South of the ITD, the southwest summer monsoon flow dominates with high intensities (> 20 - 25 ms⁻¹) over the southern parts of the Arabian Sea (off the Horn of Africa). The southwest monsoon intensifies at lower tropospheric levels (< ~700 hPa) and brings large amounts of aerosols and rainfall over India (Krishnan et al., 2013; Sijikumar et al., 2017). At the north side of the ITD, the Shamal and Levar winds are developed during the summer season.

Shamal is a strong northwest wind, with gusts up to 100 km h⁻¹ (Garzanti et al., 2013), blowing from the Tigris-Euphrates basin toward the eastern Arabian Peninsula and the Persian Gulf, and is responsible for massive dust storms (Fattahi et al., 2012; Al Senafi and Anis, 2015; Maghrabi and Al-Dosari, 2016; Hamidi et al., 2017). Shamal is usually generated by a strong baroclinic gradient between a semi-permanent anticyclone over northern Saudi Arabia and thermal cyclones over south Iran,
resulting in strong winds along the convergence zone (Shao et al., 2011). The relative positions and the intensity of these high- and low-pressure systems modulate the onset, duration and intensity of Shamal, which is also influenced by the combined effects of the subtropical and polar jet streams (Sissakian et al., 2013; Awad et al., 2014). In addition, large-scale dynamics, like El-Nino Southern Oscillation (ENSO) and the related changes in sea-surface temperature over the north Indian Ocean, have been found to control Shamal (Yu et al., 2015, 2016). Early onset of Shamal is mostly associated with La Niña phase, which favors the early development of the Iranian thermal low, while the El Niño phase inhibits its development. According to the climatological analysis by Yu et al. (2016), the summer “Shamal season” starts by 30 May ± 16 days and terminates by 16 August ± 22 days with wind gusts of ~30 m s⁻¹.

From June to September the east Iran is impacted by the strong (> 20 m s⁻¹) northern Levar (or 120-days) wind, which blows from the southern Karakum desert in Turkmenistan to the coastal Arabian Sea (Alizadeh Choobari et al., 2014; Kaskaoutis et al., 2015b). Despite its major importance on the climatic conditions, environmental and socio-economic issues in east Iran, there is no accepted definition for Levar or studies exploring its driving forces. Few recent works have shown that Levar is significantly related to changes in the pressure gradient between the Caspian Sea and the Hindu Kush mountains, with higher velocities under high CasHKI values (Kaskaoutis et al., 2016, 2017, 2018b). In synopsis, the deepening and expansion of the Indian/Pakistan, Arabian and Iranian thermal lows control the intensity and duration of these three wind regimes, while daily to inter-annual changes in dust activity are strongly linked to wind variations with important climate implications over the region (Notaro et al., 2013; Yu et al., 2015).

## 3. Dataset and methodology

### 3.1. Reanalysis data for meteorology and aerosols

The meteorological data for this study were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim atmospheric reanalysis. The data assimilation system for the ERA-Interim production is the 12-hourly 4-dimensional variational analysis (4D-Var) for the upper-air atmospheric state. The spatial resolution of the dataset is about 80 km on 60 vertical levels from the surface to 0.1 hPa with a time-step of 6 hours. **More specifically,** we used the mean sea level pressure (MSLP) and the geopotential height at 700 hPa (Z700) (0.5° x 0.5°), as well as the vector wind from 1000 hPa to 400 hPa at 6-hr intervals. To verify more precisely the wind structure, we used the ERA-Interim with horizontal resolution of 0.25° x 0.25°. The **analysis** provides the spatial distribution of the vector winds at various levels (e.g., 950 hPa, 850 hPa, 700 hPa) over the area 5° - 40° N, 30° - 75° E, as well as the time-altitude profiles of the monsoon, Shamal and Levar winds from 1000 hPa to 400 hPa during 11 - 16 July with 6-hr time step. More details of the
modelling and data assimilation system used to produce ERA-Interim can be found in
Dee et al. (2011).

The Modern-Era Retrospective analysis for Research and Applications (MERRA-2;
https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/) is a long-term global reanalysis
product for assimilation of space-borne observations of aerosols, clouds and several
other atmospheric and meteorological parameters. The MERRA-2 datasets are
produced via the Goddard Earth Observing System - Data Assimilation System
(GEOS-DAS, version 5.12.4) based on GEOS-5 climate model and the GSI analysis
scheme (Gelaro et al., 2017). The MERRA-2 products are given at a horizontal
resolution of 0.5° × 0.625° at 72 levels from the surface to 0.01 hPa. In this study, we
used the MERRA-2 products for meridional wind (850 hPa), dust column mass
density and fine-dust (PM₁₀ µm) scattering AOD₅₅₀. More details about aerosol
reanalysis from MERRA-2 can be found elsewhere (Randles et al., 2017; Jin et al.,
2018).

176

## 3.2. Satellite observations

Moderate Resolution Imaging Spectroradiometer (MODIS) AOD retrievals at 550 nm (Level 2 and Level 3, Collection 6.1) from both Terra and Aqua satellites were obtained over the study region. The MODIS AOD₅₅₀ over the bright desert terrains is retrieved by the Deep Blue (DB) algorithm that was combined with the Dark Target (DT) AOD₅₅₀ retrievals over ocean and dark surfaces (Sayer et al., 2014). Previous studies have validated MODIS against AERONET AODs at stations over the study region (Solar Village, Mezaira, Kuwait University), concluding in satisfactory agreement with errors within ± 20% (Remer et al., 2008; de Meij and Lelieveld, 2011; Yu et al., 2013; Nabavi et al., 2017). Validation of the DB-AOD against ground measurements has been performed by Shi et al. (2013), who reported a root mean-square error (RMSE) of 0.24 and 0.19 for Terra and Aqua DB-AODs, respectively, while an overall error for the DB AOD retrievals of ±0.05±0.20\*AOD was reported by Remer et al. (2013).

Level 3 (version 22) AOD at 558 nm was also obtained from the Multiangle Imaging SpectroRadiometer (MISR) sensor onboard NASA's Terra satellite with 0.5° x 0.5° spatial resolution during 10 - 19 July 2016 over the Arabian Sea and surrounding areas in order to support MODIS retrievals. MISR is specifically capable for the non-spherical dust monitoring via its 9 cameras (Kalashnikova et al., 2005; Yu et al., 2018), but its narrow swath of about 360 km does not allow for daily observations of the non-spherical AOD over the whole study region.

The Infrared Difference Dust Index (IDDI) values were obtained from Meteosat 7 observations centered at 57°E and were utilized as a powerful tool for the detection of dust emissions over bright desert terrains (Léon and Legrand, 2003). The IDDI values are provided over land under cloudless conditions at a spatial resolution of 5 × 5 km²
from Meteosat observations at 09:00 UTC over the Indian Ocean. More details about
the IDDI retrievals and the physical basis behind the thermal infrared (TIR)
temperature differences between surface and dust plume can be seen elsewhere
(Legrand et al., 2001; Kaskaoutis et al., 2015a, 2016; Rashki et al., 2015).

Finally, for examining the vertical characteristics of the dust plumes and the mixing of
dust with other aerosol types, the Cloud-Aerosol Lidar and Infrared Pathfinder
Satellite Observations (CALIPSO) vertical profiles for the total attenuated backscatter
(TAB) at 532 nm and aerosol subtypes were analyzed (Huang et al., 2008; Yang et al.,
2012). The vertical feature masks were obtained from the CALIPSO-CALIOP v 4.10
lidar data product (www-calipso.larc.nasa.gov/) for night-time (preferable) and
daytime overpasses over the study area during 11 – 16 July 2016.

213

214 **4. Results and discussion**

215 **4.1. Satellite observations of the dust plumes**

July 2016 was a classic dusty month over the Arabian Sea, with the period of 11 – 16
**July presenting** a specific interest due to the concurrent dust intrusions from different
sources and directions. Figure 2 shows the spatial distribution of the combined DT
and DB Terra-MODIS AOD<sub>550</sub> (Level 2; 10 km x 10 km) from 11 to 16 July 2016.
Iraqi plains, eastern and southern Arabian Peninsula exhibit increased AOD values
(~0.8 to 1.6), since they are characterized as “permanent dusty areas” during the
summer season (Cao et al., 2015; Nabavi et al., 2016), with high (72%) probability for
dusty conditions during the Shamal days (Yu et al. (2016). On 11 - 13 July 2016, a
dust storm affected this region, started from the Syrian-Iraqi desert and progressively
moved towards the south and southeast with highest AODs (>1.5) over the Rub-Al-
Khali and Oman deserts. This AOD spatial distribution **is similar to those observed**
**in** other studies (Ginoux et al., 2012; Jish Prakash, 2015), which identified the
southern Arabian Peninsula as the most active dust source in summer. On the same
days, dust plumes mostly coming from the east Sahara (Nubian desert) and Somalia
are identified via the increased AODs (~0.8 - 1.4) over the Red Sea and the Gulf of
Aden, although the extensive cloudiness over Somalia on the most of the days does
not allow for robust results. On 13 July a massive dust storm (AOD>3.0) was
generated over the Sistan Basin, which was further intensified and expanded to
neighboring regions on 14 July. This dust storm was moving southeastwards affecting
western Pakistan, the northernmost part of the Arabian Sea and the Indus Basin,
following the classic pathway of the Sistan dust storms (Rashki et al., 2015). **The**
**extensive cloudiness over the Arabian Sea does not allow for detection of areas**
**with dust accumulation by using daily L2 MODIS observations.**

The Meteosat-7 IDDI spatial distributions over the arid/desert lands during 11 – 16
July 2016 are shown in Fig. 3, superimposed by the CALIPSO tracks on each day.
IDDI is especially suitable for detecting massive dust emissions, near-surface thick

240

241
dust plumes and low-level transport of dust, but dust layers in the mid and upper troposphere usually have weak IDDI signal (Legrand et al., 2001). IDDI reveals very high thermal contrast (> 30 K) in the infrared (10.5 – 12.5 µm) bright temperature over Sistan and surrounding areas on 13 - 15 July, indicating a severe dust storm, which traversed along the Iran-Afghanistan borders and then shifted to the east and dissipated. Moderate IDDI values (~10 – 17 K) are also shown over Somalia (12 - 13 July) and Syrian-Iraqi desert (11 and 15 - 16 July), indicating weaker dust emissions. In contrast, the desert areas in the Arabian Peninsula do not provide a clear IDDI signal, although presence of dust was detected via the MODIS AODs, although weaker than that over Sistan (Fig. 2). The differences in the spatial distributions between MODIS-AOD and Meteosat-IDDI are attributed to the fact that IDDI represents the TIR contrast caused by the dust, which is different from the visible columnar AOD retrieved from MODIS. The IDDI values increase with the dust amount and the thermal contrast between land and overlying dust plume. Therefore, very thick dust plumes near the ground, emitted by a colder north wind such as Levar, result in larger thermal contrast with the underlying surface and in higher IDDI values, as also shown in previous studies (Kaskaoutis et al., 2015a, 2016; Rashki et al., 2015).

260

## 4.2. Prevailing meteorology

Analysis of the synoptic meteorology gives useful information about the atmospheric circulation patterns and wind regimes that favor the dust outbreaks over the sources around the Arabian Sea. The **near-surface** low-pressure system **over northwestern India and Pakistan, associated with the monsoon**, is the main characteristic during the study period (Fig. 4). This system is widely expanded to the west, forming secondary low cores over Sistan and the eastern Arabian Peninsula, while it progressively weakened from 11 to 15 July. In contrast, high MSLP conditions dominated over the Caspian Sea and surrounding areas, which intensified till 13 July and then weakened. High-pressure conditions also dominated over Ethiopia. The surface low-pressure system is interlinked with higher-pressure conditions over the eastern Mediterranean and the Middle East, which form a ridge at 700 hPa over the western Arabian Peninsula, resulting in a steep pressure gradient and strong northwesterly winds (Shamal) along the eastern Arabian Peninsula and the Persian Gulf (Hamidi et al., 2014). The increase in MSLP over the Caspian Sea from 11 to 13 July intensified the CasHKI values (Kaskaoutis et al., 2018b) from 4.2 on 11 July to 8.3 on 13 July. This is associated with an increase in the pressure gradient between the Caspian Sea and the Hindu Kush mountains, intensification of the Levar wind and outburst of a severe dust storm over Sistan (Figs. 2 and 3). The decline of the high MSLP over the Caspian Sea after 14 July (CasHKI = 3.9 on 15 July) signaled the termination of the Sistan dust storm (Figs. 2 and 3). During the whole period, the meteorological patterns at 700 hPa show intrusion of the monsoon trough from the
Indian sub-continent towards the central Arabian Sea, with a tendency of weakening following the surface thermal low. The combinations between the Indian trough (low Z700 values) and the ridge over the western Arabian Peninsula as well as the high Z700 around the Caspian Sea favor the formation of Shamal and Levar winds along the eastern Arabian Peninsula and eastern Iran, respectively. In this respect, Shamal was stronger (~12 – 15 ms⁻¹) on 11-12 July and till 06:00 UTC on 13 July (Figs. 5 and 6), associated with lower MSLP conditions and higher Z700 over the Caspian Sea. Similarly, positive 500 hPa anomalies over the Caspian Sea were found to be related to intensification of the Arabian ridge and stronger Shamal winds (Kutiel and Benaroch, 2002). It should be noted that Shamal may be also intensified by the presence of Zagros Mountains in southwest Iran that form a wind channeling, and by the enhancement of the pressure gradient due to heating of the elevated terrain (Zarrin et al., 2011; Yu et al., 2016).

The wind speed and direction during 11 – 15 July 2016 are shown in Fig. 5 and Fig. 6 for 950 hPa and 700 hPa, respectively and for two hours per day (06:00 and 18:00 UTC; LST=UTC+4:30) in order to investigate the diurnal cycle of the LLJ-like winds over the deserts (Bou Karam et al., 2009a). During all days, strong (>20 ms⁻¹) monsoon winds dominated at 950 hPa over the southwestern Arabian Sea (off the Horn of Africa) progressively weakening towards northeast (Fig. 5). These strong south-westerlies are able to erode dust from the coastal southeast Arabian Peninsula and Somalia and transport it over the Arabian Sea and India (Ramaswamy et al., 2018; Kaskaoutis et al., 2018a). The monsoon flow progressively decreased after 13 July, following the weakening of the low-pressure system (Fig. 4), while no significant differences in wind speed between 06:00 UTC and 18:00 UTC are found. During the study period, the Shamal wind generally ranged from ~7 ms⁻¹ (14 July) to ~18 ms⁻¹ (11 July) over the eastern Arabian Peninsula and the Persian Gulf, while intensification of the 950 hPa Shamal is noticed again on 15 July over the Iraqi plains. Therefore, the near-surface Shamal wind is capable of emitting and transport dust from the Iraqi plains and eastern Arabian Peninsula towards south/southeast. In the Karakum desert, Sistan Basin and surrounding regions, the Levar wind intensified on 13 - 14 July, following the increase in CasHKI, and reached at velocities of ~ 14 - 16 ms⁻¹ at 950 hPa. However, its impact seems to be rather local/regional, while is nearly vanished at the northern coast of the Arabian Sea. The analysis indicates a diurnal variation in Shamal wind at 950 hPa, with larger values during local night hours (18:00 UTC) compared to morning (06:00 UTC) (Fig. 5). This is due to the downward mixing of momentum from the nocturnal LLJ over the desert areas, as also found over parts of Sahara (Washington and Todd 2005; Todd et al., 2008; Bou Karam et al., 2009b; Schepanski et al., 2017). In general, this is also the case for Levar, since previous studies (Alizadeh Choobari et al., 2013; Kaskaoutis et al., 2015a) using model (WRF-Chem, RegCM4) simulations showed larger wind speeds over Sistan during nighttime. In addition, the mountainous topography around Sistan
324 was found to strongly affect the velocity and vertical profile of Levar, by enhancing
325 the wind speed near the surface and affecting the convection, shear and uplift of dust
326 (Kaskaoutis et al., 2015a). On the other hand, the intense surface heating over the
327 desert areas triggers the formation of local thermal low-pressure systems that
328 modulate the wind profile and convective activity (Saeed et al., 2014; Yu et al., 2015).

329 At 700-hPa level (Fig. 6), the monsoon flow is totally different and much weaker than
330 that at 950 hPa, with maximum velocities of 12 - 15 ms⁻¹ over parts of the southeast
331 Arabian Sea, while the winds have shifted from south-westerlies to westerlies. In
332 general, the morning winds seem to be slightly stronger than the evening ones over
333 the south Arabian Sea. At 700 hPa, Shamal is stronger than monsoon and on 11 – 12
334 July it reached at ~16 - 18 ms⁻¹ along the east coast of the Arabian Peninsula, with
335 lower intensities on the next days. There is no clear diurnal pattern in Shamal at 700
336 hPa, since on some days the morning wind speed was higher and **on other days the**
337 **wind maximized** in the evening. The Levar wind at 700 hPa, although not so intense
338 as near the surface, affects a much greater area reaching at north/central Arabian Sea.
339 This indicates that the dust storms originated from Sistan can impact the central
340 Arabian Sea in the mid and upper troposphere and remain there as elevated dust
341 plumes for several days (Kaskaoutis et al., 2014; 2015b). The current **analysis does**
342 not reveal any clear diurnal pattern at the 700-hPa Levar wind.

343 These contrasting wind regimes in view of the meridional components (monsoon
344 southerlies vs Shamal and Levar northerlies) converge over different parts of the
345 Arabian Sea (Fig. 7). The areas of zero meridional wind defines the position of the
346 Inter-Tropical Discontinuity (ITD), lying along the southeast Arabian coast and along
347 the north coast of the Arabian Sea at lower atmospheric levels (i.e. 850 hPa). Its
348 **position and intra-seasonal movement is a result of the convergence of the**
349 **opposing winds, while it is shifted southward during the retreat of the monsoon**
350 **in autumn.** In the Arabian Sea, the ITD lies over an area characterized by a large
351 pressure gradient (Fig. 4) and weak winds (Fig. 5) that enables the merging of various
352 dusty air masses and the accumulation of aerosols, leading to the formation of a “hot-
353 spot” aerosol-laden area. Some studies have addressed the ITD dynamics and the
354 influence on dry cyclogenesis and dust emissions due to the strong wind shear over
355 the Sahara (Bou Karam et al., 2008, 2009a, 2009b; Knippertz, 2008), but **these**
356 **mechanisms need** further investigation over the desert areas in southwest Asia. In the
357 literature, there is no study exploring possible associations and/or interrelations
358 between monsoon, Shamal and Levar winds. These wind regimes are driven by
359 different formation mechanisms, although they seem to be sensitive to the
360 Indian/Pakistan thermal low (Yu et al., 2016). In this respect, an intensification of the
361 monsoon flow over the Arabian Sea was found on the Shamal days, while warm
362 temperature anomalies over south Iran cause deepening of the thermal low-pressure
363 system and **enhancement of the** Shamal wind (Yu et al., 2016).
364
365 **4.3. Dust accumulation over the Arabian Sea**

366 Although the combined DT and DB L2 AOD<sub>550</sub> (Fig. 2) was capable of identifying
367 the major dust sources and plumes over land, the outspread monsoon cloudiness over
368 the Arabian Sea prevented the MODIS retrievals. Therefore, for dust monitoring over
369 the Arabian Sea and for identification of “hot-spot” areas of dust accumulation, we
370 used time-averaged (10 – 19 July 2016) Level 3 MISR (0.5º x 0.5º) and Terra and
371 Aqua MODIS (1º x 1º) AODs, superimposed by the composite mean (10 – 19 July)
372 vector winds at 950, 850 and 700 hPa from ERA-Interim (Fig. 8). The time-averaged
373 Terra and Aqua MODIS AODs are highly correlated (r = 0.88) over the study domain,
374 exhibiting mean values of 0.40 ± 0.33 (Terra) and 0.39 ± 0.34 (Aqua). In terms of the
375 combined DT and DB MODIS products, several factors may lead to the apparently
376 higher AODs over ocean compared to desert lands, such as the contribution of the
377 sea-salt aerosol, the discontinuity between DT and DB AOD, and the cloud
378 contamination especially over ocean. The MISR-AOD<sub>558</sub> shows a similar spatial
379 distribution with a domain-averaged value of 0.38 ± 0.27. However, due to narrow
380 swath, the MISR-AOD distribution suffers of large data missing, especially over the
381 cloudy central Arabian Sea. On the other hand, MISR has the advantage of providing
382 reliable dust recordings over the desert regions compared to MODIS DB
383 (Kalashnikova et al., 2005; Yu et al., 2018), exhibiting high AODs over the Rub-Al-
384 Khali, southern Iraq and Sistan. The **satellite sensors** reveal high dust accumulation
385 (AODs ≈ 1.5 – 2.5) over the central/north part of the Arabian Sea (~15º – 24º N, 60º –
386 67º E), an area characterized by weak winds and merging of the monsoon, Shamal and
387 Levar flows along the position of the ITD (Fig. 8). Therefore, this region can be
388 characterized as a “hot-spot” aerosol-laden pool impacted by different dust sources in
389 Somalia, Arabian Peninsula, Iran and Pakistan. However, we should not ignore the
390 influence of the sea-salt aerosols **produced by the intense monsoon winds at the**
391 **lowest boundary layer**, but their presence is mostly limited in the strong monsoon
392 area south from ~17ºN. In addition, the transported dust plumes may be coated with
393 continental pollution and, therefore, a mixing of various aerosol types is expected
394 over the “hot-spot” area, but with a clear dominance of dust, as the CALIPSO profiles
395 of the aerosol subtypes verify (see Suppl. Fig. 1). The high AODs (~1.2 – 1.6) over
396 the Indus Basin, Pakistan are attributed to the **mix** of dust with large anthropogenic
397 emissions (Alam et al., 2014).

398 The presence of the ITD near to the north coast of the Arabian Sea and the
399 convergence between the north Levar and the southwest monsoon (Fig. 8), is able to
400 uplift the dust plumes coming from the north at elevated heights (Kaskaoutis et al.,
401 2015b; Rashki et al., 2015), thus forming multiple aerosol layers in the vertical. The
402 frequency plots for the 4-days air mass back-trajectories obtained from the
403 HYSPLIT model (Stein et al., 2015) verify the different origin of the **dusty** air masses
404 between the lower (500 m, **1000 m**) and upper (**3000 m**) levels over the north Arabian
405 Sea (21º N, 62º E) (Fig. 9). Within the marine boundary layer, the air masses
originated from southwest driven by the strong monsoon flow, while at **3000 m** they
came from the north, traversing the arid/desert areas of **IPA. At 1000 m, the highest
frequency of the back-trajectories is detected over the Shamal-influenced area
along the eastern Arabian Peninsula.** Therefore, the dust plumes from Somalia and
southeast Arabian Peninsula, mixed with sea-salt, are expected to be accumulated at
the lower atmospheric levels in the “hot-spot” area, while **the Shamal** and Sistan dust
storms driven by Levar are mostly detected at higher levels (~ **1 and 3 km,**
respectively). Very similar results regarding back-trajectory pathways over the
aerosol “hot-spot” area in the Arabian Sea during summer are reported by **Jin et
al. (2018).** They found that the air masses coming from Somalia and southeastern
Arabian Peninsula at **500 m** constitute the ~50 - 90%, with a small contribution
(~7 – 26%) from the Shamal-influenced area. Furthermore, **Jin et al. (2018)**
reported that at **5000 m,** the vast majority of the air masses originated from the
**IPA region, in agreement with our case.** These dust plumes may remain for several
days over the Arabian Sea, contributing to the mid-troposphere warming and
**modulation in monsoon rainfall over India** (Solmon et al., 2015; Jin et al., 2016).
Dust deposition over the marine environment affects the nutrient balance via
deposition of iron, phosphorus and nitrogen that support phytoplankton, and
chlorophyll blooming (Singh et al., 2008; Richon et al., 2018). Furthermore, deposited
dust may have serious effects on agriculture and forest ecosystems in the Zagros
Mountains (Moradi et al., 2017; Varnosfaderani et al., 2017).

427

4.4. Dust - wind interrelations

In this section, we interrelate the satellite AOD distributions with horizontal and
vertical profiles of the dominant winds (monsoon, Shamal, Levar) at specified
domains over the study region. The winds correspond to 6-hr datasets from ERA-
Interim reanalysis, while the daily Terra and Aqua MODIS AODs were interpolated
to the same 6-hr intervals during 11 – 16 July 2016. In this respect, Figure 10a shows
combined plots of AOD<sub>550</sub> with vector wind at 925 hPa over specified domains of the
Arabian Sea (see Fig. 8a) impacted by monsoon. The AOD shows a remarkable
south-to-north **increasing gradient** (from 5° N to 25° N), with highest values (> 2.0)
on 15 and 16 July, when the various dusty air masses merged over the “hot-spot”
aerosol-laden area (see Fig. 8). The days and **the** area of maximum AODs are
characterized by weak winds (below 5 ms<sup>-1</sup>), while the southern latitudes **influenced**
by the strong monsoon flow (>20 ms<sup>-1</sup>) exhibit AODs ~0.6, indicating dusty-
background conditions. Note also the strong gradient of decreasing monsoon wind
along 16° N – 19° N, where the wind speed from ~20 ms<sup>-1</sup> drops to ~5 ms<sup>-1</sup>, defining
the position of the ITD, as verified in Fig. 8a. Figure 10b emphasizes on the area with
the highest monsoon (D3 in Fig. 8a) showing the time-altitude wind profile. Monsoon
is especially strong around 950 - 900 hPa with wind gusts of ~24 ms<sup>-1</sup> and is
practically vanished **above 700 hPa (<6 ms<sup>-1</sup>**). No remarkable tendency in daily and
diurnal monsoon variability is shown during the examined days, while the monsoon's intensity slightly lowers in the evening.

The longitudinal AOD variability **averaged over** the latitudes 23° N – 30° N (D1 in Fig. 8b) reveals a remarkable west-to-east increase due to the southeast movement of the Shamal dust plumes and their convergence with the monsoon air masses and dust storms from Sistan over the “hot-spot” area on 15 - 16 July (Fig. 11a). The winds in the D2 (see Fig. 8b), shown as contours in Fig. 11a, are rather moderate (~6 - 11 ms⁻¹), with higher intensities on 11 - 12 July at 45° E – 50° E (Shamal influence) and on 13 - 14 July at ~62° E (southern residues of Levar). In addition, a gradient of decreasing wind is seen at ~63° E, along the northern flanks of the ITD over the south Indus basin, favoring the aerosol accumulation (see Fig. 8b). The vertical profile of Shamal (Fig. 11b) over the D3 (Fig. 8b), reveals decreasing intensities after 13 July, especially at levels above 800 hPa, while on 11 - 12 and 14 July, Shamal winds of ~10 ms⁻¹ are also seen between 700 - 600 hPa. In general, the **stronger** Shamal is shown around 950 - 900 hPa, exhibiting a remarkable diurnal cycle with maximum **velocities** during the nighttime, due to downward mixing of momentum from the nocturnal LLJ (Bou Karam et al., 2009b).

Figure 12 focuses on the north-eastern part of the Arabian Sea and the arid IPA region, where Levar wind and Sistan dust storms dominate (Kaskaoutis et al., 2015b; Rashki et al., 2015; Behrooz et al., 2017). The strong Levar (~20 ms⁻¹) at 850 hPa around 32° N – 33° N (Fig. 12a) practically vanishes along the north coast of the Arabian Sea (~25° N) and the high aerosol laden area is dominated by weak winds preventing dilution and favouring accumulation (AOD> 2.0). Figure 12a reveals a strong impact of the Sistan dust storm on 14 July on the AOD values over the northeast Arabian Sea (< 25° N) on 15 - 16 July. This justifies previous results (Kaskaoutis et al., 2015b), which revealed that the Sistan dust storms usually need 1 - 2 days to reach the north Arabian Sea. Focusing on the strong-Levar domain (D3 in Fig. 8c), the vertical profiles **show a large increase in the Levar intensity** on 13 - 14 July (as also seen in Figs. 5, 6). Levar maximizes at ~850 hPa and exhibits larger intensities during the nighttime, as also seen in previous studies (Alizadeh Choobari et al., 2013; Kaskaoutis et al., 2015a), while the local topography may strongly modulate the wind profiles and, therefore, detailed analysis with fine spatial resolution is needed.

Figure 13 summarizes the time-vertical profiles of the total attenuated backscatter (TAB) coefficient at 532 nm for nighttime and daytime (12 July) CALIPSO overpasses over the study region (see Fig. 3). In general, the vertical extend of the dust plumes was up to ~5 km over parts of the Arabian Peninsula, the Arabian Sea and **IPA region**. Very high TAB values (~4 – 6x10⁻³ km⁻¹sr⁻¹) occurred at lower tropospheric levels (< 2 - 3 km) over Sistan and surrounding regions on 13 and 15 July (so the high IDDI in Fig. 3). In contrast, the dust plumes over the central/eastern Arabian Peninsula were not so intense and were vertically expanded to higher
altitudes on 11, 14 and 16 July, thus justifying the very low signal in IDDI values (Fig. 3). CALIPSO profiles over the Arabian Sea reveal the presence of a rather permanent dust-aerosol layer extending up to ~4 - 5 km during the summer season (Prijith et al., 2013), which contributes about **38% – 54% to the aerosol loading** (Jin et al., 2018). The time-vertical CALIPSO profiles for the aerosol subtypes (Suppl. Fig. 1), show that the dust and polluted/dust aerosol types (yellow and brown, respectively) dominated over the study region. The polluted/dust type (dust **mixed with** anthropogenic **emissions**) dominated at northern latitudes (Turkmenistan and central Asia), as well as in parts of the southeast Arabian Peninsula and the Arabian Sea due to emissions from the oil refineries.

On certain days, the CALIPSO TAB profiles reveal the existence of multiple aerosol layers in the vertical (white circles in Fig. 13), which are examined **using** the time-altitude cross sections of the wind along the CALIPSO overpasses (Fig. 14). On 12 July, two distinct dust-aerosol layers lying near the surface (below ~1.5 km) and between 3 and 5 km, are observed over the area ~23.5° – 25° N, 63° – 64° E. The lower dust plume **was** associated with the monsoon flow, carrying dust from Somalia and the southeast Arabian Peninsula, **while** the upper one **corresponds to** dusty air masses from the north. This was justified by the air-mass back trajectories (Fig. 9), while the vertical wind profile (Fig. 14) shows weak (< 4 ms⁻¹) intensities over this region, identifying the position of the ITD and the “hot-spot” aerosol-laden area. A similar case is observed on 15 July over a neighboring area (~21° – 23° N, 62° E) in the north Arabian Sea, where dusty air masses from opposing sectors merged, **resulting in** a high AOD>2.0 in the vertical (Figs. 8, 10-12). The wind profiles along the CALIPSO tracks reveal opposed meridional wind components between monsoon, Shamal and Levar with distinct areas of weak winds in between, defining the position of the ITD (Fig. 14). Furthermore, **the** areas impacted by **the** Shamal or Levar winds (highlighted in Fig. 14) reveal increased TAB values near the surface (Fig. 13), **indicating** dust emissions of various intensities. In addition, the increased TAB values near the sea surface in the southwestern parts of the Arabian Sea on 11, 13 and 15 July (Fig. 13) are mostly composed by marine (sea salt) aerosols (see Suppl. Fig. 1), produced by the intense monsoon winds (> 20 ms⁻¹, see Fig. 14).

**The MERRA-2 spatial distributions of** the columnar dust mass density (in g m⁻²) and scattering AOD₅₅₀ for the fine-mode (PM₁) dust, averaged during 11 – 16 July 2016, are shown in Figure 15a, b, superimposed by the 850 hPa and 700 hPa vector winds, respectively. The supplementary Figures 2 and 3 provide the respective daily **spatial distributions** of these parameters. MERRA-2 **reanalysis** shows **great similarities** in the spatial distribution for both parameters, with highest time-averaged values over the Rub-Al-Khali and Oman deserts and secondary ones over the Ad-Dahna desert and the Indus Basin in Pakistan. Moderate-to-high values of **the** dust loading and fine-dust scattering AOD₅₅₀ are also shown over the Iraqi desert, east Sahara, south Red Sea, Gulf of Aden, Persian Gulf and along the western and
northern coasts of the Arabian Sea due to the strong continental influence. MERRA-2 seems to overestimate the dust load and scattering over the Arabian Peninsula compared to Sistan, in contrast to satellite (MODIS, MISR, CALIPSO) observations, while it satisfactorily detects the “hot-spot” aerosol area over the north Arabian Sea. However, the MERRA-2 reanalysis product corresponds only to dust, in contrast to the satellite sensors that detect the whole atmospheric aerosols. Furthermore, the dust-scattering AOD$_{550}$ (~0.03 – 0.18) is much lower than the AODs (>2.0) from MODIS and MISR, since it corresponds only to light scattering from the fine (PM$_{1}$) dust particles, which contributes much lesser to the total aerosol extinction (Dumka and Kaskaoutis, 2014). On the other hand, the MERRA-2 daily reanalysis (Suppl. Figs. 2, 3) represent pretty well (i) the south-eastwards propagation of the Shamal dust plumes driven by stronger winds on 11 – 13 July and the increased dust loading over the Rub-Al-Khali on the same days, (ii) the intense Sistan dust storm and its south-eastward propagation on 13 – 14 July, (iii) the high dust loading over Pakistan on 15 - 16 July, (iv) the new dust emissions along the Arabian Peninsula on 15 - 16 July, which are consistent with the daily MODIS AOD observations (Fig. 2). This renders the MERRA-2 aerosol reanalysis as a powerful tool for examining the dust-aerosol properties and lifecycle over the south Asia, although the MERRA aerosol data underestimate the dust AOD compared to satellites (Jin et al., 2018).

548

## 5. Conclusions

This study examined the synoptic meteorological patterns and the wind regimes that are associated with high accumulation of dust over the central/north part of the Arabian Sea during the summer season. 11 – 16 July 2016 was selected as a representative period, since contrasting dust plumes of various intensity, originated from the deserts in the Arabian Peninsula, Somalia and Sistan (Iran), impacted the Arabian Sea. The Indian/Pakistan thermal low-pressure system, extended over south Iran, north Arabian Sea and the Arabian Peninsula with secondary lows dominated at sea level, while high-pressure systems existed over the Caspian Sea, east Mediterranean and east tropical Africa. At 700 hPa, an extent ridge of southwest-to-northeast axis dominated from east Sudan and the southwest Arabian Peninsula to Iran, also connected with the high-pressure system over the Caspian Sea, while the Indian sub-continent was affected by the summer monsoon trough. These atmospheric circulation patterns initiated contrasting wind regimes, namely the southwest monsoon flow, the Shamal and Levar winds, dominated with varying intensity over the Middle East and southwest Asia. The strong (>20 - 25 ms⁻¹ at 950 hPa) southwest monsoon off the Horn of Africa, was able to emit dust from the deserts in Somalia and southeastern Arabian Peninsula and to carry it over the central and north parts of the Arabian Sea. These dust plumes were merged with those coming from Iran (Sistan dust storm) triggered by the strong (~20 ms⁻¹ at 850 hPa) Levar wind, as well as those originated from Iraq and the deserts in the east Arabian Peninsula, which were
transported by the northwest Shamal (~ 10 - 14 ms⁻¹ at 950 hPa) wind. The end result was the large dust-aerosol loading (MODIS/MISR AODs > 1.5 - 2) over the central/north Arabian Sea (~15º - 24º N, 60º - 67º E). This “hot-spot” aerosol-laden area was characterized by weak winds (< 5 ms⁻¹) and convergence of the southerly monsoon flow with the northerlies Shamal and Levar, defining the position of the ITD, which seems to play an important role in dust accumulation over the Arabian Sea. The CALIPSO vertical profiles revealed presence of multiple aerosol layers along the position of the ITD in the north Arabian Sea, with the lower plumes (~ 500 m) to be originated from Somalia and southeast Arabian Peninsula, while the upper ones (~ 3000 m) from the north (Iran-Pakistan-Afghanistan). At 1000 m, a significant fraction of the dusty air masses originated from the Shamal-influenced area in the eastern Arabian Peninsula. During the study period, the dust plumes over the Arabian Peninsula were mostly detected at elevated heights (up to ~5 km), while the Sistan dust storm on 13 - 14 July was travelling below 2 - 3 km over the land areas. ERA-Interim reanalysis of the wind profiles along the CALIPSO overpasses verified the weak-wind area along the position of the ITD and the contrasting meridional wind regimes. Wind profiles also explained the dust outbreaks over Sistan and parts of the Arabian Peninsula driven by the Levar and Shamal winds, respectively. MERRA-2 reanalysis of columnar dust mass density and fine-dust scattering AOD₅₅₀ overestimated the dust presence over the southeast Arabian Peninsula compared to Sistan, opposing to satellite (MODIS, MISR, CALIPSO, Meteosat) observations. However, MERRA-2 detected pretty well the dust accumulation area over the north/central Arabian Sea, as well as the daily spatial distribution of dust.

594

## Acknowledgments

This work is supported by the project "Le Sistan, source singulière d'aérosols désertiques: sous quelle influence dynamique locale et synoptique? 2016 - 2018" funded by CNRS, France (Ref No: AO2016-997442). ERA-Interim reanalysis is gratefully acknowledged for providing the meteorological fields over southwest Asia through https://www.ecmwf.int/en/forecasts/datasets/archive-datasets/reanalysis-datasets/era-interim. Analyses and visualizations used in this paper were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We acknowledge the MODIS, MISR and MERRA-2 mission scientists and associated NASA personnel for the production of the data. Many thanks go to Eumetsat services that provided the raw Meteosat imagery for IDDI retrievals free of charge. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT model through the READY website (http://www ready.noaa.gov).

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<figure><img src="image_3.png" /><figcaption>**Figure 1:** Topographic map of the study region indicating the main deserts.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Figure 2:** AOD<sub>550</sub> spatial distributions (10 km x 10 km) via combined Dark Target and Deep Blue v6.1 Terra-MODIS retrievals over southwest Asia during 11 – 16 July 2016.
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Spatial distribution of the Meteosat-7 IDDI retrievals over land in southwest Asia during 11 - 16 July 2016, superimposed by the CALIPSO overpasses on each day. The white areas are cloud contaminated without IDDI estimates.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4:** MSLP and geopotential height at 700 hPa synoptic maps during 11 – 15 July 2016 from ERA-Interim reanalysis at 12:00 UTC.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5:** Vector winds at 950 hPa for 06:00 UTC (left) and 18:00 UTC (right) during 11 – 15 July 2016 from ERA-Interim reanalysis. The coloured scale represents the wind speed (ms⁻¹).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6:** Vector winds at 700 hPa for 06:00 UTC (left) and 18:00 UTC (right) during 11 – 15 July 2016 from ERA-Interim reanalysis. The coloured scale represents the wind speed (ms⁻¹).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7:** Composite mean of the meridional wind over south Asia at 850 hPa during 11-16 July 2016 from MERRA-2 **reanalysis**. The area of near-zero meridional wind defines the position of the ITD.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 8:** MISR AOD<sub>558</sub> (a), Terra (b) and Aqua (c) MODIS AOD<sub>550</sub> spatial distributions, superimposed with the **ERA-Interim** vector winds at 950 hPa (a), 850 hPa (b) and 700 hPa (c), averaged for the period 10 – 19 July 2016. The position of the ITD is also defined. The domains are used for the analysis shown in Figs. 10-12.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

**Figure 9:** Trajectory frequency plots taken from HYSPLIT for the 4-days air-mass back-trajectories ending at 500 m (a), **1000 m** (b) and **3000 m** (c) over the north Arabian Sea (21° N, 62° E) on 15 July 2016. The frequency (%) corresponds to the number of endpoints per grid (1° x 1°) per the total number of trajectories (N=40).

<figure><img src="image_12.png" /></figure>

**Figure 10:** (a) Time-latitude variation of the AOD<sub>550</sub> (longitude-averaged at 60° – 67° E; D1) along with vector wind at 925 hPa (longitude-averaged at 51° – 60° E; D2). (b) Time-altitude (1000 – 400 hPa) variation of the monsoon wind profile in the domain 8° – 15° N, 51° – 60° E (D3). The domains D1, D2, D3 are shown in Fig. 8a. The wind corresponds to ERA-Interim reanalysis. The AOD<sub>550</sub> is from daily Terra and Aqua MODIS observations temporally interpolated for 6-hr intervals. White areas correspond to missing AOD.
<figure><img src="image_13.png" /><figcaption>**Figure 11:** (a) Time-longitude variation of the AOD<sub>550</sub> (latitude-averaged at 23° – 30° N; D1) along with vector wind at 925 hPa (latitude-averaged at 29° – 31°; D2). (b) Time-altitude (1000 – 400 hPa) variation of the Shamal wind profile in the domain 25° – 31° N, 47° – 50° E (D3). The domains D1, D2, D3 are shown in Fig. 8b. The wind and AOD<sub>550</sub> retrievals are as in Fig. 10.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 12:** (a) Time-latitude variation of the AOD<sub>550</sub> (longitude-averaged at 60° – 67° E; D1) along with vector wind at 850 hPa (longitude-averaged at 61° – 62°; D2). (b) Time-altitude (1000 – 400 hPa) variation of the Levar wind profile in the domain 31° – 34° N, 61° – 62° E (D3). The domains D1, D2, D3 are shown in Fig. 8c. The wind and AOD<sub>550</sub> retrievals are as in Fig. 10.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure 13:** Time-altitude TAB cross sections at 532 nm along the CALIPSO overpasses over the study region during 11 – 16 July 2016. Pressure, potential temperature and temperature levels are superimposed on the TAB profiles. White circles emphasize on low- and upper-level aerosol plumes.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 14:** Vertical cross sections of the ERA-Interim vector wind (time in UTC) from 1000 hPa to 400 hPa along the CALIPSO overpasses over the study region (see Fig. 2). The major winds (monsoon, Levar, Shamal) and their directions are indicated on each day. The red thick lines correspond to the position of the ITD (lowest wind speeds). The “North” and “South” indicate the pathway (ascending or descending) of the CALIPSO overpass.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure 15:** Spatial distribution of the columnar dust mass density (in g m⁻²) (a) and fine-dust (PM₁) scattering AOD₅₅₀ (b) over the southwest Asia averaged for the period 11 – 16 July 2016, according to **MERRA-2 reanalysis**. The superimposed winds are at 850 hPa (a) and 700 hPa (b) levels averaged for the same time period.</figcaption></figure>
Contrasting wind regimes and source regions that accumulate dust over the Arabian Sea during the summer season – The July 2016 case

<figure><img src="image_18.png" /></figure>