Extremely high aerosol loading over Arabian Sea during June 2008: The specific role of the atmospheric dynamics and Sistan dust storms

The aerosol field over Arabian Sea is strongly influenced by the Asian pollution outflow during the post-monsoon and winter seasons and by dust storms originated from Middle East, Arabia and southwest Asia during pre-monsoon and monsoon seasons (Tindale and Pease, 1999, Dey and di Girolamo, 2010), resulting in the highest annual aerosol optical depth (AOD) of about 0.6–0.8 during June–August (Kaskaoutis et al., 2011). Satellite observations have shown a strong south-to-north gradient in AOD, with values above 1.0–1.2 in the northernmost part associated with larger contribution of aerosols of desert origin (Satheesh et al., 2010, Prijith et al., 2013). The vertical profile of aerosols also revealed the significant continental dust outflow, which progressively increases in altitude (>2–3 km) along its transport over central and southern Arabian Sea (Prijith et al., 2013). The dominance of the dust aerosols over central and northern parts of the Arabian Sea was also revealed from ship-cruise campaigns (ARMEX, ICARB) during the pre-monsoon (Kalapureddy et al., 2009) and monsoon (Moorthy et al., 2005) seasons. The aerosol field over Arabian Sea, its variations and the dusty clouds that are usually formed are interlinked with the southwest Indian summer monsoon (Rahul et al., 2008, Manoj et al., 2011, Manoj et al., 2013, Vinoj et al., 2014), while previous studies have shown some teleconnections with El-Nino Southern Oscillation (ENSO) (Gadgil et al., 2003, Abish and Mohanakumar, 2013). Furthermore, the seasonal change of the Inter-Tropical Convergence Zone (ITCZ) controls the meteorological and atmospheric dynamics over Arabian Sea (Krishnamurthi et al., 1998), while Satheesh et al. (2006) found large negative aerosol radiative forcing at surface associated with an overall top of the atmosphere cooling.

The Sistan region, which is located in the southeastern Iran along the Afghanistan and Pakistan borders, has been considered as a major dust source in southwest Asia (Middleton, 1986, Prospero et al., 2002, Léon and Legrand, 2003, Ginoux et al., 2012; Goudie, 2013) due to the extreme intensity of the northerly Levar or "120-day" wind during the June–September period (McMahon, 1906). Several papers have investigated the dust activity, outflows and relation with the Levar wind over the region (Rashki et al., 2012, Rashki et al., 2013a, Rashki et al., 2013b, Rashki et al., 2014, Alizadeh Choobari et al., 2013, Ekhtesasi and Gohari, 2013, Rezazadeh et al., 2013). The dust outbreaks originated from Sistan take place in the northernmost lowland of the region, which is covered by shallow (<4 m in depth), marshy and ephemeral lakes (Hamouns, approximately 160 km long and 8–25 km wide with nearly 4500 km² surface area) that are fed from the discharge of the Helmand river in Afghanistan. Rashki et al. (2013c) and Sharifikia (2013) revealed the strong linkage between the dust activity and water coverage in the Hamoun dry-bed lakes. Due to extreme hot-dry conditions during the summer months and, depending on the snowfall and snowmelt in southern Hindu Kush mountains and on the precipitation variability over the region, the lakes usually get dried, as occurred in several years during the last decade (Rashki et al., 2013c). When the lakes get dried, a saline alluvial clay material is left at the dried beds, which is easily eroded by the strong Levar wind (>15 m s⁻¹), thus forming intense dust storms that cover Sistan and downwind areas, also affecting Karachi (Alam et al., 2011) and northern part of the Arabian Sea.

The dust-storm occurrence over Sistan has a significant inter-seasonal and intra-annual variability due to the reasons mentioned above, thus contributing to significant fluctuation in the aerosol loading over the downwind areas, one of which is the northern Arabian Sea. In the present study, we highlight the significant role that the Sistan dust storms play in the aerosol loading over Arabian Sea, focusing on June 2008, when an extremely high aerosol loading covered its northern and central parts. This work supplements the results obtained by Prijith et al. (2013), who analyzed the atmospheric and meteorological conditions over Arabian Sea during June 2008 providing some reasons for the exceptionally high AOD, but without reporting the main dust source region, the topographic and meteorological dynamics that favored the enhancement in dust activity. Thus, the role of the Sistan basin and the associated dust storms in the aerosol loading over Arabian Sea remain unexplored. These issues are analyzed and discussed in the current work by the synergy of ground-based (Zabol station) meteorological data, synoptic meteorological reanalysis (NCEP/NCAR), satellite (Landsat, MODIS) remote sensing, HYSPLIT air-mass trajectories and model (SPRINTARS) simulations.