high-pressure system centered over Caspian Sea and central Asia and negative ones over Hindu Kush. Similar pattern was observed for the 700 hPa pressure level. This suggests that during the dust-storm days the west-to-east pressure gradient, which is the inaugural force for the Levar wind and the low-level jet, is further enhanced. Actually, wind-speed records at Zabol revealed much stronger northerlies during the dust-storm days compared to the monthly climatology. This meteorological dipole in anomalies was also active during specific case studies, like the frequent Sistan dust storms during June 2008 (Kaskaoutis et al., 2014b) and during the recent case 1-3 July 2014 (unpublished data).
Figure 5: Anomalies of MSLP (above) and Z700 (below) from the mean climatological situation (1981-2010) for dust storms over Sistan during the period 2001-2012 [Reproduction by Kaskaoutis et al. 2014a].
Sistan dust storms and influence over northern Arabian Sea
Recent studies by Kaskaoutis et al. (2014b) and Rashki et al. (2015) examined the dust transport pathways and the role of Sistan in dust loading over north Arabian Sea. Figure 6 shows the 5-days HYSPLIT forward trajectories during dust storms originated from the Sistan Basin in June and July. The results reveal that the most affected areas from the Sistan dust storms are the southeast Iran, southwest Afghanistan and south-central Pakistan. The air masses are initially having a southward direction due to Levar wind and when they reach the northernmost Arabian Sea they are progressively shifting to east-northeast, mostly affected the Indus basin. The northern Arabian Sea
is also strongly affected, while the north-western India and the central Arabian Sea are affected in certain circumstances. The synoptic atmospheric circulation over south Asia during the summer monsoon (Rashki et al., 2014) is the control factor for the pathway and the "shift" of the Sistan-originated dust storms. The south-western monsoon air flow interacts with the northerly Levar flow along the Oman Sea and coastal north Arabian Sea, where the ITCZ lies during this season. The convergence zone seems to play a crucial role for the north-eastwards shift of the Iranian dusty air masses, while the steep increase (up to 3-4 km) in their altitude pathway towards Arabian Sea is attributed to the uplift along the convergence zone. Thus, the position and the movement of the ITCZ seems to play a crucial role concerning the influence of the Sistan dust storms over Arabian Sea. The ITCZ seems to differentiate the dusty air masses coming from Iran and Arabia over the northern Arabian Sea by uplifting the Iranian air masses to higher altitudes and usually above the dusty air masses coming from the Arabian Peninsula.
Figure 6: 5-day forward air-mass trajectories originated from Sistan Basin at 500 m agl on the dust-storm days for the June-July months during the period 2001-2012. The trajectory pathways are colour-scaled according to the altitude [Reproduction by Rashki et al. 2015].
The severe dust aerosol loading over Arabian Sea in June 2008 examined in detail by Prjith et al. (2013) was then verified that it was mostly attributed to enhanced dust activity and several (18) dust outbreaks from Sistan over the marine environment as well as to favourable meteorological conditions that allowed the influence of the Sistan dust storms over central part of the Arabian Sea (Kaskaoutis et al., 2014b).
References
- Kaskaoutis, D.G., A. Rashki, E.E. Houssos, A. Mofidi, D. Goto, A.Bartzokas, P.Francois and M. Legrand (2014a). Meteorological aspects associated with dust storms in the Sistan region, southeastern Iran. Climate Dynamics (in press), doi: 10.1007/s00382-014-2208-3.
- Kaskaoutis, D.G., A. Rashki, E.E. Houssos, D. Goto, P.T. Nastos (2014b). Extremely high aerosol loading over Arabian Sea during June 2008: the specific role of the atmospheric dynamics and Sistan dust storms. Atmos. Environ., 94, 374-384
- Prijith, S.S., K., Rajeev, B.V., Thampi, S.K., Nair, M., Mohan, (2013). Multi-year observations of the spatial and vertical distribution of aerosols and the genesis of abnormal variations in aerosol loading over the Arabian Sea during Asian Summer Monsoon Season. J. Atmos. Solar-Terr. Phys., 105-106, 142-151.
- Rashki, A., D.G. Kaskaoutis, C.J.deW. Rautenbach, P.G. Eriksson, M. Qiang and P. Gupta (2012). Dust storms and their horizontal dust loading in the Sistan region, Iran. Aeolian Research, 5, 51-62.
- Rashki, A., D.G. Kaskaoutis, A.S. Goudie and R.A. Kahn (2013). Dryness of ephemeral lakes and consequences for dust activity: the case of the Hamoun drainage basin, southeastern Iran. Sci. Total Environ., 463-464, 552-564.
- Rashki, A., Kaskaoutis, D.G., Rautenbach, C.J.de W., Flamant, C., Abdi Vishkaee, F., 2014. Spatio-temporal variability of dust aerosols over the Sistan region in Iran based on satellite observations. Natural Hazards, 71, 563-585.
Dimitris G. Kaskaoutis
Assistant Professor, School of Natural Sciences, Department of Physics, Shiv Nadar University
