The Caspian Sea–Hindu Kush Index (CasHKI): A regulatory factor for dust activity over southwest Asia

Dust life cycle (emission, transport and deposition) is strongly interlinked with atmospheric circulation and climate dynamics (Washington, R., et al., 2006, Pey, J., et al., 2013, Salvador, P., et al., 2014, Kaskaoutis, D.G., et al., 2015a). In this respect, variability in Sahelian rainfall during summer and the North Atlantic Oscillation (NAO) during winter seem to be related with dust outflow from west Sahara to subtropical Atlantic (Prospero, J.M. and Lamb, P.J., 2003, Chiapello, I., et al., 2005, Ginoux, P., et al., 2012). Especially for the atmospheric and climatic conditions over SW Asia, Mediterranean Sea and Europe, the North Sea–Caspian Pattern (NCP) was found to play an important role, on modulating either the wind patterns or the recorded temperature and precipitation (Kutiel, H. and Benaroch, Y., 2002, Brunetti, M. and Kutiel, H., 2011, Ghanghermeh, A., et al., 2015). Rodríguez et al. (2015) found that variability in Saharan dust outflow to subtropical Atlantic and the monsoon rainfall over Sahel are linked to the intensity and variations of the North African dipole (NAFDI) composed of the 700-hPa geopotential height anomalies between Morocco and Mali. Therefore, synoptic and dynamic meteorology control dust emissions and life cycle, while intense dust plumes can modify weather by large attenuation of solar radiation (Badarinath, K.V.S., et al., 2010, Antón, M., et al., 2012, Valenzuela, A., et al., 2015), suppression of precipitation, evaporation of clouds and decreasing cyclonic activity (Rosenfeld, D., et al., 2001, Dunion, J. and Velden, C., 2004). Although these issues are fully addressed over Sahara via the synergy of ground-based and satellite remote sensing in the framework of several in-situ measurements and experimental campaigns (e.g. Barkan, J., et al., 2005, Washington, R. and Todd, M.C., 2005, Engelstaedter, S., et al., 2006, Todd, M.C., et al., 2008, Schepanski, K., et al., 2009, Alonso-Perez, S., et al., 2011, Calastrini, F., et al., 2012, Israelevich, P., et al., 2012, Marsham, J.H., et al., 2013), some few recent studies have focused on this field over southwest (SW) Asia and Arabian Sea (Alizadeh Choobari, O., et al., 2013, Prijith, S.S., et al., 2013, Kaskaoutis, D.G., et al., 2014a, Kaskaoutis, D.G., et al., 2015a).

Arid areas (deserts) in SW Asia contain active dust sources resulting in frequent and massive sand/dust storms, especially during the summer (June, July, August, September — JJAS) season (Middleton, N.J., 1986, Ekhtesasi, M.R. and Gohari, Z., 2013, Rezazadeh, M., et al., 2013, Rashki, A., et al., 2013, Shahraiyni, H.T., et al., 2013, Cao, H., et al., 2015, Moridnejad, A., et al., 2015). Dust mobilization over Middle East, Arabia and SW Asia is controlled by contrasting meteorological regimes. For example, the passage of frontal systems and the east-Mediterranean trough pressure gradient are associated with dust activity over Middle East and Arabia (Hamidi, M., et al., 2013, Awad, A. and Mashat, A.-W., 2013, Awad, A. and Mashat, A.-W., 2015), while the Indian thermal low and the Levar wind are responsible for dust emissions over SW Asia (Alizadeh Choobari, O., et al., 2014, Kaskaoutis, D.G., et al., 2015a). However, assessing the atmospheric circulation patterns, which are associated with modulation and long-term trends in dust activity over Middle East and SW Asia has not been evaluated so far except for some case studies (Maghrabi, A., et al., 2011, Najafi, M.S., et al., 2014, Jish Prakash, P., et al., 2014). Changes in the wind regime between contrasting monsoon years associated with variations in the El-Nino Southern Oscillation (ENSO) seem to affect the dust outflow over the Arabian Sea (Abish and Mohanakumar, 2013). Larger dust-aerosol loading over the Arabian Sea is mostly related to normal rather than weak monsoon years (Rahul et al., 2008), while recent works (Vinoj, V., et al., 2014, Solmon, F., et al., 2015, Das, S., et al., 2015) using satellite observations and model simulations found a positive feedback between Arabian dust outflow and rainfall over India.

The present work is an attempt to highlight the key role of meteorological dynamics on modulating dust activity over SW Asia and the North Arabian Sea (NAS). A recent work (Kaskaoutis et al., 2015a) revealed that the dust storms over Sistan basin in eastern Iran were associated with positive anomalies in mean sea level pressure (MSLP) and Geopotential Heights at 700 hPa (Z700) over CS and negative anomalies over the HK mountain range. A similar synoptic meteorology pattern (i.e. MSLP high over CS and MSLP low over HK) dominated during the long-range transport of a dust storm along the eastern Iran borders during 1–3 July 2014 (Kaskaoutis et al., 2015b). In that study, Kaskaoutis et al. (2015b) associated the meteorological dipole (i.e. MSLP anomalies over CS and HK) with modification in wind speed over eastern Iran and dust outflow over NAS, revealing a strong linkage between them. The anomalies in MSLP between CS and HK are now quantified into a new meteorological index, the so-called Caspian Sea–Hindu Kush Index (CasHKI). Variations in CasHKI intensity are examined against satellite observations (Meteosat, OMI, MODIS, TRMM) and model (MIROC-SPRINTARS) simulations of dust emissions, outflows and rainfall distribution over south Asia, in order to examine the impact of CasHKI variations on atmospheric dynamics and modulation of dust activity. Section 2 describes the datasets used in the current analysis and the CasHKI definition, while Section 3 contains the main results of the relation between CasHKI and atmospheric dynamics over SW Asia. Section 4 summarizes the results and itemizes open issues for further research.