Climatology of the Sistan Levar wind: Atmospheric dynamics driving its onset, duration and withdrawal

Abstract

Levar wind is the dominant meteorological and climatic feature in east Iran, blowing from Central Asia to the northern coast of the Arabian Sea. It is also known as “120-days wind” due to its mean duration in the summer season. Although violent and responsible for massive dust storms in southwest Asia, long-term climatology and atmospheric dynamics that facilitate genesis and dissipation of Levar have not been well documented. This study uses the two-phase regression method for determining the onset, dissipation and duration of Levar during a 41-years period (1972–2012), based on identification of the change points in wind data series at Zabol meteorological station. The mean duration of the Levar period is estimated to 135 ± 24 days, with a remarkable inter-annual variability. The mean Levar onset is determined around 22 May ±23 days, while the withdrawal on 8 October ±25 days. On average, the onset and dissipation of Levar occur between wind speeds of 4.6–7.7 ms−1 in spring and 3.9–6.7 ms−1 in autumn, respectively. A comprehensive analysis is performed for the first time to examine meteorological dynamics that are associated with onset, duration and withdrawal of Levar. The early onset years are characterized by stronger winds during May, while no considerable changes in wind are detected between early and late withdrawal years. Changes in the mean sea-level pressure (MSLP) dipole between the Caspian Sea and India/Pakistan forces the onset of Levar. Therefore, early (late) onset (withdrawal) of Levar are driven by changes in MSLP over these areas, while local dynamics and topography also play an important role. Levar seems to be modulated by CasHKI (Caspian Sea Hindu Kush Index), while changes in the Caspian-Sea High (majorly) and in the Indian/Pakistan monsoon thermal low (secondarily) affect the Levar intensity.