Climatology and variability of the Middle Eastern summer Shamal wind : Implications to dust storm variability
Thursday, 18 December 2014
The climatology and variability of Shamal wind, an important regulator of summertime Middle Eastern dust storm activity, have been poorly characterized in the scientific literature. The Middle Eastern Shamal is a strong low-level northwesterly wind, which can lift dust from the Syrian and Iraqi deserts and transport it to the Persian Gulf and Arabian Peninsula. Despite its critical role as a dust storm driver, the variability and controls of summertime Shamal wind has not been addressed by previous studies. The spatial distribution and seasonal cycle of the Shamal wind during 1970-2013, along with its interannual variability, covariability with dust storm activities, and potential link to sea surface temperatures (SSTs) in the tropical Pacific Ocean and Mediterranean Sea, are studied using hourly station records, reanalysis data, and remotely-sensed data. According to a wind-direction and speed-based criterion for Shamal days, the summertime Shamal behaves like a summer monsoon with variability in its onset, termination, and distinct break periods. Based on a multi-station criterion for Shamal onset and termination, the onset occurs on May 23 ± 8 days (one standard deviation), and the termination date is Sep 2 ± 21 days. The highest wind speed on a Shamal day occurs around the noon along the west coast of the Persian Gulf, corresponding to a peak in horizontal pressure gradient. The onset of Shamal is associated with the development of low pressure over Iran and high pressure over the eastern Mediterranean and northwestern Arabian Peninsula - an eastward expansion of the high over subtropical Atlantic. El Niño events support the delayed onset and weaker intensity of the summer Shamal. Intensified summer Shamal winds are associated with anomalously cool Mediterranean SSTs, which enhance the high pressure over the eastern Mediterranean. The impacts of El Niño-Southern Oscillation and Mediterranean SSTs on summer Shamal are validated by an independent statistical method, the General Equilibrium Feedback Assessment. Periods of enhanced Shamal wind intensity favor increased summertime floating dust frequency (temporal correlation = 0.77,1975-2013), dust storm frequency (temporal correlation = 0.57, 1975-2013), and aerosol optical depth (temporal correlation = 0.73, 2000-2013).