A comprehensive investigation of the dust-storm characteristics over the Sistan Basin in eastern Iran revealed that the dust-storm days during the summer season were associated with an atmospheric teleconnection pattern consisted of high-pressure values over the Caspian Sea and lower than normal pressure over the Hindu Kush – Pamir Mountains.
During the summer season, the area along the east Iranian borders with Afghanistan and Pakistan is strongly impacted by the strong (>20 ms-1) northerly Levar or 120-days wind, which causes massive dust storms on certain circumstances, rendering Sistan as the major dust source in southwest Asia and one of the windiest desert environments over the globe (Rashki et al., 2013). In addition, there was an evidence that the highest wind speeds in Sistan were associated with positive anomalies in surface pressure over the Caspian Sea and slight negative ones over the Hindu Kush.
The Caspian Sea – Hindu Kush Index (CasHKI) has been recently introduced as a regulatory factor for the wind regime and dust activity over the southwest (SW) Asia. The CasHKI is defined as the difference between the spatial averaged mean sea-level pressure (MSLP) daily anomaly, from the 1981–2010 climatology, over the Caspian Sea (40–50° N, 50–55° E) and Hindu Kush (35–40° N, 70–75° E) domains [CasHKI = MSLPanomCS − MSLPanomHK]. The MSLP dataset is obtained from the Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis project.
Figure: Schematic approach of the low CasHKI (left) and high CasHKI (right) modes and associated changes in mean sea-level pressure, Levar wind and dust activity over southwest Asia. Credit: Dimitris G. Kaskaoutis
It was found that on a long-term (~30 years) basis, the CasHKI values, either during the whole period or for each month separately, follow the Gaussian curve, ranging between -25 hPa and +35 hPa, while during the summer months, this range is much lower i.e., -12 hPa to + 14 hPa (Kaskaoutis et al., 2016, 2017, 2018). Initially, the CasHKI values were examined against the dust activity over the SW Asia during the summer months (June to September), when the dust activity over Sistan maximizes. It was found that the highest CasHKI values were statistically significant associated with an increase in dust activity, either observed via satellites (MODIS-AOD, OMI-AI) or simulated via models (SPRINTARS, RegCM4). In contrast, the dust storms were very rarely occurred on days with low CasHKI values.
In synopsis, abnormal high-pressure values over the Caspian domain associated with concurrent lower than normal pressure values over the Hindu Kush mountains resulted in an increase in the surface pressure gradient between these regions, which intensifies the northern Levar wind. Higher wind intensities enable for dust erosion from the alluvial playas in Sistan Basin and the end result is the frequent and intense dust storms that impact the SW Asia (southern Iran, Afghanistan, Pakistan) and the northern part of the Arabian Sea. Initially, the CasHKI impact on modulation of the dust activity over the SW Asia was evaluated during the summer dusty period (Kaskaoutis et al., 2016), but further research proved that the same mechanism is also active even for the rare dust storms during the wintertime (Kaskaoutis et al., 2017).
The high CasHKI values were also found to be associated with an intensification of the Indian summer monsoon trough and a deepening of the Indian/Pakistan thermal low, but no statistically significant association between CasHKI and the Indian summer monsoon was found. During the wintertime, the high CasHKI mode is related with a decrease in temperature along east Iran due to the intensification of the northern cold air masses from central Asia, while a rather negligible linkage was found between CasHKI and large-scale dynamics, like El-Nino Southern Oscillation. Further research is needed for associating CasHKI with other teleconnection patterns, like the North Sea – Caspian Pattern (NCP), the monsoon system, and/or ocean-atmospheric dynamics over the Indian Ocean.
These findings are described in the article entitled Long-term variability and trends in the Caspian Sea – Hindu Kush Index: Influence on atmospheric circulation patterns, temperature and rainfall over the Middle East and Southwest Asia, recently published in the journal Global and Planetary Change. This work was conducted by D.G. Kaskaoutis from the National Observatory of Athens, E.E. Houssos from the University of Ioannina, F. Minvielle, I. Chiapello, and M. Legrand from the University of Lille, A. Rashki from the Ferdowsi University of Mashhad, and U.C. Dumka from the Aryabhatta Research Institute of Observational Science.
References:
- Kaskaoutis, D.G., E.E. Houssos, A. Rashki, P. Francois, M. Legrand, D. Goto, A. Bartzokas, H.D. Kambezidis, T. Takemura, 2016. The Caspian Sea – Hindu Kush Index (CasHKI): a regulatory factor for dust activity over southwest Asia. Global and Planetary Change, 137, 10-23.
- Kaskaoutis, D.G., A. Rashki, E.E. Houssos, M. Legrand, P. Francois, A. Bartzokas, H.D. Kambezidis, U.C. Dumka, D. Goto, T. Takemura, 2017. Assessment of changes in atmospheric dynamics and dust activity over southwest Asia using the Caspian Sea – Hindu Kush Index. International Journal of Climatology, 37 (Suppl.1), 1013–1034.
- Kaskaoutis, D.G., E.E. Houssos, F. Minvielle, A. Rashki, I. Chiapello, U.C. Dumka, M. Legrand, 2018. Long-term variability and trends in the Caspian Sea – Hindu Kush Index: Influence on atmospheric circulation patterns, temperature and rainfall over the Middle East and southwest Asia. Global and Planetary Change, 169, 16-33.
- Rashki, A., D.G. Kaskaoutis, A.S. Goudie, R.A. Kahn, 2013. Dryness of ephemeral lakes and consequences for dust activity: the case of the Hamoun drainage basin, southeastern Iran. Science of the Total Environment, 463–464, 552–564.