شناسایی منطقۀ همگرایی دریای عرب و نقش رودباد تراز پایین سیستان در شکل‌گیری آن

Sistan wind is one of the most important atmospheric phenomena of warm period of year in Iran plateau which creates environmental different impacts on its dominated region. In the northern hemisphere during summer monsoon, ITCZ moves to higher northern geographic latitude. South Asia summer monsoon system is one of the phenomena which arises from this movement and a tongue extension of Gang low moues towards west, its domination over southern region of Iran, and its penetration to Khozestan plain and Mesopotamia is also one of this movement outcome. In the meantime, in atmospheric low level, Azores high tongue, in this region of the world also has to remain in north territories of Iran (that is over Caspian sea and its surrounding) more than world-wide average. So that, with the pressure gradient between the Caspian sea high and Gang low generates the Sistan wind system, in a way that can be said that this is northern trade wind which blows in the region. The period used in this study was for 2480 days in 22 years (2012-1993) from May until end of September. The atmospheric circulation types were extracted using daily mean of the 850 hPa geopotential height data for these days between 15°–80°E, 5°–50°N, with a 2.5° (lat) × 2.5° (lon) spatial resolution. These data were retrieved from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis archive. Then the agglomerative hierarchical cluster analysis with the ward algorithm and Euclidean distance were used to identify atmospheric circulation types over Iran in mentioned period of years. Because using hierarchical cluster analysis can take maximized within-group similarity and minimized between-group similarity in data, the groups of days with similar characteristics were determined. Then the calculated within-group correlations were used to identify representative days. The day with highest within-group correlation was representative day of atmospheric circulation types. Finally, 5 atmospheric circulation types were identified in this period in which 2 cluster showed the mechanism of in Sistan winds in the southeast of Iran. Then for representative day of atmospheric circulation types in 2 mentioned clusters, wind speed and direction, as well as the wind convergence in levels of in 1000, 925, 850 and 700 hPa and also the convergence in atmospheric vertical profiles were analyzed. This paper identifies that, this wind has the features of LLJ and its core is often at 850 hPa level which is titled and is known as Sistan LLJ. Its speed continuation also stretches to low levels and to the earth surface, as a result it creates Sistan wind near the surface. On the other hand, along with ITCZ belt anomalous movement to northern hemisphere, the southern trade winds also enter northern hemisphere to reach ITCZ belt and after acquiring humidty from Indian ocean and Arabian sea, they enter Indian subcontinent. Now, the assumption is tested that between these two kinds of blowing systems (Sistan LLJ and southern trades) a region of convergence must be created, then with the continuation of the above convergenced, wind sysrem is identified over north of Arabian sea, Pakistan and Iran coasts. This convergence region which establishes at the time of  Sistan LLJ balances with southern trade winds, which determines its location geographic latitude. So that, if Sistan LLJ blows slowly, this convergence region moves to more southern geographic latitudes. also its vertical extension in atmosphere is to the extent that its generating factors (Sistan LLJ and southern trades) exist and when one of these factors, does ascend to the higher level of atmosphere, this convergence region disappears. So that, naturally its establishment can be observed from the earth surface to 750 hPa level, and it often disappears above this level. This convergence region can also be a reason for the issue that during the fact Sistan LLJ the northern trade winds converge with southern trades on Arabian sea. The other indications of these winds are being originated from subtropical high and blowing toward tropical convergence region, so that the Indian monsoon is in fact the southern trades and Sistan LLJ that blow from two sides towards ITCZ.