Synoptic and thermodynamic analysis of spatial movement of sub-tropical jet stream in Sudan low activity (case study: wet years in Fars Province, Iran)

The present study used two methods of circulation to peripheral and peripheral to circulation. To study and analyze the spatial movement of sub-tropical jet, the data of the website http://www.esrl.noaa.gov was used. The maps of jet for a 21 year statistical period (from October to May) were produced using Grads software, and the axis of jets for every month were drawn on the basic maps produced in ARCGIS10.4 software. Afterwards, the monthly precipitation data of nine synoptic stations in Fars province during the crop year of 1996-2017 was extracted, and the samples were selected. Examination of jet axis in different months showed that jet axis has relatively much northward and southward movement. From the early October, gradually the jet streams locate in proper places following the eastern movement of Arabian high pressure, and provide proper condition for atmospheric instability. In January and December, the jet is located in its best place. The results of the synoptic patterns of heavy rains showed that from a few days before the start of precipitation, a deep ridge develops over Africa and central Mediterranean to the north of the latitude of 60 degree and cause fractures in the Rossby waves. The resulting sever thermal gradient causes a deep trough over the eastern Mediterranean which is intensified following the advection of cold sub polar over Sudan system, and proper moisture advection over the southern warm seas of sub-tropical jet stream, and creates sever instability over the region.