The Siberian High-pressure (HP) has various effects on Iran’s climate. Climate warming, especially in the last century, has raised the possibility of changes in the Outbreaks of the Siberian High-pressure extending toward Iran (OSH). In this study, to test the mentioned hypothesis, daily sea level pressure maps for 50 years (1972 to 2021) during the month in which the SH appeared more strongly in the monthly composite maps were downloaded from the NCEP/NCAR atmospheric data reanalysis database. The selection of this month was based on the highest central pressure intensity of the high (intensity index) and its greatest spatial extent (spatial index) compared to the other six months. The axes of the OSHs extending toward Iran were classified into four categories: “continuous and reaching,” “continuous and non-reaching,” “discontinuous and reaching,” and “discontinuous and non-reaching.” The design of their synoptic patterns showed a clear decrease in the frequency of “continuous and reaching” OSHs. Continuity refers to the directness of the OSH, and a reaching OSH is one that extends to the borders of Iran or penetrates into the country. Using two criteria— “the integrated advance of cold and dry air from the high-pressure center toward Iran” and “verification of the OSH entering or reaching Iran’s borders”—it was found that cold and dry air travels from Central Asia toward Iran through three main pathways: 1. The pathway of cold and dry air transfer from the western slopes of the Altai Mountains, then across the Turan Plain, and finally into central Iran. 2. The pathway of air transfer from the Dzungarian Plain in western China, then along the Tian Shan Mountains, through Afghanistan, and finally into the north of Sistan-va-Baloochestan Province. 3. The pathway of air transfer from the Kazakh steppes, through the Caucasus corridor (west of the Caspian Sea), reaching Azerbaijan, and extending along the Zagros Mountains. The frequency of OSHs has indicated a decline in five separated decades along all three pathways during the past half century. This decrease is more evident in the third pathway, namely the Caucasus corridor, which already had a lower frequency from the beginning.

The aim of this study is to synoptic analyze of the number of frost days in a year of Khuzestan province. For this purpose, using the minimum daily temperature data of 12 stations during the statistical period of 1992 to 2017, the Meteorological Organization of the country, 54 days of frost was identified. Sea level pressure, Geopotential Height, Zonal and meridian wind and temperature of 500 hPa data with size of 2/5 * 2/5 degree arc from the National Oceanic and Atmospheric United States of America were extracted. On the matrix of the variance of sea level pressure data in 54 days, the analysis of the basic components was performed and 10 components which identified 83% variance of the sea level pressure. Then, by applying the hierarchical cluster analysis method, the integration method was applied to the scores of the 10 components and 5 patterns of sea level pressure were identified. The results showed that frost phenomenon in Khuzestan province occurs from November to march and its trend is decreasing during the statistical period. Also northern and western parts of the province have allocated the most frequency of frost. Also the synoptic condition analysis of troposphere showed that 5 sea level pressure pattern with different make ups lead to pervasive frosts of Khuzestan province. Weak and moderate frosts formed by the influence of Siberian and European cold high pressure systems. But severe frosts occur with spread of Iceland low pressure to Iran, along with strong cold pressures. Meanwhile, the powerful Siberian high pressure is present in most of the patterns, which its interaction with sub polar and Icelandic low pressure, plays the most role in the most severe frost in the province of Khuzestan. Also in the middle level of troposphere penetration of deep troughs from northern latitudes and east European huge blockings has the most role, which has advection of cold air from the side west of troughs on the country and during the intensity of the frost added to its continuity.