اثر گرمایش جهانی بر مرکز چرخندزایی شرق مدیترانه و ارتباط آن با ناهنجاری بارش نیمه‌ی غربی ایران

Introduction

  Variability is one of inherent properties of the climate system. In addition to the dynamic nature, the climate system is extremely intertwined nature also, so that its components interact with each other and eventually they are changing each other. In general, the climate of a geographic location is controlled by various factors, including Atmospheric Action Centers (AAC's). AAC's behaviors have an effective role in short and long term changes in weather and climate conditions and components. AAC's of Mediterranean Sea especially its cyclogensis centers in east side of sea have an effective role in Iran precipitation regime. From this point of view‌, this study looking forwards the effects of temperature and pressure components of east Mediterranean cyclogensis centers in rainfall anomalies of west of Iran.

Material and methods

   In this study have been used two data sets: first, database of Meteorological Organization of Iran, based on 1961 to 2010 data series and then, database of U.S. Environmental Studies Organization based on 1948 to 2014 data series. From the total of months were concerned in this study, 55 months have been selected because their abnormality. By using Pearson method the correlation between precipitation amounts of Iranian stations and temperature and pressure on atmospheric levels of MSL, 850, 500 and 300 hpa was calculated for east of Mediterranean Sea.  Then significant correlations were interpolated for whole of west of Iran using IDW methods in ArcGIS and the patterns of iso-correlation countors  were plotted  as a series of maps.


Discussion and Results

   Based on results of research the widespread abnormalities of rainfall in west of Iran have correlated more with temperature and pressure changes of upper level synoptic patterns of east section of Mediterranean Sea. Likely, the changes in behaviors of upper levels patterns in east Mediterranean AAC's in cold season and their effects on deepening  of Mediterranean Trough and creation the positive vorticity advection in eastern parts of trough have more importance than the patterns in lower levels of atmosphere. In addition , because upper level patterns are far from reach of local ones in low levels, their impacts is more activeness in long distances from Mideast Mediterranean to west of Iran. Also based on results of this study the changes of pressure and  temperature components in eastern part of Mediterranean Sea is increasing and significant especially in case of upper levels ACC's. Vertical profiles of abnormalities in all levels of atmosphere confirms increasing of tropospheric temperature and geopotential height over the past three decades (1981-2010)  than the normal period (1949-1980). On this basis the different levels of atmosphere from MSL to 300 Hpa have experienced  an increase range from 11 to 15 gpm, that for level of 300 Hpa it reach to 35 gpm. But upper parts of atmosphere in levels of 20 to 10 Hpa the feature of decreasing is dominant.  In case of temperature of atmospheric levels it was shown that in lower-to-mid levels exists an increase about 0.3Co whereas in upper levels was seen a decrease approximately 1 Co.

Conclusion

   Based on results the widespread anomalies of rainfall in west of Iran show strong inverse correlation with temperature and pressure changes in Mid-east AAC's of  Mediterranean sea at all atmospheric levels. In Northwest of Iran, above mentioned correlations are weak and insignificant. Also the significant  increasing trend of temperature and pressure in Mideast of Mediterranean  area in relation with its powerful and inverse correlation with rainfall of west of Iran and with concern to developing trend of Global Warming , confirms the more probability of drought  occurrence in west and whole of Iran.

  Although according to results of other researches, the rainfall of Iran hasn't already showed any significant trend, but the synoptic studies on mentioned factors especially the cyclogensis centers in east part of Mediterranean sea, shows an increase in atmospheric pressure and consequent more occurrence of droughts in Middle East and Iran in some future decades.