It is well-known that regional weather and climate around the globe are strongly influenced by large-scale atmospheric circulation patterns. The centers of action corresponding to different levels of atmosphere play an essential role in controlling the climate of different climatic regions around the globe. Over the years several efforts have been made to identify the main centers of action and large-scale atmospheric circulation patterns leading to precipitation events and to study how their variability can affect the frequency and intensity of precipitation. Hence, using weather types or circulation patterns one is able to investigate and explain the physical causes for the frequency and intensity variation of precipitation over a region.
Reviewing synoptic studies in relation to Iran have suggested that in spite of many subjective circulation classifications being implemented using observational data mainly on the monthly basis; few attempts have been done objectively using reanalysis data, especially on the basis of daily data. Hence, this paper aims to identify the main centers of action and circulation patterns in relation to winter precipitation variability over Iran.
To recognize winter atmospheric circulation patterns over the Middle East, the mean daily 500 gph for December, January, February and March were retrieved and used from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis archive, covering the period from January 1965 to 2000, accounting for 4355 days during a period of 36 years. Subsequently, daily precipitation rates for some selected days were also retrieved from the NCEP/NCAR reanalysis archive in order to assess the influences of the identified circulation patterns on precipitation in Iran.
To classify the 4355 days and extract the main circulation patterns, S-mode PCA was applied to the data matrix and the resultant 9 leading PCs were retained based on the scree test. The retained PCs were then rotated using the varimax criterion. By plotting the rotated PC loadings the centers of action for 500 hpa level that control the winter climate of the Middle East were identified. To analyze the corresponding synoptic characteristics of the identified centers of action, 10 days with the highest PC score (positive phase) were selected for each PC. The composite map of the selected days and the corresponding vorticity maps for 500 and 1000 hpa levels were represented as the winter circulation patterns. Finally, by composing the precipitation rates associated to the each aforementioned selected 10 days, the relationship between identified synoptic circulation patterns over the Middle East and the winter precipitation in Iran was investigated.
The results indicate that the spatial pattern of winter precipitation in Iran, with the exception of the southern coastal areas of the Caspian Sea, is properly governed by the 500 hpa circulation patterns. The results also show that the large spread water deficit and dry periods in Iran are related to the northward displacement of the Arabian high pressure at mid troposphere level over the western part of the Middle East. Moreover, the results indicate that the deepening of the westerly wave and the increasing positive vorticity in the area between Iran and the southern part of the Red Sea, accompanied with the development and/or reinforcement of a high pressure in the area between eastern Saudi Arabia and the central part of the Red Sea is responsible for widespread precipitation occurrences over vast areas of western and southwestern parts of Iran. Investigation of the relationship between synoptic circulation patterns and the regional scale winter precipitation in Iran shows that the precipitation occurrence in the Caspian region is mostly related to the position and strength of the lower atmosphere high pressures rather than the mid troposphere circulation patterns. This is evident if we consider that in about 4 out of 9 identified circulation patterns the southern coastal areas of the Caspian Sea get remarkable precipitation due to the predominance of a high pressure system and an increase in the negative vorticity in mid troposphere over the western part of the Caspian Sea as well as the development and/or prolonging anticyclonic circulation and northerly flows over the Caspian Sea.