blocking systems

According to the complexity of planetary-scale mechanisms affecting extreme snowfall events, the necessity of recognizing and studying the effective components of these climatic mechanisms are presented. Due to the role of the polar vortex of northern hemisphere in tropospheric variability in winter and also, in variability of the structure of the teleconnection patterns on intensity and weakness of the polar vortex, revealing the mechanism of impact of the two mentioned components on synoptic patterns leading to extreme snowfall events in Iran was considered in this study. According to the environmental approach in this study, first the snowfall days were identified and the atmospheric circulation patterns related to these days were investigated. For this purpose, daily data on rainfall and winter temperature (December-March) of fifty Iranian synchronous stations in the statistical period 1951-2016 were received from the Meteorological Organization of Iran. Atmospheric data and teleconnection indices were also obtained from the NCEP / NCAR. Finally, five periods were identified, during which snowfall was recorded in at least six stations and their average snowfall reached more than 20 mm. Of these event periods, which were in January and February, three periods related to January were examined in this study, which occurred in 1972, 1988 and 2008. In the second stage, atmospheric patterns were drawn in combination and for the average of the event periods, in three scales of hemisphere, synoptic and expanse of Iran, in the GrADS system Then, the amount and status of the teleconnection indices including: NAM, AO, NAO and PNA were analyzed in the period of extreme snowfall events. The emphasis of this study is on the effect of mid-tropospheric conditions in terms of altitude, temperature and altitude on snowfall events in Iran, while the patterns of pressure and temperature of the earth's surface and the humidity of the lower troposphere have also been considered. In hemispheric-scale combined patterns, the emphasis has been on showing the status of the tropospheric polar vortex in relation to teleconnection indices. In the third step, by doing a combined analysis, the effect of polar vortex on the change in teleconnection indices and their effectiveness on the formation of synoptic patterns leading to extreme snowfall events in Iran was revealed. According to the research findings, the NAM index, the strength of the center of the polar vortex and the AO index have shown the shape of the vortex in terms of its concentration or elongation. The PNA index has also been associated with the strength of northwestern Europe and the North Pacific anticyclones, which have contributed to the formation of blocking systems due to the northward deflection of the extending trough of polar vortex from their axial direction. The NAO index also represents the emergence of systems in the Euro-Atlantic section. The results showed that the necessary dynamic conditions for all three events have provided the emergence of blocking patterns due to weakening and distortion of the polar vortex. These events are often associated with disconnection of jet stream in the Eastern Hemisphere, especially in the Euro-Atlantic area. Hence, NAO and PNA indices, directly and NAM and AO indices indirectly affected the conditions required for extremesnowfall events in Iran.

This research is an attempt to investigate the blocking systems over the Atlantic Ocean and especially over the Mediterranean sea, which leads to surface cyclogenesis in the east of the Mediterranean Sea, and the influence of their effects over the Iranian region. At the first step of study, Mediterranean topography, local Mediterranean winds and cyclogenesis have studied; then structure and types of existing blocking systems on the synoptic weather maps have been introduced with figures.
In the second step of the study, all of the blocking systems that were formed over the Atlantic Ocean were identified from 1989 to 1997, the results of which are shown in the table included in the paper. Of course this part of the work was done with the help of Prof. Colucci from USA and data are given from NCEP.
Atmospheric blocking systems can lead to a stagnation of weather patterns where the patterns remain for several days or even weeks at the same location during the blocking system. This case can lead to flooding, drought, above normal temperatures, below normal temperatures and other weather extremes. Therefore, it is important to recognize a blocking pattern in its initial development. With this awareness, one can forecast up to several days in advance with a high degree of accuracy.