عنوان مقاله [English]
Based on the report by Islamic Republic of Iran Meteorological Organization (IRIMO) published as part of a greater report on the state of the world climate in 2011 by the American Meteorological Society (Blunden and Arndt, 2012), large parts of Iran, from central to northern and northeastern areas, have experienced significant negative anomalies of surface temperature together with positive anomalies of precipitation in autumn 2011. The temperature and precipitation anomalies have been determined with respect to the climatological mean values over the period 1960 to 2010. As the establishment of a prolonged period of cold weather in Aban 1390 (23rd Oct. to 21st of Nov. 2011) together with abundant precipitation in the form of both rain and snow played a great role in shaping the climate anomalies of autumn 2011 in Iran, this study aims to investigate the large-scale dynamical processes involved in the climate anomalies of this period. Such studies are particularly important, when the increase in the frequency of extreme climate anomalies in recent years and its possible link with global warming is noted. To this end, the NCEP/NCAR reanalysis data are used for the concerned period and the long-term mean fields (from 1950 to 2010). The main analysis tools used are the analysis of the anomalies of geopotential height and temperature in the lower and middle troposphere, jet speed and relative vorticity in the upper troposphere, the computation of the blocking index (BI) introduced by Wiedenmann et al. in 2002, and the energy diagnostics. The latter includes the eddy kinetic energy, ageostrophic geopotential flux and its convergence, total flux and its convergence, baroclinic generation, baroclinic conversion, and barotropic conversion.
The results for the year 2011 indicate the action of two consecutive blocking systems, which extended their central ridges over Europe with their troughs stretched over the North Atlantic and the west of Asia. The two blocking systems were peaked in the 3rd and 21st of Aban 1390, with respectively moderate and high intensities as measured by BI. In addition, the obtained results show that a branch of Siberian high-pressure system extended to the west of Asia associated with a positive relative vorticity anomaly in the north of Iran, lead to vigorous cold air advection to the North and Northwest of Iran. The increase in eddy kinetic energy over a band stretched from the North Atlantic to the Mediterranean and Black Seas in Aban 1390 was associated with an increase in the strength as well as the zonal and meridional extensions of the subtropical jet. Concerning energy diagnostics, the positive anomalies of the ageostrophic and total flux convergence over Iran indicate that the country was a favorable region for receiving large amounts of energy. Also, the flux vectors demonstrate that the main passage of this energy to Iran was through a north–south extent that included an emission area over the Black Sea. This was further confirmed by the analysis of baroclinic generation, which showed a positive anomaly over the Black Sea. The analysis also shows that the low-frequency phenomena and teleconnection patterns, including the positive phases of the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), and the positive phase of the East Atlantic–West Russia (EA–WR) may have played a part in shaping the climate anomaly over Iran in Aban 1390.