عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Blocking is one of the most important weather systems in mid-latitudes that have significant effects on atmospheric air flow and regional weather conditions. In this study, climatology of Asian and European blockings was investigated for the period of 1950–2010. The data used in this study were 500-hPa geopotential heights from the National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset with 2.5°×2.5° resolution. The blocking events were detected using two indices; the first one, a one-dimensional index, was the new version of the Tibaldi and Molteni (1990) index presented by Barriopedro et al. (2005) and the second one, a two-dimensional index, was presented by Diao et al. (2006). Then, applying these indices, the characteristics of detected blockings such as frequency, duration, intensity, position of center and extension were obtained and compared with each other.
In the period of this study, 1054 and 729 blocking events were identified using the first and second indices, respectively. The spatial and temporal distribution analysis showed that blocking was most prevalent in the longitude band between 30 °W and 47.5 °E and most of the maximum intensity occurrence was in the east of Atlantic sector between 30 °W and 5 °E.
It was found that blocking events were more frequent in the Atlantic-European sector than in the other sectors in winter, spring and autumn, but more persistent in the Atlantic-European sector than in the Asian sector for all seasons. Additionally, the maximum amplitude of intensity was 3.0 for the first index and 150 gpm for the second index. A comparison of the blocking events in the Asian sector with the Atlantic-European sector indicated that the intensity of winter events in the European sector was higher than that of the other seasons. This may be due to synoptic-scale eddies and planetary waves which are more active and stronger in winter compared to the other seasons.
The trend analysis displayed that the number of blocking events in the period of this study was decreasing slightly in the entire region, including Iran. The number of events was significantly more in the west and northwest of Iran than in the central and eastern parts.
Also, the results of two indices showed that the latitude band of blocking occurrence and its maximum frequency was similar in the different seasons for the first index, whereas this band changed for the second index. The maximum frequency of blocking event was seen in latitude 55 °N for winter, spring and autumn and in latitude 62.5 °N for summer. Thus, it indicated that blockings occurred in a higher-latitude band in summer than the other seasons. In addition, it was found that there was a good consistency between the longitude band of blocking event, including the Asian and European sectors, and its frequency in summer and autumn using the two indices. In the other seasons, the longitude band of the Asian and European blocking events, especially in the eastern part of the Asian sector, was different for the two indices. The other noticeable point was that the activity centers of blocking events in winter, spring, and autumn obtained by the second index were located more westward of the Atlantic-European sector and more eastward of the Asian sector than those obtained by the first index.
The main conclusion was that since the Diao et al. (2006) index is a two-dimensional one, it is able to identify better and more precisely the blocking events and their characteristics as compared to the Barriopedro et al. (2005) index.