An Investigation of Iran’s Precipitation Anomalies in Relation with Atmospheric Blocking

Abstract

Atmospheric blocking is a synoptic-planetary scale system that disrupts the prevailing westerlies. An important feature of this phenomenon is that in addition to being large-scale, it is quasi-stationary, which could lead to climatic anomalies within regions upstream and downstream of the main occurrence. In the present study, using CBS, blocking occurrences were identified for region 60° W to 50° E for a period of 1978–2007. To determine precipitation anomalies, standard scores were used. For doing so, standard scores higher than 0.5 and lower than −0.5 were defined as precipitations with anomalies. Standard scores defined covering more than half of all stations, were considered as positive and negative precipitation anomalies for Iran. The coincidence of positive and negative precipitation anomalies with dates of blocking occurrences has revealed using components of atmospheric circulation such as zonal and meridional wind, vorticity advection and omega. The results indicated that if a southern trough of dipole blocking places over a region, it will lead to precipitation in longer period. In addition, the quasi-stationary nature of blocking increases vorticity advection and will retain the positive value of vorticity during a blocking occurrence in front of a cyclone.