Identification and Climatology of Cut-off Lows over IRAN and Relationships with ENSO and NAO

• Main Authors: Ghasem Azizi, mohammad Moradi, hossein Rezaei
• Format: Article
• Language: fas
• Published: Afarand Scholarly Publishing Institute 2018-04-01
• Series: تحقيقات جغرافيايی
• Subjects: historical geography; rishahr; old era; assanid dynasty.
• Online Access: http://georesearch.ir/article-1-288-en.html

Cut-off low pressure systems are defined as closed lows in the upper troposphere that have become completely detached from the main westerly current. Cut-off low is one of the phenomena affecting the climate of Iran. This article attempts to examine the extent of this effect and its relation to some of the Indects Teleconnection. The characteristics of Cut-off Lows in Iran are studied for the period 1976–2015. To identify these systems, the Raul Nieto algorithm was used with automatic detection capabilities. The systematic identification of Cut-off Lows is realized by applying an original automated scheme using mean daily geo-potential height, wind and air temperature at 500 hPa NCEP. Cut-off Lows are more frequent during spring in the IRAN, with a maximum of frequency in March. Over the period 1976–2015, the annual Cut-off Lows frequency exhibits a positive trend. In this study, the most cutoff lows lasted only 2 days. In the seasonal study of lifetime, it was revealed that in the winter and spring, the highest survival was 2 days, and in the summer and autumn, it was one day. Cut-off Lows Systems are largely influenced by the ENSO and North Atlantic Oscillation (NAO). During La Nina events Cut-off Lows are found to be more frequent and during El Nino events Cut-off Lows are found to be less frequent in Iran. In general, NAO negative phase exhibits a lower number of Cut-off Lows systems. During positive NAO months, a high occurrence of Cut-off Lows is detected for cold seasons on the other hand, during negative NAO months this high density of Cut-off Lows is detected for warm seasons. Finally This study provides an observational case study of a cut-off low that developed over North of Iran early February 1991. This Cut-off Low is formed in the eastern arm of the Omega Block. © 2018 Geographical Researches