Mid-Latitude Cyclones Climatology over Caspian Sea Southern Coasts – North of Iran

Abstract The southern coasts of Caspian Sea is subject to synoptic/mesoscale weather systems ranging from locally enhanced sea breeze formation and small local front systems to synoptic scale cyclones. The intensity and structure of these weather systems cover a wide spectrum, which include cold-cored weather systems from the Siberian highs as well as warm-cored mid-latitude cyclones, hereinafter referred to as Westerly systems. Heavy coastal rain and combined effects of wind and sea state accompanying these cyclones threaten life and property in the southern coasts of Caspian Sea. This study presents climatology of cyclones over the southern coasts of Caspian Sea covering a ten year period 1996-2005. Altogether 57 cyclones were formed during the ten year period. A noticeable seasonality is observed in evolution of cyclones over the entire Caspian region, a majority of the 57 cyclones (73.7%) were developed during winter and fall seasons while the remaining (26.3%) occurred during spring and summer seasons. Most of the cyclones were of low intensity, out of 57 cyclones observed during the ten year period 16 (28%) were deep depressions and 24 (42.1%) were cyclonic storms. Altogether 5 super cyclonic storms were observed during the period, out of which 4 were observed during winter and fall seasons. Mid-tropospheric, large-scale processes and local features were responsible for the initial development of all weather systems. The Mediterranean Sea plays a significant role in cyclogenesis and propagation of the systems the Caspian region. Further, a Mediterranean cyclonic system formed during October 2001 was studied in detail using backward trajectory Lagrangian model: Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT). The HYSPLIT model outputs confirmed the observed synoptic features for the weather system of the case study.

Key Words: High-Pressure Systems, Mid-Latitude Cyclones, Moisture Convergence, ECMWF, APHRODITE, HYSPLIT Model, North of Iran