عنوان مقاله [English]
The southern coast of Caspian Sea, located between Alborz Mountain range and the Caspian Sea, is considered as the smallest climatic zone in Iran (Alijani, 2007). Factors such as the constant passage of troughs and ridges of the western waves, adjacency to the strongest high pressure center of northern hemisphere, and having been located between the largest internal body of water on the earth and the north, and the Alborz high mountains to the south have led to the formation of one of the most diverse, yet prominent climactic zones of Iran at the southern coast of the Caspian Sea. Accordingly, the highest extents of annual, seasonal, daily and even hourly precipitation of the country has been recorded in this region. Meanwhile, with its clear regional diversities and temporal precipitation changes, this zone has witnessed the most intense and durable rainfalls across the country (Mofidi, Zarin, & Karkhaneh, 2014).
The majority of studies conducted on precipitation at the southern coast of the Caspian Sea have been focused on heavy rainfalls; subsequently, the following question can be posed: Can there be an index to study these precipitations so that there would be a significant relation with widespread precipitations at the southern coast of Caspian Sea whilst demonstrating a significant behavior relative to other teleconnection indices of precipitations at this region? By analyzing and summarizing the synoptic features causing heavy and widespread precipitations during the fall season at the southern coast of the Caspian Sea, the present study seeks to offer a new index using their intersection in line with the study of these precipitations.
Review of Literature
The literature shows that the involvement of convective, orographic, and dynamic factors result in rainfalls at this region. In one of the earliest investigations, Ganji (1975) considered the orographic ascension of humidity of the Caspian Sea from the northern slopes of Alborz mountain range as the cause of precipitations at the Caspian region. Khoshhal Dastjerdi (1997) identified 115 synoptic systems from ground surface to the level of 200 hectopascals during a 20-year period (1970-89), resulting in precipitations if over 100 mm at the southern coast of the Caspian Sea. He concluded that precipitations of over 100 mm occur at coasts due to the entrance of strengthened fronts of the sea breeze through synoptic systems; moreover, the increase in the precipitation is the result of elevation at northern slopes of Alborz caused by the reinforcement of anabatic winds through said system and the ascension of warm and humid weather through sea breeze fronts to the valleys atop the slopes.
To conduct a synoptic examination of heavy precipitations at the Caspian region and to present an index for analyzing these precipitations, daily rainfall data at 8 synoptic stations, located in Iranian Northern provinces including Guilan, Mazandaran, and Golestan were used. To this end, daily precipitation data from 1986 to 2010 were collected from the Iran Meteorological Organization. In this study, the variable thresholoding method was used for each station to indicate heavy and widespread precipitation. Days on which the precipitation at the station was more than the long-term average were considered heavy precipitation; if heavy rain was recorded in more than 70% of stations on that day, then it was considered as heavy widespread precipitation. Next, the synoptic state and the dominant structure of climate flow were studied. Consequently, data including mean sea level pressure, geo-potential height, and orbital and meridional wind components for 17 climate levels during the aforementioned days were collected from the National Center for Atmospheric Research/ National Center for Environmental Prediction (NCEP/NCAR). The data was employed to produce and analyze mixed sea level pressure maps, wind vector field, relative vorticity, and geo-potential height changes for different periods. Finally, the main synoptic patterns were indicated. Given the need for the classification of the maps as well as the clustering’s ability to reduce the data and find the real groups, the compact hierarchical clustering method was employed using Ward’s agglomerative method.
Results and Discussion
Considering the presented definition of heavy and widespread precipitation, 104 events of these precipitations from 1986 to 2010 in the region were identified. Based on the results of sea level pressure clustering for the precipitations, three distinct, significantly different synoptic patterns were identified. Accordingly, three high pressure Siberian and migrating patterns are the causes behind heavy and widespread precipitation at the region during fall. The results of the synoptic analysis of heavy and widespread precipitation days at the southern coast of the Caspian Sea showed that one of the main causes of the occurrence of such rainfalls is the formation of pressure gradient over the Caspian Sea in which the Northern streams are led towards the southern coast. However, this factor required quantification in order to confirm this claim. Therefore, the mean linear pressure in the north of the Caspian Sea was reduced from the mean linear pressure near the southern coast so as to obtain the pressure gradient value which demonstrated wind conditions over the Caspian Sea.
The results showed that heavy and widespread precipitations occur at the same time as the high pressure pattern settles on the northern half of the Caspian Sea in such a way that a high pressure present over the sea results in the formation of northern streams above warm waters of the Caspian Sea which in turn leads to humidity absorption from the bottom and unsustainability at the end of the path.
Synoptic examinations on this type of precipitation showed that in case all factors result in the formation of northern wind and the synoptic conditions of the upper level enables unsustainability, heavy and widespread precipitations will occur. Exploring the CGP index with other active teleconnection and Siberian high patterns and precipitations at the stations suggest a stronger relation between these indices and all the aforementioned factors compared to the former indices.