The investigation of atmospheric circulation patterns during wet and dry spells over the southern coast of the Caspian Sea

The prevailing synoptic pattern and the characteristics of the regional atmospheric circulation were studied in order to understand the predominant mechanism which governs the dry and wet spells over the southern coast of the Caspian Sea (SCCS). Monthly precipitation data from 20 weather stations located in the SCCS were analyzed for a 30-year period (1974-2003). Over this period, 10 widespread dry and 10 widespread wet months were identified for the Caspian region by investigating the spatial and temporal distributions of precipitation. Gridded precipitation data for the Middle East, provided by Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of the Water Resources (APHRODITE) on 0.5×0.5 degree grid along with the NCEP/NCAR reanalysis dataset on a 2.5×2.5 degree global grid were employed to determine the characteristics of the atmospheric circulation and to identify the predominant synoptic patterns during the dry and wet spells. The results indicated that the existence of a very strong ridge in the middle troposphere along with the formation of a strong anticyclone in the lower atmosphere over the northern-side of the Black Sea can be considered as the main features of the wet spells over the SCCS. The findings also indicated that the wind direction at the lower atmosphere over the Caspian Sea can be used as an appropriate criterion to separate the wet and dry spells over the SCCS so that the northerly prevailing wind during the wet spells is replaced with the southerly prevailing wind during the dry spells. Additionally, the occurrence of the dry and wet spells in the region is associated with the formation of a sea-saw pattern in vertical velocity values over the Caspian Sea. During the wet spells, formation of the descending prevailing motions over the north half of the Sea simultaneously leads to predomination of the ascending prevailing motions over the southern part. Meanwhile, during the dry spells, a reverse pattern resulting from the ascending and descending motions dominates over the Caspian region. The findings also show that it is reasonable to use the sea-level pressure changes in the Black Sea region as an appropriate index (predictor) to interpret the spatial and temporal variations of precipitation over the SCCS. In this way, during the wet spells, an increase in the sea level pressure over the Black Sea and its consequent positive feedbacks including an anticyclonic circulation over the Black Sea, northerly wind strengthening over the Caspian Sea, and air ascending over the southern part of the Caspian Sea will eventually cause an increase in precipitation over the SCCS. A reverse condition will be found in the dry spells. The investigation of the cyclones and anticyclones frequency and track during dry and wet spells indicates that the northern corridor of the Black Sea plays a very significant role in occurrence of the dry and wet spells over the SCCS. It is found that during the wet spells, the activity of the anticyclones in the northern corridor of the Black Sea is considerably increased. On the other hand, during the dry spells, a decrease in the activity of anticyclones in the north corridor is associated with a significant increase in the activity of cyclones in the region. In addition, an increase in the activity of anticyclones over the Middle East region and the Siberian high-pressure activity over the eastern-side of the Caspian Sea are taken into account as the main features of the dry spells.