تبیین علل کاهش یافتن مقدار و شدت بارش‌‌های زمستانه در قیاس با بارش‌‌های پاییزه در سواحل جنوبی دریای خزر

The causes of wintertime precipitation decrease in the Southern Coast of Caspian Sea (SCCS) compared to its counterpart in autumn, is investigated with the use of synoptic stations daily dataset of SCCS and gridded NCEP/NCAR and NOAA dataset. The structure of atmospheric circulation and synoptic and physical conditions over the Caspian Sea region which are dominate in the winter and autumn precipitation events are analyzed. The results revealed that the anticyclone which is located over the north and/or west of Caspian Sea is highly reinforced compared to its counterpart in autumn in the all wintertime precipitation synoptic patterns. Also, there is a considerable increase of anticyclonic circulation intensity in lower troposphere over the Caspian Sea in all patterns, which the mean value of it in wintertime high pressure pattern is reached to twice of its value in autumn. The investigation of vertical velocity over the Caspian Sea region indicates an outstanding Sea-Saw behavior between the northern and southern Caspian Sea during autumn. As such, the maximum ascend and descend of air flow is seen at 45? and 37.5? N, respectively. During winter, as the autumn Sea-Saw pattern is weakened, the frequency and intensity of ascending air is decreased over the SCCS. The repeated suitable settlements of anticyclone centers over the northern Caspian Sea in autumn, and their considerable meridional variations of them in winter caused the obvious seasonal differences in ascending air and hence in precipitation occurrence. The investigation of SST and latent heat flux over the Caspian Sea showed that there is a significant positive Correlation between the precipitation amounts in SCCS and the SST and latent heat flux. As the cold period prevails, because of the SST decrease there is a decrease in the evaporation over the Caspian Sea and consequently there is a decrease in the precipitation of SCCS. The investigations also revealed that there is a strong and significant negative correlation between the northerly wind magnitude at the lower troposphere and SST, latent heat flux, and precipitation of SCCS during the cold period. Therefore, the more the intensity of northerly winds increase over the Caspian Sea, the more latent heat values, SST and precipitation intensity decrease in return. The results showed that the anticyclones which are settled over the northern and western parts of Caspian Sea are stronger during winter. Consequently, the pressure of lower troposphere is considerably increases over the southern part of Caspian Sea. Such an increase is associated with the noticeable meridional variability of wintertime anticyclones. The changing of dominate wind direction in the most important pattern for precipitation in the Caspian region, and also the decrease of SST and latent heat flux. The aforementioned conditions are caused by the decrease of amount, intensity and numbers of precipitation days during winter.