Sea breeze is a local circulation taking place over the coastal areas. This phenomenon is as a result of the thermal dissimilarity between the land and sea. The study of SB characteristics in relation to synoptic-scale flows is important due to their influence on precipitation, air pollution transport and diffusion and intensity of convection. The focus of this paper is on sea breeze (SB) characteristics during May-September and the effect of synoptic prevailing winds on this phenomenon in the Caspian Sea Southern Coasts. The data set including 168-days sea breeze for a period of 17-years (1990-2006) were determined over the southern coasts of Caspian Sea. All days under the study were investigated in order to determine the type of flows in the sea level and at the 850-hPa, and their impacts on the sea breeze as well as sea breeze developing towards the land. In order to obtain the best results, we employed the outputs of TAMP model as well. The wind regimes were classified into 2 categories (onshore and offshore). Finally, the main characteristics of sea breeze were tested include Mean lag of the SB passage, wind speed and direction at the onset time, Mean duration of the SB and sea breeze developing towards the land. It is found that in comparison with onshore synoptic flows, offshore flows postpone arrival and termination of SBs. On the contrary, the strong onshore flows accelerate the start of sea breeze. Three simulations presented in the case study confirmed the effect of sea surface colder temperature and thermal difference between land-sea across the coast on the weakness and strength of the sea breeze circulation; it means that when the offshore synoptic wind flow is mild and gentle, the thermal gradient of sea-land appears which is necessary for the beginning or end of the sea breeze circulation. The results showed when the prevailing synoptic winds oppose the sea breeze, the effectiveness and power of the sea breeze depends strongly on the inter-diurnal variations of the synoptic pressure gradient and temperature gradient between the land and sea.
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