Numerical Modelling of sound velocity profile in different layers in the Persian Gulf Masoud Sadrinasab a , Karim Kenarkoohi b a Khorramshahr University of Marine Science and Technology, Khorramshahr, IRAN. b Malek Ashtar University, Shiraz, Iran. Dr. Masoud Sadrinasab, masoud.sadri@gmail.com, fax no: +98 632 4234409 Abstract: The three-dimensional variability of sound speed in the Persian Gulf is investigated. In this study, a three-dimensional hydrodynamic model (COHERENS) is employed in a fully prognostic mode to derive sound speed profiles in the Persian Gulf, an evaporation-driven inverse estuary that is governed by the import of surface water from the adjacent ocean and the export of saline bottom gulf water through the Strait of Hormuz. During spring and summer, a cyclonic overturning circulation establishes along the full length of the Gulf. During autumn and winter, this circulation breaks up into mesoscale eddies, laterally stirring most of the Gulf’s surface waters. Results of the model show that sound speed in the Persian Gulf depends mainly on the temperature in the surface layer whereas the bottom layer as well as the southern part of the Gulf depends on temperature and salinity. Maximum sound speed occurs during the summer in the Persian Gulf which decreases gradually moving from the Strait of Hormuz to the north western part of the Gulf. A gradual decrease in sound speed profiles with depth was commonly observed in almost all parts of the Gulf. However, an exception occurred in the Strait of Hormuz during the winter. The results of the model are in very good agreement with previous observations. Keywords: Persian gulf, Numerical modelling, Sound velocity, Strait of Hormuz.