Investigation of salinity and temperature of Persian Gulf water by FVCOM Model

Identifying shallow water masses is much more complex than deep water; because physical and chemical changes in shallow water occur faster and the properties of the water mass are lost. Since in the absence of air-sea interaction, water masses have different properties, so in this study, due to temperature and salinity conservation, the properties of Persian Gulf water mass are identified. One of the main characteristics of the Persian Gulf is its very shallow depth with an average depth of 35 meters. The maximum depth is located in the Strait of Hormuz with a depth of about 100 meters. The Strait of Hormuz is located in the northwestern part of the Indian Ocean and is the junction of the Persian Gulf with the Sea of Oman.

In this research, a 3D open source ocean model FVCOM in 20 layers was used to model the water exchange between the Persian Gulf and Sea of Oman (47˚E to 59.45˚E and 22˚N to 32˚N) without considering the wind stress on the region, the flow pattern of Persian and Oman Gulfs and the water mass of Persian Gulf in the four seasons of spring, summer, fall and winter should be studied. FVCOM uses the finite volume method to discretize hydrodynamic equations in a triangular grid. Specialized SMS software version 10 was used to generate the computational network. A non-uniform computational network with a horizontal resolution of at least 950 m to 4700 m was used in the model and the bathymetery information was interpolated with 30-second accuracy from the GEBCO-2019 data on this network. Water level fluctuations on open boundaries correspond to the location of the open boundary nodes of the computational network located at 59.45˚E, which was extracted from the TMD and entered as the main configuration of the model. Temperatures and salinities profiles of open boundaries from HYCOM model output (freely available) were also used at standard depths. The model was run in the absence of air-sea interaction and the role of wind stress for 6 years. HYCOM and satellite data were used to calibrate and validate the model, respectively.

The saline waters of the Persian Gulf to the mouth of the Strait of Hormuz are well visible in the surface layers and are not seen in the surface layers from the mouth of the Persian Gulf to the Sea of Oman. In the middle layers of the Oman Sea, there are waters with the characteristics of the waters of the Persian Gulf, which can be concluded that the surface waters that existed in the west of the Strait of Hormuz have been transferred to the lower or middle layers and flowed to the Oman Sea. Considering the temperature and salinity conservation in the water mass, the temperature and salinity of the Persian Gulf in the Strait of Hormuz were studied and the results show that the Persian Gulf water mass (38psu) in the summer season through the south of the Strait of Hormuz as a subsurface flow and in the winter season this water mass penetrates to greater depths and moves further away from the Strait of Hormuz. The Persian Gulf water mass exits through the southern part of the Strait of Hormuz as a subsurface flow. In the summer season, there is a water mass with 37psu salinity in the depth range of 50 to 80 meters and 40 km off the coast of Oman, whose exact position is latitude 25.4˚N and longitude is 56.50˚E to 56.69˚E and in the cold season, it moves away from this area and advances to lengths greater than 57˚E and penetrates to a depth of about 130 to 150 meters and more depths.

Due to the high rate of evaporation in the Persian Gulf, to replace the evaporated waters of the Persian Gulf, it is necessary for water to enter the Persian Gulf from the Strait of Hormuz. The salinity results of summer show this phenomenon well and the water entering the Persian Gulf is diverted to the coast of Iran and the movement of these waters from the coast of Iran to the northwest of the Persian Gulf continues. There are years, but it is more intense in summer than in other seasons.