Seasonal changes of the Persian Gulf water mass in the Gulf of Oman

The Gulf of Oman, with an area of about 94000 km2 and a maximum depth of more than 3000 m, is located between latitudes 22 and 26 degrees of north and longitudes 56 and 62 degrees of east. This gulf plays the role of a waterway that connects the Persian Gulf to the Indian Ocean and is the place where the saline water of the Persian Gulf is exchanged with the less salty water of the Indian Ocean. This research aims to investigate the Persian Gulf Water mass (abbreviated as PGW) in the Gulf of Oman and investigate its seasonal changes.

In order to identify PGW, in the studied area, the international temperature and salinity data set of the World Ocean Atlas with the abbreviation WOA18 and the spatial accuracy of 0.25° longitude and latitude at the depths of zero, 150 and 300 meters were used seasonally. The surface maps were ploted for the mentioned depths in Ocea Data View (ODV) software. A vertical cross section map was also illustrated in the middle of the Gulf of Oman to determine the thickness of the PGW, the extent of its expansion, and the depth of its placement (based on the salinity line of 36.45 psu).

The results show that the density on the surface is mostly a function of temperature. Winter is the coldest (23.24˚C) and densest (25.12 kg/m3) season and summer is the warmest (32.42˚C) and least dense (22.41 kg/m3) season in the surface layer. Summer experiences the highest and lowest salinity at the surface (36.45 psu<ssurface<36.66 psu) and at a depth of 150 m (36.12 psu<s150 m<36.45 psu). On the contrary of the surface layer, where winter is a cold season and summer is the hot season, at the depth of 150 m, summer is the cold season and winter is the hot season, which can be attributed to the differences of the seasonal thermocline up to the depth of 150 m. At the depth of 150 m, the density pattern in summer and autumn is a function of salinity, but in spring and winter, the density pattern is a function of temperature. In addition, in summer, sub-mesoscale high-pressure cyclones near Muscat can be seen on the surface, which are not clearly seen in other seasons. Also, the results clearly show that PGW is not seen in the surface layer and flows subsurface from the southern side of the Strait of Hormuz towards the Gulf of Oman. The depth of placement and extent of PGW has seasonal and spatial changes, and by moving eastward in all seasons, the thickness of PGW decreases due to mixing with the surrounding waters. The results obtained from the vertical cross section AB, in the middle of the bay, show that PGW is observed during different seasons from the depth of 150 to 375 meters, which is shallower in spring and summer and has less depth buoyancy than in autumn and winter.</s</s
In the side results of this research, several fronts such as Ras Al-Had, Fins and Al-Ramis were observed in the quantity of temperature, salinity or density, which had seasonal changes. These fronts were especially noticeable in layers such as 150 m and 300 m depth because the existence of subsurface fronts is less reported in the region. Also, the results of the research showed dense and less dense cores, some of which are formed due to the eddies in the area and others due to the hydrodynamics of the area. For example, the low salinity core (13.36 psu) near Chabahar can be due to the occurrence of the upwelling of the area, which brings the less saline waters of the middle of the gulf to the surface and causes a decrease in salinity.

The results of this research clearly showed that PGW has seasonal changes and is located at shallower depths in spring and summer than autumn and winter. Examining the same surface maps, especially at the depth of 150 m, showed that PGW, in addition to the southern coast, can also be seen on the northern coast, which requires a more detailed investigation regarding the possibility of its presence on the northern coast or its multiple branches.