Seasonal variability of hydrographic structure in Sharm Obhur and water exchange with the Red Sea

Abstract

Sharm Obhur is a narrow coastal inlet about 10 km long. The maximum depth at the entrance is about 35 m, which decreases gradually towards the head. Nine field trips were conducted for hydrographic survey in the Sharm during April 2015–January 2016 covering pre-summer transition, summer, pre-winter transition and winter seasons. In each trip, eight stations along the central axis of the Sharm were occupied for the measurement of temperature and salinity. In addition, an Acoustic Doppler Current Profiler (ADCP) mooring was deployed near the entrance (at station 2) during 18 February–26April 2015. The vertical structures of temperature and salinity show two distinct layers—a relatively low saline surface layer and a high saline bottom layer. The thermohaline properties increase from the entrance towards the head in all the seasons except for a slight decrease in temperature during December. Near the head, the observed maximum temperature and salinity are 33.22 °C (August) and 40.36 psu (April), respectively, while the observed minimum temperature and salinity are 25.05 °C and 38.97 psu, respectively, during January. The water exchange between the Sharm and the Red Sea shows two-layer structure, with a surface inflow and a deep outflow which is typical of basins where evaporation exceeds precipitation. The pressure gradient generated by the increasing density towards the head pushes the relatively low saline surface water from the Red Sea to the Sharm with a gradient in surface salinity influenced by the evapouration and heat exchange. Near the head, it sinks and returns as a deep water flow. The estimated flushing time of the Sharm varies between 7 and 12 days with an average of 9.5 days.