عنوان مقاله [English]
A thorough appreciation of the dynamic effects of bottom friction on water mass flow requires an understanding of the boundary layer characteristics, known as Ekman layer. Ekman layer on a sloping bed includes upslope or downslope flows that may intensify mixing or change the thickness of the Ekman boundary layer. The bottom friction can also reduce the fluid current, thereby reducing the Coriolis force and destroying the geostrophic current, named as the process of spin down.
This study is conducted to investigate the impact of bottom slope on the distribution of physical parameters of water and the resulting phenomena, in order to identify the characteristics of the Ekman layer on the bottom slopes of the western Strait of Hormuz. The research utilizes the CTD (conductivity, temperature, and depth) field data of the western Strait of Hormuz in 2005 gathered by the National Institute of Oceanography of Iran. The vertical profiles of temperature, salinity, density, tidal currents and the horizontal cross sections of density were made by using Excel and MATLAB software.
The results indicate that the current on the bottom of Strait of Hormuz is of an upslope type, and the pycnoclines become almost perpendicular to the slope with the increase of slope of the bottom. The minimum Ekman flux spin down is estimated of two hr and is related to a station with the maximum slope. Greater bottom slope and stronger stratification are found to remarkably shorten the shutdown timescale of the Ekman layer. We estimated the eddy diffusivity, , between zero and 0/005 m2s-1. A positive correlation was also found between eddy diffusion coefficient and Ekman layer parameters, which is indicative of the non-stationary flow and mixing due to Ekman layer stoppage.
The horizontal scale that characterizes the dimension of a boundary current is the Rossby radius of deformation. Since baroclinic flow involves a number of internal modes, there will be a Rossby radius of deformation for each mode. The average Rossby radius of deformation in the south of the island for the first baroclinic mode is estimated at about 5.6 km, and buoyancy frequency in the Strait of Hormuz is 0.02 ( ). Spin down time ( ) of the stations located in the south of Qeshm for the first baroclinic and barotropic modes were also calculated. Results show that the greater the depth of the boundary layer on the bottom, the larger the spin down time for the first baroclinic mode. When Ekman layer is arrested in the barotropic mode, it seems that the spin down time for the first baroclinic mode becomes very large; thus we can conclude that the spin down does not happen in the arrested Ekman layer. Typical thickness of the Ekman layer in the Strait of Hormuz varies between 0.5 and 22 meters.
Since the most important driving force of water up the slope is the tide, the tidal level changes about the long mean for the periods of ten years, one year, one month and one day are examined in the area. The results show that in the northern Strait of Hormuz, the maximum tidal range is about 4.73 meters and the average neap and spring tides are 2.15 and 3.53 meters, respectively. The calculated tidal currents and the related profiles for all the stations indicate that the maximum tidal speed is seen at the top of the bottom mixed layer. As the bottom is approached the speed of tidal current decreases remarkably and turbulence dominates which represent the state of the surface layer.