After the tidal current created by gravitational force of the Sun and the Moon, the second most dominant reason for surface ocean currents is circulation of the air in the closest layers of the atmosphere to the Earth (wind). Currents in Gulf of Oman are affected by Arabian Sea and Indian Ocean. Reviewing available literature indicates that due to 120-day-duration monsoon and tropical storms in the Arabian Sea and Indian Ocean, surface ocean currents are extremely affected by winds. Considering significance of ocean currents in Makran Coasts of Iran, a comprehensive 3D numerical modeling for evaluation of ocean surface currents and thermohaline circulations is accomplished. In order to gain appropriate perception of different effective phenomenon, first of all, tidal currents are modelled and after that wind and salinity concepts and values are added to the model.
The presented modeling covers parts of the Arabian Sea, Gulf of Oman and Makran coasts located in furthermost south-east of Iran. Currents measurements are also conducted in south-east part of Iranian Coasts along the Makran coast. Deployment (layout) of current meters is based on the segment of not only covering all the study area but also help studying the characteristics of the currents in the region of interest. Measuring stations are distributed in east from Gwadar Bay (at the boundaries of Iran and Pakistan) to Meydani Estuary in the west of the monitored region. Based on the available measurement data, simulations are carried out for 2015/04/01 to 2015/09/15 time interval.
A 3D modelling of currents, based on solving four fundamental equations of Momentum, Continuity, Salinity and temperature has been performed. Irrotational vortices both in surface and depth are also accounted in simulations. Initial and boundary conditions are chosen in a manner to ensure the highest possible spatial and temporal resolution. Water level and current speed boundary conditions are respectively extracted from OTPS Global Model and HYCOM Global Model 2D in depth. 3D time-series data of salinity and temperature are also provided from HYCOM. Atmospheric parameters required for current modeling are adapted from wind models previously executed in WRF for the study area. Being benefited from the available measurement data, model was optimized and calibrated.
First, a tidal current modeling was carried out by excluding impressions of other parameters due to the significance of astronomical tide and its unique feature. For this first step, results of numerical model and measurement were compared. Comparing results of simulation of tidal currents with measurement data revealed major differences between values of total currents of the region of interest and tidal currents gained from numerical modeling. Therefore, considering impressions of the other influential factors in current modeling is inevitable.
Hence, in the next step, wind and salinity-temperature forces were added to the model, respectively. Taking this approach, results of numerical simulation and measurements demonstrated more compatibility. Modeling tidal currents and wind currents shows that major currents of the region consists of wind driven currents; but in this condition, cross-shore current speeds are not estimated precisely. Taking into account the properties of salinity and temperature points out that cross-shore current speeds are mostly influenced by these two features. With more precise cross-shore constituents resulted from models, the effect of thermohaline circulations are displayed. The intensity of this phenomenon is in direct proportion with wind speed strength in the study region. Approaching the Strait of Hormoz, the saline water exiting Persian Gulf causes a thermohaline gyre in high depths close to Arabian Sea. These thermohaline circulations show great impacts on the overall currents of the study region.
The overall current speed and its components (u & v) extracted from 3D model is in a fairly good agreement with measurement data; the complexities of the ocean current is appropriately modelled. For example, in summer time monsoon, current’s principal direction is mocked fairly. Currents roses achieved from measurement analysis proved the accuracy of model in estimating the ocean current’s dominated direction.
In summary, it can be concluded that tidal currents have not got pivot role in studied region. Wind driven currents are preeminent currents. Salinity-thermohaline circulation currents cannot be ignored. Cross-shore currents are the currents mostly influenced by salinity-temperature alterations and this impact can be an indication of the great significance of upwelling phenomena and its demand to be evaluated in Makran Coasts.