Study of changes physical parameters in Chahbahar Bay water in winter monsoon (2006-2007)



In this study variations of field data such as temperature, salinity and density in and across the transects of Chahbahar Bay were analyzed using CTD data acquired between winter 2006 and spring 2007 (winter monsoon).The results showed that thermocline layer intensify in mouth of Bay between 2m and 6m with 7.81 ?C decrease before the Gonou hurricane occured (mid-May 2007). Below thermocline layer to 24m depth, temperature ranged between 24 ?C to 25.5 ?C with small seasonal variation. A week after weakening of wind related hurricane, density and salinity distributions throughout the Bay showed strongly stratified conditions. This stratification were generated by, the inflow of Oman sea low salinity water toward Chahbahar Bay coast, decreasing of wind speed and mixing that produced a vertically uniform density (salinity) gradient perpendicular to Bay mouth. Surface layer salinity after the hurricane was 1.6 psu lower than before it. This decrease was caused by precipitation increase and Ekman transport toward coast. Average salinity of 37.59 psu to 34.07 psu was observed above and below the halocline, respectively. Decrease of salinity with depth was related to subsurface low salinity Oman water (average depth between 25m to150m, salinity less than 36.5 psu). Upwelling from seabed upward to 9-10 m depth was caused by westerly winds. Balancing of water salinity by Oman Sea current caused density and salinity vertical gradients in the mouth of Bay that were less than that in north of the Bay. Investigation of the results of statistical tests showed that horizontal and vertical density variations were mostly due to the water temperature and not salinity. Result of Randomized Complete Block Design test shows that circulation of chahbahar bay water is cyclonic in winter and April and anticyclonic in May and June.
Material and method: The CTD data was collected at 24 stations with 3.6 km distance along 9 transects in the Bay (fig 1). Four of the transects were perpendicular to the coast and five transects were parallel to coast. Sampling of temperature, salinity and density were continuously done in middle months of winter 2006 and spring 2007 seasons. The CTD was adjusted to collect data with a time interval of one second. Metrological data consisted of wind velocity, air temperature; humidity that were obtained from a station located in the Metrological Center of Chahbahar. Meteorological station collected time series of data at 1 hour interval for period of 6 months between January 2006 and June 2007. Some corrections were done on these data to transfer them into 10m of sea surface data. In spatial data analysis normalization of measuring data has been investigated by one-sample Kolmogrov-Smirnov nonparametric test, and correlation of these data was determined by Pearson correlation test. Finally regression test was done between temperature, salinity, density and depth.
Conclusions: In this study, monthly variations of physical properties of water such as temperature, salinity and density were determined in Chahbahar Bay by CTD sampling in winter monsoon (winter 2006 and spring 2007), and the effect of meteorological conditions such as Gonou hurricane have been analyzed.
In winter, increase of turbulent kinetic energy and decrease of sun radiation caused the mixed layer depth to increase as far as seabed. In spring the thermal structure of the Bay indicated that the thickness of mixed layer decreased to 2m, indicating the deepening of the surface mixed layer due to meteorological seasonal changes due to the hurricane. In spring, before the hurricane, the temperature structure indicated a thermocline located between 2m and 6m depth with 7.81?c temperature decrease across it. This layer caused by 4?c increasing of air temperature during April and May. Density and salinity fields after hurricane showed more vertically uniform distributions of these parameters that were strongly stratified everywhere also a for this event thermocline intensifies in mouth of Bay before hurricane.
Before hurricane salinity contours showed upwelling in the Bay that has led by the subsurface low salinity Oman water. Vertical salinity variations were very small and ranged mainly between 35.99 and 36.97 with very slight seasonal variation. Salinity vertical gradient in coastal stations were more than that of the mouth of Bay. this decrease was due to the inflow of Oman Sea water. Salinity in the halocline is slightly more than the salinity in the surface mixed layer. Due to an increase in temperature, density was slightly less than winter. Salinity data before hurricane showed that this parameter decreased with depth. The time series of wind velocity and temperature showed that upwelling has cansed the movement of the seabed upward to 9-10 m depth. Investigation of results of statistical tests clearly showed that density variations trend was mostly due to the water temperature and not salinity, therefore thermohaline circulation in Bay was controlled by temperature. Also water circulation patterns were Cyclonic in winter and Anticyclonic in spring.