Background and Objectives: The Arctic and Antarctic play very important role in the thermohaline circulation and Earth’s thermal balance. Due to the very important role of polar sea ice and the significant reduction of ice in these two oceans, climate changes’ effects and global warming are more pronounced in these areas. Reducing the sea ice thickness and increasing its transparency, exposes ocean water to sunlight and leads to an increase in water surface temperature which has intensified melting of polar sea ice in recent years. The ice-free ocean has been exposed to direct winds and common polar storms, not only drastically changing the water column, but also intensifying the melting of surface ice. On the other hand, because the ocean surface layer is the interface layer between the atmosphere and the deep part of the ocean, it is of special importance. In this study, monthly variations of the mixed layer depth and its physical properties in Franklin Bay in the Arctic Ocean are investigated.
 Methods: The data used in the present study are part of the data obtained during the CASES international program conducted between September 2003 and August 2004 in order to investigate the ecological consequences affected by the landfast ice variation due to the climate change. The data used in this study were sampled twice a day at a fixed station in Franklin Bay between late December to early June. In this study, the mixed layer depth is estimated based on the density profile using the threshold method with threshold values of 0.05, 0.2, 0.4 and 1 kg.m-3. In order to assess the most appropriate threshold value the estimated depths were compared with visual mixed layer depths using a number of statistical indicators. Thereafter, monthly and seasonal variations of the mixed layer characteristics including temperature, salinity, density and depth were investigated.
Findings: The results showed that in the study area the most appropriate threshold value for estimating the mixed layer depth is 0.4 kg.m^-3. The findings also showed that in Franklin Bay, the mixed layer depth and its characteristics have monthly and seasonally variations. These variations are impacted by meteorological variations such as the temperature and wind intensity and direction. The mixed layer depth started to increase from the beginning of the observation until it reached its maximum value in April and thereafter decreased. On the other hand, the temperature, salinity and density of the mixed layer started to decrease, increase and increase, respectively, from the end of autumn, and after reaching its minimum, maximum and maximum value in April, it started to increase, decrease and decrease. The temperature of the mixed layer during the winter and spring was close to freezing point and in spring it was colder than the winter. The salinity range during the winter was larger than that during the spring and in general, salinity in spring was higher than that in the winter. In addition, observing the wind intensity and direction showed that the prevailing easterly winds between 12 and 20 December 2003 leads to an ice edge upwelling in the Franklin Bay. Upwelling brought relatively warm and salty waters to the surface and the mixed layer was totally destroyed during this period.
Conclusion: Oceanic mixed layer depth is affected by various parameters such as air temperature, ice freezing/melting, intensity and direction of the wind, as well as other oceanic processes such as upwelling. As the air temperature gradually decreases salt rejection due to the ice formation starts. Increasing salinity at the water surface leads to the increased surface layer density which brings convection throughout the cold season, as well as increasing the mixed layer depth. The mixed layer depth reached its maximum in early April, then started to reduce with higher temperature which causes ice to melt. Ice melting results in an increase in surface fresh water that making surface water stratified which leads to decrease the mixed layer depth.