Seasonal variations of the water column structure and estimation of the mixed layer depth based on the temperature using threshold method in Babolsar and Ramsar regions

摘要

The physical processes play an important role on the biochemical phenomenal in the seas and oceans. The Mixed layer is the surface layer in which due to the air-sea exchange, the physical parameters such as temperature, salinity and density are almost constant. The layer beneath the mixed layer where the gradient of the physical parameters is large, is called thermocline, halocline and pycnocline, respectively in the temperature, salinity and density profiles. The deep part is the deepest layer where the physical parameters are nearly constant. Because the mixed layer acts as an interface between the atmosphere and deeper layers of the sea, its depth is not only influenced by weather but also strongly impacts the climate change. The mixed layer depth (MLD) has an important role in biochemical processes, gas exchanges, transferring heat, mass and momentum between the atmosphere and the sea. In this study seasonal and spatial variations of the MLD as well as the temperature and the salinity profiles are investigated in the Southern Caspian Sea in the Babolsar and Ramsar regions based on the Conductivity-Temperature-Depth (CTD) measurements conducted during fall, spring and summer 2012. According to the observations, despite the fact that the range of variations of the temperature and the salinity in the Babolsar and Ramsar is comparable, during the spring the salinity fluctuation inside the halocline is larger in Babolsar. It is worth to mention that the salinity fluctuates highly inside the halocline, contrary to the classic definition that the salinity increases with depth inside the halocline. The MLD has been estimated using the threshold method with four different threshold values (0.05, 0.5, 1 and 1.25 (°C)). In order to avoid erroneous estimation of MLD (very extreme values), each temperature profile is also carefully examined by visual investigation. Then visual inspection and statistical analysis approaches have been employed to assess the most appropriate threshold value. To this end, calculated MLDs using different threshold values have been plotted against visual MLDs. Large number of points away from line of 45° shows that the calculated MLDs using related threshold value is biased against visual MLDs. While the largest number of points around 45° line demonstrates that the MLDs estimated by both methods are similar to each other and the considered threshold value is an appropriate one. The results reveal that the seasonal hybrid algorithm with threshold values of 0.5 (°C) for fall, 1 (°C) for summer, and 1.25 (°C) for spring gives the best estimation for the MLDs. The calculated MLDs show that the MLD is maximum in the fall and minimum in the spring which is in agreement with Jamshidi et al. (2010). The reason for a deeper MLD in the summer compared to the spring can be related to the high evaporation during this season, which leads to salinity increase at the surface and augmentation of the convection. Spatial comparison of the MLDs in Babolsar and Ramsar regions shows that the MLD is slightly deeper in Ramsar and the gradient of the temperature just below the mixed layer in Ramsar is larger compared with that in Babolsar. The vertical structure of the mixed layer can be sub-divided into three principle types: the classical, stepwise and inclined types. The classical and stepwise type profiles are similar to the results reported by Tai et al. (2017) conducted in the principle northern South China Sea. The classical type has quasi isothermal mixed layer followed by a steep thermocline which is the most observed in the fall. In the stepwise type, the temperature decreases inside the mixed layer with one or more small steps before drastical decrease in the seasonal thermocline. The stepwise type has been observed more often during the summer. Finally in the inclined type which is occurred in the spring, the MLD's temperature gently decreases with depth followed by an abrupt decrease of the temperature in the thermocline.