Variation of the water sound speed in Southeastern Caspian Sea based on the field measurements

In this study, the seasonal variations of the sound speed in the water column and consequently the temperature and salinity in Babolsar area were investigated based on the field measurements. Data used in this study were carried out in two hot (summer) and cold (winter) seasons of 2019, from inshore to far offshore. The mixed layer depth was calculated using the seasonal combination threshold value of 1.25 (C°) for summer data and 0.1 (C°) for winter data. The results show that in the summer, due to the hot weather and increased stratification, the mixed layer depth is shallower than what is observed in winter. In winter, vertical convection increased due to the decrease in air temperature which leads to the deep mixed layer depth and in most of the stations whole water column is completely mixed. Consequently, whole column water surrounded with the sonic layer channel during the winter while in summer any sonic channel was observed. The surface temperature in summer is significantly higher than winter. Also, the thermocline slope is very strong in summer and very weak in winter.