Background and Objectives: Different types of instability occur at sea. Layered structures are one of the most important physical phenomena in marine environments. Static stability and double diffusion are associated with density changes with respect to depth and temperature and salinity gradients, respectively. Due to the difference in temperature and salinity in the water column, marine areas are inherently capable of double diffusion convection. Their study is important because of the effect they have on some oceanic processes (including: changing water stratification, the formation of internal waves, the speed of sound) and even on marine life. In this research, the static stability and double diffusion of the waters of the Caspian coast are investigated.
Methods: In this research, field data of temperature, salinity and density of coastal areas of Mazandaran from Ramsar to Babolsar have been used. Measurements were taken in summer (July and August) and late autumn (December). Temperature and salinity parameters directly from field measurements and other parameters (density, buoyancy frequency, density ratio and turner angle, etc.) using temperature and salinity data and related relationships by Excel program ( Excel) is calculated. The profiles of temperature, salinity, density, buoyancy frequency and turner angles in the hot and cold seasons have been studied and compared.
Findings: Salinity inversion In most stations that have occurred, the thickness of the inversion layer varies from 3 to 10 meters. Most salinity changes in the inversion layer of about 0.5 psu occurred in summer. In the cold season, salinity inversion occurs at lower depths than in the warm season, which can be due to changes in the external factors of the Caspian Sea, including evaporation and rainfall. Also, slight temperature inversion was observed only in two stations with a thickness of 1.5 m and 3 m with temperature changes of 0.3 and 0.7 ° C, respectively. Due to the short range of salinity changes, density changes are mainly affected by temperature. The density ratio in salinity inversion stations is often negative, ie double diffusion convection has not occurred. However, in some stations, especially in the eastern part of the study area, the density ratio is positive and greater than 1, which indicates the occurrence of the phenomenon of double diffusion convection of the weak finger regime.
Conclusion: In the Mazandaran beach, due to the absence of inpatient complications, including Absheron, no specific temperature inversion has been observed. The density ratio in these areas is often negative, indicating stability in the water column. In the eastern part of the study area, a larger buoyancy frequency was recorded, which shows greater stability in this area. In deep layers, however, the buoyancy frequency tends to be zero (neutral stability). Also, the diffusive regime was not observed in the studied stations, but in the east of the research area, the finger regime occurred in several stations.