On the seasonal variability of the vertical physical structure of the water column in the continental shelf, South-Eastern Caspian Sea

Abstract

The seasonal and spatial variability of the vertical water structure as well as the mixed layer depth (MLD) are examined using CTD data measured during 2011-2012 in the south-eastern Caspian Sea in four different areas of the Mazandaran continental shelf. Based on the conservative temperature and potential density profiles, the mixed layer structure is divided into three different patterns, namely classical, stepwise and inclined. Mixed layer with the classical, stepwise and inclined patterns are respectively the most frequent patterns during fall, summer and spring. The classical and inclined patterns are directly influenced by the sharp decreasing and increasing trend of air temperature while the stepwise pattern is formed in transient season (i.e., summer) impacted by the combination of different atmospheric forces. The MLD is estimated using threshold and gradient methods based on the conservative temperature and potential density profiles with different threshold and gradient values. Based on different criteria, the most appropriate method for estimating the MLD in the study region is the threshold method with a threshold value of 1 °C for the temperature profiles and a threshold value of 0.5 kg.m−3 for density profiles. Results show that during fall (spring) the mixed layer is the deepest (shallowest). The salinity inside the mixed layer is high during fall and summer while the mixed layer in spring is fresher. Vertical structures of the water column in considered regions are also different even in the same season. For instance, in Noshahr, the thermocline is formed in a closer distance from the coast compared to that in Ramsar and Babolsar.