ESTIMATION OF VERTICAL MIXING IN THE NORTH AEGEAN SEA BASED ON ARGO FLOAT DATA (original) (raw)

The North Aegean Sea is considered as a region of deep water formation of the Mediterranean. Dense water formation events are known to take place rather infrequently and in the time intervals between such events, the bottom waters are excluded from interaction with other water masses through advection. In order to examine the evolution of deep waters during those periods at shorter-than-annual time scales, new, high-frequency data from a profiling ARGO float were analyzed. The specific MedArgo float (nr.6901884) was trapped during 2014-2015 within the deep Athos basin, and remained there for 13 months. The analyzed hydrographic profiles point out, that the Black Sea Water surface layer is an effective isolator between the deep layers of the North Aegean and the atmosphere, absorbing large amounts of heat and buoyancy and hindering dense water formation. Also, the temporal evolution of the averaged potential density anomaly possibly indicates an alternation ofreveals a short-termed dense-water formation that affected the layers shallower than 600 m, that was followed (after March 2015) and by a longer-termed stagnation period lasting till November 2015. only in the surface layer and the intermediate layer. It seems that the water masses deeper than 600m remain almost constant, without being affected by the dominant seasonal cycle. It could be possible to conclude, that the replenishment of deep basins takes place in longer-than-annual cycles, since the dominant seasonal cycle seems to affect only the upper water masses. It is noteworthy, that the vertical eddy diffusion coefficient K_S (based on the observed rate of change of salinity) indicated a positive conductivity sensor drift of the profiling float. Furthermore, the conductivity corrections verified a remarkable drift in salinity of 0,0019 to 0,0113 due to the increasing conductivity sensor drift rate of 4,6187*10-5 mmho day-1. After the calibration correcting for the sensor’s drift, the eddy diffusion coefficients K_σθ, K_S, K_T were found to range between 6-7×10-5 and 2-3×10-3 m2s-1 for the deeper than 400m waters. Considering that the deep basins of the North Aegean are practically isolated below the 400 m threshold during the stagnation period, the good agreement of the three diffusion coefficients Kc and the fact that the properties of deeper water masses slide along the θ-S curve towards lower densities suggests that the dominant process in vertical diffusion is turbulent mixing.