Hydrodynamic response of Thassos Passage (N. Aegean Sea) to Nestos River discharge and meteorological forcing

The hydrodynamic response, in terms of the competitive stratification-destratification processes and the along- and cross-shore circulation in Thassos Passage, was studied. This passage is a narrow and shallow channel, located in the vicinity of Nestos River mouth, connecting the eastern and western Thracian Seas. Stratification-mixing conditions were studied in terms of the water column potential energy anomaly ϕ, and the quantification of the along- and cross-channel dynamic equation terms, utilizing the results of a regional three-dimensional model. Circulation conditions were assessed by separating the tidal and non-tidal seasonal signals recorded from a bottom-mounted ADCP, and cross-correlating residual circulation to the meteorological influence. Results showed that short-term mixing events, produced under strong south-west winds (8-14 m s−1), and stratification incidents under high river discharge conditions (>100 m3 s−1), may alter the potential energy anomaly of the water column with a time-lag of 3-6 h. Such ϕ-changes were attributed to the along- and cross-channel advective and straining terms, with significant spatial variability, due to the Thassos Island sheltering effect. The along-channel potential energy anomaly flux was computed heading westwards, having a mean value of 22.5 J m−2 s−1. North-northeast winds tend to increase the sub-tidal level in the channel, while south and south-west winds are responsible for its reduction. At the northern part of Thassos Passage, the development of an elongated sandbar enhances frictional effects, favoring the development of a clockwise rotation system, supported further by southern wind events. Cross-correlations of residual flows and wind effects indicated a wind transfer coefficient of 8%, rotating the wind-induced surface flow by 14o rightwards. Overall, the impact of seasonal water column stratification on the water flow of Thassos Passage was limited, due to the reduced Nestos River runoff after its damming.