Surface water renewal and mixing mechanisms in a semi-enclosed microtidal domain. The Barcelona harbour case

•The renewal and mixing is investigated in a micro-tidal environment.•The forcing conditions that enhance water renewal and mixing have been characterized.•The benefits in terms of water renewal of having two entrances one is discussed.

Water renewal and mixing are highly related to the evolution of the ecological status in harbour areas and are crucial for environmental harbour management. For the first time, these processes have been studied in the Barcelona harbour with a high resolution 3D hydrodynamic model. This harbour is representative of a semi-enclosed domain with a complex coastline in a micro-tidal environment (like most Mediterranean harbours). The tracking of numerical Lagrangian particles deployed in the flow was used to parameterize the surface water renewal mechanisms. The use of Finite Size Lyapunov Exponents has proven to be a relatively easy and efficient way to characterize the mixing patterns. The complex geometry of the harbour and the meteo-oceanographic forcings lead to intricate hydrodynamics that define spatial heterogeneity of water renewal and mixing. The most favourable conditions for enhancing surface water renewal and mixing have been identified in idealized scenarios and realistic simulations. In general, renewal is enhanced near the harbour mouths and strongly limited in the inner areas. However, under particular forcing conditions, the residence time can decrease even in the most sheltered areas. The presence of two mouths in the harbour seems to favour water renewal in comparison to harbours with only one mouth. Mixing is mainly induced by the action of wind forcing, while external shelf currents are much less efficient. The study of two realistic simulations suggests that harbour response to real forcing cannot be deduced from the combination of simple idealized scenarios. The time evolution of the forcings and the transient response of the system play a key role in defining the residence time patterns.