A combined modeling and EOF approach to characterize the hydrodynamics of a semi-enclosed bay

Semi-enclosed coastal basins are highly vulnerable ecosystems where anthropogenic pressures are often amplified by intricate, topologically-constrained hydrodynamics that lead to low water renewal and pollutant trapping. Understanding their circulation drivers is therefore critical for effective environmental management. In this study, we investigate a representative semi-enclosed basin by combining a high-resolution, three-dimensional numerical model with Empirical Orthogonal Function (EOF) analysis to deconstruct and interpret the system’s circulation. The results show that EOF analysis effectively isolates the system’s primary drivers. The leading modes of surface circulation are unequivocally linked to wind forcing, revealing a clear seasonal modulation of wind-driven currents. The analysis of vertical density structures and interannual analysis demonstrate that seasonal stratification and open sea currents are the dominant controls on water exchange; strong summer stratification inhibits renewal, whereas winter mixing promotes vigorous baroclinic circulation. Higher-order modes of the vertical density transects successfully identify exchange patterns at the openings, revealing that a portion of the system’s variance is also explained by the presence and dynamics of the pycnocline, which modulates internal mixing and recirculation. This work highlights the intricate balance between external forcing (wind), internal dynamics (stratification), basin geometry and external currents in shaping circulation. It validates the combined use of high-resolution modeling and EOF analysis as a powerful approach to provide a clear and quantitative framework for identifying the key physical processes governing the hydrodynamics of semi-enclosed coastal systems.