Diagnosis of residual flow drivers in estuaries with the densimetric tidal Froude number

Khosravi, Maziar; Fortunato, A. B.; Lopez-Ruiz, A.; Valle-Levinson, A.; Garel, Erwan

http://hdl.handle.net/10400.1/29314

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Residual circulation in estuaries drives salt intrusion, material transport, and ecosystem functioning. Yet the ability to diagnose its dominant forcing mechanism remains a challenge. While previous studies suggest that the densimetric tidal Froude number () may distinguish tidal from baroclinic forcing, its reliability across varying estuarine conditions and spatial scales requires systematic evaluation. This study carries out such evaluation with a suite of process-oriented numerical simulations in an idealized estuary–shelf system that features Gaussian cross-channel bathymetry. Here, is treated as a diagnostic indicator reflecting the dynamics established by either barotropic or baroclinic forcing. The model configuration spans a parameter space that includes variations in tidal amplitude, channel depth, lateral slope, and river discharge. Residual circulation regimes are diagnosed using the ratio between the barotropic and baroclinic terms of the tidally averaged, axial momentum equation obtained from model outputs. Diagnoses are made together with the transverse structure of residual flows, allowing direct comparison with predictions. This study demonstrates that can reliably diagnose the dominant driver of residual circulation when the density length scale is defined over the shortest feasible axial distances. Crucially, the results show that the skill of is highly sensitive to this spatial scale: its performance deteriorates when the density gradient is computed over lengths that extend into regions with heterogeneous forcing. Across all simulations, functions as a robust bulk indicator of tide-dominated, density-dominated, or regimes influenced by both forcings only when the density scale is restricted to zones where the axial density gradient is approximately constant. Such zones consistently occupy the seaward side of the peak salinity gradient. Within this region, emerges as a relevant diagnostic tool and offers process-oriented guidance for results that require identifying the prevailing residual-flow driver. The experiments suggest configuration-dependent ranges associated with regimes dominated by tides, density gradients, or both