Fortnightly variability of the lateral structure of residual flows along non-rotating estuaries

Residual circulation in estuaries plays a crucial role in controlling salt intrusion, material transport, and ecosystem functioning, making its understanding essential for coastal management worldwide. Despite its importance, the mechanisms driving the transverse structure of residual flows remain insufficiently explored, particularly in relation to the fortnightly tidal variability. This study shows how the main driver of residual flow may switch from baroclinic dominance at neap tide to barotropic dominance at spring tide. Three-dimensional, process-oriented numerical simulations are carried out in an idealized, non-rotating, estuary–shelf system, represented by an 80 km-long semi-closed channel with a 1 km-wide Gaussian-shaped cross-section. This setting is inspired by the Guadiana Estuary (Portugal/Spain), where previous observations have reported a switch of the residual circulation between spring tide and neap tide. To mimic such fortnightly pattern, the model is forced by freshwater inflow at the head and M 2 and S 2 tidal harmonics at the ocean boundary. Harmonic amplitudes were selected based on a sensitivity analysis. The along channel and temporal variability of the lateral structure of axial residual flows is represented using a non-dimensional parameter derived from the residual inflow that may develop in the deep channel. The simulation results show residual flow transverse structures consistent with theoretical expectations for a baroclinic driver in neap tides and a barotropic driver in spring tides, as confirmed by analyses of the residual momentum equation terms. In these cases, the near-bottom residual flow in the channel is relatively strong and its direction indicates the dominant driver. The study also reveals a previously undocumented transverse structure described by inflows near the channel bed and over the shoals. This structure develops in relation to the Stokes drift when baroclinic and barotropic forcings are relatively balanced across a section. • Fortnightly switch of residual flow drivers in non-rotating estuaries • New transitional lateral residual structure emerges under near-balanced forcing • Identification of residual flow patterns with a dimensionless parameter