Tidal ellipse variability, wind-driven circulation and plume classification in the ROFI of the dam-regulated Guadalquivir estuary

Abstract

This observational study provides a comprehensive analysis of the spatial and temporal variability of tidal currents, wind-driven circulation and plume behaviour in the region of freshwater influence (ROFI) of the Guadalquivir estuary (Gulf of Cadiz, SW Spain). The study disentangles the relative importance of tidal and wind mixing, wind shear stress, buoyancy and rotation on the inner shelf and classifies the plume dynamics of this estuary. Measurements were primarily obtained from five current meter profilers deployed along a cross-sectional arc near the mouth of the estuary, complemented by satellite imagery and reanalysis data products. The observations reveal significant vertical and temporal variability in the tidal ellipses, with semi-major axes showing subsurface maxima during spring tides. Tidal ellipse rotation varies between surface and near-bottom layers due to topo-bathymetric constraints, friction, and stratification, which is promoted by the freshwater discharge. The extent of the plume is explained by the magnitude of the discharge from the dam and residual wind-driven currents. The plume is characterised by small mouth Kelvin numbers and small Froude numbers both during low and high freshwater discharges. Under low discharge conditions, small-scale processes dominate, resulting in a flow dominated by inertia and tides (Kelvin number close but below 1, and small tidal index). These plumes are nearly symmetric, surface-advected and attached to the estuary mouth where the vertical mixing is strong. High discharge events lead to the dominance of large-scale processes where rotational and wind-driven circulation dominate (large Kelvin and small Rossby numbers). These plumes show larger radius of deformation and extend further along the coastline, but remain surface-advected. Southerly winds drive these large-scale plumes northwards, while westerly winds extend them south-southwestwards. Northerly winds detach the plume from the coast, inducing anticyclonic rotation. This study improves the understanding of ROFI systems, providing valuable insights into the behaviour of coastal shelf environments and informing future environmental management efforts.