Fortnightly Switching of Residual Flow Drivers Produced by Seawater Intrusion in a Long, Non-rotating Estuary

Abstract Towed and moored ADCP and salinity measurements were collected in Summer 2023 at two cross-channel transects of the 80-km-long Guadiana Estuary, at 4 km and 20 km from the mouth, during low (< 10 m 3 /s) river discharge conditions. The dataset indicates that the lateral structure of axial residual flows changed from vertically sheared to laterally sheared with tidal forcing. These structures resemble theoretical expectations for a density-driven or eddy viscosity-shear covariance (ESCO) circulation at neap tide and for a tidally driven circulation produced by longitudinal advection (tidal stress) at spring tide. The tidal variability of the residual flow magnitude and of the contributions of unidirectional or two-layer vertical profiles indicate that the primary driver of the residual circulation changes between neap tide and spring tide. Seawater (i.e. with salinity > 35 kg/g) intrusion was several kilometres, equivalent to ~ 1/8 of the estuary’s length. As a result, the horizontal density gradient was weakest near the mouth where the residual circulation was barotropic, produced by tidal stress at spring tide and possibly by an ESCO mechanism at neap tide. At 20 km upstream, the dominant driver switched from tidal stress at spring tide to a baroclinic (and potentially ESCO) circulation at neap tide. At this location, scaling analyses of the tidally averaged momentum equation using the densimetric tidal Froude number supported the idea of dominance of tidal stress at spring tide.