Switching of residual exchange flows in a thermal low-inflow estuary

A low-inflow estuary showed a regime shift: from a ‘typical’ regime in spring to a ‘thermal’ regime in summer, with water density decreasing landward in both seasons. In spring, low salinity at the head relative to the mouth dominated the density gradient, favoring a positive estuarine circulation. In summer, residual circulation varied between the two sampling campaigns. In 2022, a thermal density gradient induced a laterally sheared gravitational circulation, resembling theoretical tidal residuals in ‘short’ channels; a densimetric tidal Froude number placed this campaign within a transitional regime, where neither tides nor density clearly dominated. In 2024, under stronger tidal forcing, residual circulation departed from this typical pattern on both flanks of the cross-section. This departure coincided with a nearby channel bend, whose relative influence we assessed through two curvature-based indices: a curvature Rossby number and a proposed curvature Froude number ( Fr c ), both larger in 2024 than in 2022. These indices indicate, through scaling analysis, that curvature-driven accelerations were comparatively more important in 2024, consistent with mechanisms described in other curved channels, although a direct momentum balance was unviable with the measured data. A month-long record from a moored instrument further showed that subtidal flow near the shoal did not follow the typical outflow direction expected over that zone, but responded more consistently to density than to tides, in qualitative agreement with the pattern observed in 2024. These findings motivate further observational studies and provide information for the ecological management of thermal low-inflow estuaries susceptible to natural or anthropogenic change.