Tidal processes and their spatial and temporal variability in the mid-field Guadalquivir ROFI

In the Region Of Freshwater Influence (ROFI), located between sea exposure and estuary, characteristic physical processes of both estuarine and shelf seas overlap and impact on shelf circulation, sediment transport and ecosystem (Simpson, 1997). Although freshwater discharge typically exhibits the highest variability (Horner-Devine et al., 2015), this work focus on the tidal variability within the ROFI, which is often overlooked. This work addresses the spatial and temporal variability of tidal elevations and currents at the mid-field Guadalquivir ROFI (SW Spain), which is semidiurnal in character. Observations from five current-meter profiles, which were moored pointing upwards from June 2008 to December 2009 are analyzed. These instruments were placed along an arc, from south to north, and closing the estuary mouth.  

 

The analysis of the observations indicates that tides in the Guadalquivir ROFI have a close-to standing wave behavior. This is induced by the reflection at the continental margin of the northward-propagating tidal Kelvin wave. Regarding the M2 and M4 tidal constituents, which are relevant for residual sediment transport, their relative phase difference shows that, although the inner estuary is flood-dominant, in the mid-field ROFI zone ebb currents are slightly stronger than flood currents. Fortnightly variations are observed in the ratio of the M4 and M2 tidal amplitudes. Remarkably, the minimumvalues occur during the transition periods from neap to spring tides, whereas the maximum values are observed during neap tides. These results might suggest that there is still some influence of the tidal jet in this region.

 

Regarding the vertical structure of tidal currents, the M2 inclination varies with depth, being maximum near the bottom at all moorings. Tidal currents inclination also varies with time in the southern part of the ROFI. Moreover, in the southern part of the ROFI, current ellipses in the upper layer of the water column rotate clockwise whereas near the bottom currents rotate anti-clockwise, as revealed by the sign of the eccentricity. However, the eccentricity of tidal ellipses is uniform throughout the water column in the northern part of the ROFI. This along-coast variability of the vertical structure of the tidal ellipses suggests that the buoyant outflow circulates preferentially southwards, most likely driven by the prevailing winds.