Interannual and seasonal variability of stratification and mixing in a high-discharge micro-tidal delta: Magdalena River

Abstract The spatio-temporal variability of stratification and mixing at the mouth of the Magdalena River Delta (MMRD) was studied for the warm (El Nino), cold (La Nina), and neutral phases of El Nino–Southern Oscillation (ENSO), using the Richardson number (RL), potential energy anomaly (φ), buoyancy frequency (β), turbulent kinetic energy production (P), and bottom shear stress (τxz). These parameters were obtained from the MOHID numerical model outputs, which were implemented and calibrated for the MMRD. Analysis of the spatio-temporal variability of φ and β indicated stratification at different mouth zones, and the spatio-temporal variability of RL and P indicated a mixing process. Results show that during the dry season, the mixing zone of the MMRD presents its maximum intra-annual variability, and during El Nino conditions, it has its highest interannual variability. Further, the maximum penetration of the salt wedge occurs during El Nino - dry season conditions. On the other hand, during the wet season, the salt wedge is located at the mouth entrance, independent of the ENSO phase. During La Nina conditions, the mixing zone restricts its intra-annual variability to around 0 km of the MMRD.