Spatial variability of turbulent mixing in a highly stratified system

This study investigates the lateral variability of circulation, salt distribution, and mixing processes along the highly stratified, micro-tidal Rio Grande Channel, which connects the Patos Lagoon and the Atlantic Ocean. High-resolution observations of current velocities (ADCP), water properties (CTD), and turbulent kinetic energy dissipation (microstructure profiler) were collected at three cross-sections near the lagoon mouth. The field campaign was carried out during a period of seaward flow conditions, and the cross-sections capture conditions upstream of the salt intrusion limit (freshwater/vertically homogeneous), at the landward limit of salt intrusion (the tip of the salt wedge), and downstream of the salt intrusion (highly stratified). While lagoons are often considered shallow, well-mixed systems dominated by barotropic dynamics, our results revealed pronounced stratification, with baroclinic processes playing a central role. Secondary flows driven by baroclinic pressure gradients and acceleration due to curvature and Coriolis enhanced mixing in the cross-sections. Bottom-generated mixing was evident across all transects; however, vertical shear of the horizontal current velocities at the pycnocline emerged as the primary driving mechanism in the third (most-downstream) cross-section. Stratification suppressed mixing between upper and lower layers at the midstream and downstream cross-sections, with the pycnocline damping turbulence, yet showing elevated mixing immediately above and below its interface. Notably, despite a channel-funneling effect that increased flow velocity and shear toward the mouth, mixing did not intensify as expected due to the strong stratification. These findings are particularly relevant for understanding dynamics in other microtidal estuaries, especially in choked lagoons and constricted channels.