Density gradient effect on circulation patterns and mixing processes: Observations at the convergence front of a salt wedge, microtidal estuary

Abstract

The convergence front in estuaries is a crucial area influenced by dynamic mixing processes which vary from one tidal cycle to another, with density gradients playing a key role in shaping circulation patterns. This significance is particularly pronounced in salt wedge estuaries, where discharge holds greater influence than tides. Despite this, there is a limited amount of research based on high-resolution in-situ data to analyze circulation and mixing processes affected by density gradients in the convergence front of a salt wedge estuary. In this study, continuous measurements of current velocities and CTD profiles were conducted over three tidal cycles to analyze the intratidal variability of circulation patterns (u, v, w) and mixing parameters in the convergence front. Stratification parameter (ηs) and Richardson numbers (RL and Ri) were calculated to analyze the mixing/stratification variability within a tidal cycle. The observations revealed a persistent Subsurface Velocity Maximum (SVM) in the longitudinal component, coinciding with density gradient increases > 4 kg m−3 along the water column. Similarly, lateral circulation patterns were influenced by those increases in density gradient, resulting in eastward-directed currents and the formation of two circulations cells. As expected in a salt wedge estuary, stratification occurs during flood and High Water (HW), when the density gradient increases due to intrusion of the salt wedge. Notably, in the Magdalena River estuary, when the discharge is below 3800 m3 s−1, the salt wedge remains consistently present irrespective of the tidal phase, resulting in a permanently stratified water column with ηs values between 1.60 – 1.95. It is expected that the steady presence of stratified conditions during periods of low discharge leads to an increased accumulation of sediments. These findings provide valuable quantitative insights into the complex interplay of density gradients, circulation patterns, and stratification dynamics in salt wedge estuaries.