Understanding the circulation in the deep, micro-tidal and strongly stratified Congo River estuary

The Congo River estuary is characterised by a deep canyon that connects the river to the deep ocean by cutting through the continental shelf. Its estuary is influenced by high river discharge and micro-tidal conditions, with a large depth and limited vertical mixing. This restricts the supply of oxygen from the surface waters to the more saline bottom waters, leading to hypoxic and anoxic zones. We study the dynamics of the Congo River estuary by applying the multi-scale baroclinic coastal ocean model SLIM 3D (www.slim-ocean.be) to this topographically challenging environment. By allowing a high degree of flexibility in the representation of both the complex geometry and the strong stratification, SLIM 3D is able to simulate riverine, tidal and gravitational processes that drive the estuarine circulation. Model results compare favourably with in-situ data in the estuary, suggesting that the exchange flow is correctly simulated. The latter is characterised by a two-layer dynamics. The combination of the large river discharge, the strong stratification and the large depth results in a moderate freshwater Froude number and a very small mixing number. It makes the Congo River an outlier in state-of-the-art estuarine classifications, closer to fjords than salt wedge estuaries. Furthermore, using the age as a diagnosis sheds light on the spatial variability of the estuarine waters ventilation. Local maximum of renewing water age located just below the pycnocline is exceeded by old dense oceanic waters which stagnate at the bottom of the canyon for more than two months due to the small vertical mixing. It helps explain the hypoxic and anoxic conditions observed at the bottom of the submarine canyon. •The multi-scale SLIM 3D correctly reproduces the estuarine two-layer dynamics.•Small vertical mixing restricts oxygen influx and favours hypoxia and anoxia.•Renewing water age highlights the slow ventilation in the deep submarine canyon.•The Congo River estuary is an outlier in state-of-the-art estuarine classifications.