Exchange flow and mixing in ice-covered estuaries

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The land-fast ice cover on the surface of Arctic estuaries in winter leads to a fundamental dynamical change in their hydrodynamics. Instead of tide-independent wind forcing, an oscillating tidal stress acts at the surface, with a magnitude that is comparable to the normally dominating bed stress. For ice-free estuaries, there exist well-established theories to explain estuarine circulation and mixing, two strongly interlinked processes that determine the structure and position of the salt wedge as well as the sediment transport patterns. For ice-covered estuaries such theories are missing, despite the fact that some of the world's largest estuaries are situated in the Arctic. It is expected that the ice cover during winter will have a substantial impact on the mixing properties within the estuary, such that the source waters for river plumes ejected into the Arctic Ocean will be significantly modified. Although preliminary water column model studies with and without ice cover show a reduced estuarine circulation due to ice cover, it is not clear if this is also the case in reality where variable along-estuary density gradients play an important role. Winter situations in Arctic estuaries may have among others the following major opposing impacts: (i) eddy viscosity - shear covariance as driver of estuarine circulation should be strongly weakened, leading to weaker estuarine circulation; (ii) increased turbulence due to ice cover should lead to decreased stratification and thus to shorter estuaries, increased horizontal density gradients and possibly increased estuarine circulation; (iii) strongly reduced freshwater runoff during winter should result in decreased stratification, reduced horizontal density gradients and thus decreased estuarine circulation. The present project intends to shed light into this yet unexplored research field by means of idealised and realistic numerical modelling. The focus estuary will be the Gulf of Ob, the world's longest estuary, which is discharging into the Kara Sea of the Arctic Ocean. Idealised two- and three-dimensional model experiments covering a large parameter space will help exploring the influence of ice cover on estuarine hydrodynamics. Furthermore, realistic multi-annual hindcast simulations are planned for the Gulf of Ob. These new insights are needed to estimate the impacts of a rapidly warming Arctic on the dynamics of Arctic estuaries.