Exchange flow and mixing in ice-covered estuaries

冬季北极河口表面的陆地固结冰导致了其水动力学的根本动力变化。与潮汐无关的风强迫不同，振荡潮汐应力在地表起作用，其大小与通常占主导地位的床应力相当。对于无冰河口，已经有了完善的理论来解释河口环流和混合，这两个密切相关的过程决定了盐楔的结构和位置以及沉积物的输运模式。对于冰雪覆盖的河口，尽管世界上一些最大的河口位于北极，但这样的理论是不存在的。预计冬季的冰盖将对河口内的混合特性产生重大影响，从而使喷入北冰洋的河流羽流的源水发生重大改变。尽管有和没有冰覆盖的初步水柱模型研究表明，由于冰覆盖，河口环流减少，但尚不清楚实际情况是否也是如此，河口密度梯度的变化起着重要作用。北极河口冬季情况可能产生以下主要的相反影响：(1)作为河口环流驱动因素的涡动黏度-切变协方差应被强烈减弱，导致河口环流减弱；（ii）由于冰层覆盖而增加的湍流应导致分层减少，从而缩短河口，增加水平密度梯度，并可能增加河口环流；（iii）冬季淡水径流的大幅减少应导致分层减少，水平密度梯度减小，从而减少河口环流。本项目旨在通过理想化和现实的数值模拟来揭示这一尚未探索的研究领域。重点河口将是鄂毕湾，这是世界上最长的河口，它流入北冰洋的喀拉海。覆盖大参数空间的理想二维和三维模型实验将有助于探索冰盖对河口水动力的影响。此外，还计划对Ob湾进行实际的多年后播模拟。要估计北极快速变暖对北极河口动态的影响，需要这些新的见解。

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.