Exchange flow and mixing in ice-covered estuaries

冬季北极河口表面的陆固冰覆盖导致了河口水动力的根本性变化。而不是潮汐独立的风强迫，振荡的潮汐应力作用在表面上，其大小与通常占主导地位的床应力相当。对于无冰河口，存在完善的理论来解释河口环流和混合，这两个强烈相互关联的过程决定了盐楔的结构和位置以及沉积物输运模式。尽管世界上一些最大的河口位于北极，但对于被冰覆盖的河口，这样的理论是缺失的。预计冬季的冰盖将对河口内的混合特性产生重大影响，从而使喷射到北冰洋的河流羽流的源沃茨发生重大变化。虽然初步的水柱模型研究有和没有冰盖显示减少河口环流，由于冰盖，目前还不清楚，如果这也是在现实中的情况下，沿河口的密度梯度发挥重要作用。北极河口的冬季情况可能会产生以下主要的相反影响：（i）作为河口环流驱动因素的涡动粘性-切变协方差应被大大削弱，导致河口环流减弱;（ii）由于冰盖引起的湍流增加应导致分层减少，从而缩短河口，增加水平密度梯度，并可能增加河口环流;（iii）冬季淡水径流量的大幅减少应导致分层减少，水平密度梯度降低，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阐明这一尚未探索的研究领域。重点河口将是世界上最长的河口鄂毕湾，它正在向北冰洋的喀拉海排放。理想的二维和三维模型实验覆盖大的参数空间将有助于探索冰盖对河口水动力的影响。此外，还计划对鄂毕湾进行现实的多年度后报模拟。需要这些新的见解来估计迅速变暖的北极对北极河口动态的影响。

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.