In the Bay of Biscay (north-east Atlantic), long-living eddies and the frontal activity that they induce substantially contribute to mesoscale and submesoscale dynamics. Tides and river plumes also contribute to frontal activity. Biological productivity is sensitive to river plume fronts and to external forcings (tides and wind). Considering the importance of river plumes, we study here the structure, stability and vertical mixing processes in such river plumes (similar to those generated by the Gironde river). Restratification budget is considered here for evaluating stirring (frontogenetic/frontolytic) or vertical mixing (parametrised here from Ertel potential vorticity mixing) processes. Using high-resolution idealised numerical simulations, we analyse the evolution of the bulge and of the coastal part of this plume and we conduct sensitivity experiments to the river discharge, to southwesterly winds and to M2 tides. The bulge and the coastal current are stable (unstable) in case of moderate (high) river discharge, due to mixed barotropic/baroclinic instabilities. In the unstable case, near surface symmetric and vertical shear instabilities develop in the coastal current and in the core of the bulge where the Rossby number is large. When southwesterly winds blow, the river plume is squeezed near the coast by Ekman transport. The river plume is then subject to frontal symmetric, baroclinic, barotropic and vertical shear instabilities in the coastal part, north of the estuary (its far field). Conversely, in the presence of M2 tides, the river plume is barotropically, baroclinically and symmetrically unstable in its near field. Interior vertical mixing is induced by advective (stirring) and frontogenetic processes. Frontogenesis is dominant in the far-field (in the presence of southwesterlies) or in the near-field (when M2 tide is active). Frontogenesis is important in the far-field region in unforced river plumes (both with moderate and high river discharges). Potential vorticity is eroded in the far-field when southwesterlies blow. This is primarily due to the frictional processes which are dominant at the surface. This study has identified the instabilities which affect a river plume in different cases, and the local turbulent processes which alter the stratification.

中文翻译：

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河流羽流中的不稳定性和垂直混合：在比斯开湾的应用

在比斯开湾（大西洋东北部），长期存在的涡流和它们引起的锋面活动对中尺度和亚中尺度动力学有很大贡献。潮汐和河流羽流也有助于锋面活动。生物生产力对河流羽流前沿和外部强迫（潮汐和风）很敏感。考虑到河流羽流的重要性，我们在这里研究了这些河流羽流的结构、稳定性和垂直混合过程（类似于吉伦特河产生的羽流）。此处考虑再分层预算以评估搅拌（锋生/锋解）或垂直混合（此处从 Ertel 位涡混合参数化）过程。使用高分辨率的理想化数值模拟，我们分析了这个羽流的凸起和沿海部分的演变，并对河流流量、西南风和 M2 潮汐进行了敏感性实验。由于混合的正压/斜压不稳定性，在中等（高）河流流量的情况下，凸起和沿海流是稳定的（不稳定）。在不稳定的情况下，近地表对称和垂直剪切不稳定性在海岸流和罗斯比数大的凸起的核心中发展。当西南风吹来时，埃克曼运输将河羽挤压到海岸附近。然后，河流羽流在河口以北的沿海部分（其远场）受到锋面对称、斜压、正压和垂直剪切不稳定性的影响。相反，在 M2 潮汐的存在下，河流羽流呈正压，在其近场中斜压和对称不稳定。内部垂直混合是由对流（搅拌）和锋生过程引起的。锋生在远场（存在西南风时）或近场（当 M2 潮汐活动时）占主导地位。锋生在远场地区的非受迫河流羽流（具有中等和高河流流量）中很重要。当西南风吹来时，远场的潜在涡度被侵蚀。这主要是由于在表面占主导地位的摩擦过程。本研究确定了在不同情况下影响河流羽流的不稳定性，以及改变分层的局部湍流过程。锋生在远场（存在西南风时）或近场（当 M2 潮汐活动时）占主导地位。锋生在远场地区的非受迫河流羽流（具有中等和高河流流量）中很重要。当西南风吹来时，远场的潜在涡度被侵蚀。这主要是由于在表面占主导地位的摩擦过程。本研究确定了在不同情况下影响河流羽流的不稳定性，以及改变分层的局部湍流过程。锋生在远场（存在西南风时）或近场（当 M2 潮汐活动时）占主导地位。锋生在远场地区的非受迫河流羽流（具有中等和高河流流量）中很重要。当西南风吹来时，远场的潜在涡度被侵蚀。这主要是由于在表面占主导地位的摩擦过程。本研究确定了在不同情况下影响河流羽流的不稳定性，以及改变分层的局部湍流过程。当西南风吹来时，远场的潜在涡度被侵蚀。这主要是由于在表面占主导地位的摩擦过程。本研究确定了在不同情况下影响河流羽流的不稳定性，以及改变分层的局部湍流过程。当西南风吹来时，远场的潜在涡度被侵蚀。这主要是由于在表面占主导地位的摩擦过程。本研究确定了在不同情况下影响河流羽流的不稳定性，以及改变分层的局部湍流过程。