Dynamics of oceanic western boundary currents at high resolution (CASE)

西部边界流在大西洋的经向翻转环流中起着关键作用。在目前的气象局模型中，它们解决得很差，也很难理解。西部边界流的大部分基本理论是基于垂直侧壁和横向摩擦边界层。相比之下，西部边界流在海洋中的流动沿着一个倾斜的底部边界与底部边界层与相对停滞的沿海shoones.The湍流通量的作用，在设置西部边界流的结构知之甚少。关键流程包括：抑制强水深梯度、向上梯度动量通量和向下梯度浮力通量（或通过等效涡流形式应力的向下动量通量）之间的混合。西部边界流被认为是大量涡流能量耗散的区域，这两者都是由于向西向传播涡流的墓地（Zhai等人，2010，Nat. Geosci.）西边界流的斜压不稳定是大涡能量源。对这些涡流能量汇的更好理解对于理解涡流-平均流相互作用的详细性质是重要的（马歇尔等人，2017年，Geophys，Res.，Lett.; Mak等人，2018年，提交给J. Phys. Oceanogr。以及边界流内由此产生的底流混合，这对于AMOC的短路以及因此在大西洋中的海洋热输送是重要的。有相当多的证据表明，边界层分离与粘性边界层外部的流动减速（“外部流”）有关（例如，马歇尔和坦斯利，2001年，物理海洋学杂志）。然而，在海洋中，这应该是底部边界层沿沿着倾斜海底的减速。在我们的分离问题的方法的范式转变是必要的，远离分离从垂直侧壁分离的表面强化电流从倾斜的底部边界。分离问题的另一个关键因素是墨西哥湾流离开北大西洋陆架并穿过DWBC时涡旋拉伸的作用，DWBC在其下方通过时下降约800米。其中包括：边界流及其中尺度曲折/涡旋的对称和斜压不稳定性，相对应力效应-与洋流切变相关的Ekman泵送特别重要，Ekman输送使冷水流过暖水，导致边界流一侧的表面混合层不稳定。Bell（2018，Q.罗伊流星Soc.）提供了一个统一的框架内分析无数的不稳定性西部边界电流和eddys.Progress在数值模式和计算资源意味着它现在是可行的运行有限的区域西部边界电流模式在极高的空间分辨率，解决中尺度和（短时间窗口）的亚中尺度。这些模型应该能够代表陡峭的水深合理令人满意，并提供洞察力的动态管理的路径，墨西哥湾流周围的大银行，这是一个已知的来源，在SST的错误在气象局models.We建议建立在方法Gelderloos等人。（2011年，J物理。并建立了一个非常高分辨率的佛罗里达海流和分离的墨西哥湾流（包括可能的大浅滩）模型。水深测量将采用理想化、半现实和现实的配置，并将使用返回通道和海绵将流体返回赤道，并以计算有效的方式建立流入/流出边界条件（更多细节见Gelderloos等人）。将使用各种涡度趋势诊断和能量转换。

Western boundary currents play a critical role in the Atlantic's Meridional Overturning Circulations. In the current Met Office models they are poorly resolved and poorly understood. Most of the underlying theory of western boundary currents is based on vertical sidewalls and lateral frictional boundary layers. In contrast, western boundary currents in the ocean flow along a sloping bottom boundary with a bottom boundary layer with relatively stagnant coastal water onshore.The role of turbulent fluxes in setting the structure of western boundary currents is poorly understood. Critical processes include: suppression of mixing across strong bathymetric gradients, up-gradient momentum fluxes and down-gradient buoyancy fluxes (or downward momentum fluxes via the equivalent eddy form stresses). The western boundary currents are believed to be a region of substantial eddy energy dissipation, both due to being the graveyard of westward propagation eddies (Zhai et al., 2010, Nat. Geosci.) and due to the large eddy energy source through baroclinic instability of the western boundary currents. Improved understanding of these eddy energy sinks is important both for understanding the detailed nature of the eddy-mean flow interactions (Marshall et al., 2017, Geophys, Res., Lett.; Mak et al., 2018, submitted to J. Phys. Oceanogr.) and also the resultant diapycnal mixing within the boundary currents, important for short-circuiting of the AMOC and hence ocean heat transport in the Atlantic.There is considerable evidence that boundary layer separation is related to deceleration of flow ("the external stream") just outside the viscous boundary layer (e.g., Marshall and Tansley, 2001, J. Phys. Oceanogr.). However, in the ocean this should be deceleration of the bottom boundary layer along sloping sea floor. A paradigm shift in our approach to the separation problem is required, moving away from separation from a vertical sidewall to separation of a surface-intensified current from a sloping bottom boundary. Also critical to the separation problem is the role of vortex stretching as the Gulf Stream leaves the North Atlantic shelf and crosses the DWBC which descends by about 800m as it passes beneath.There are a myriad of sub-mesoscale processes at play in the surface mixed layer of the boundary currents. These include: symmetric and baroclinic instabilities of the boundary current and its mesoscale meanders/eddies, relative stress effects - the Ekman pumping associated with shear in ocean currents being of particular importance, Ekman transport fluxing cold water over warm leading to destabilising the surface mixed layer on one flank of the boundary current. The recent work of Bell (2018,Q. J. Roy. Meteor. Soc.) provides a unified framework for analysing the myriad of instabilities within the western boundary currents and eddies.Progress in numerical models and computational resources mean that it is now feasible to run limited area western boundary current models at extremely high spatial resolution, resolving both the mesoscale and (for short time windows) the sub-mesoscale. These models should be able to represent the steep bathymetry reasonably satisfactorily and provide insight into the dynamics governing the path of the Gulf Stream around Grand Banks, which is a known source of error in SST in the Met Office models.We propose to build on the methodology of Gelderloos et al. (2011, J. Phys. Oceanogr.) and set up a very high-resolution model of the Florida Current and separated Gulf Stream (including, potentially Grand Banks). The bathymetry will assume both idealised, semi-realistic and realistic configurations, and a return channel and sponges will be used to return the fluid equatorward and establish the inflow/outflow boundary conditions in a computationally efficient manner (see Gelderloos et al. for more details). A variety of diagnostics of vorticity tendencies, and energy conversions will be used.