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

西边界流在大西洋经向翻转环流中起着至关重要的作用。在目前的Met Office模型中，它们没有得到很好的解决和理解。西方边界流的基本理论大多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沿倾斜的底部边界流动，底部边界层在近岸相对停滞的近岸水域。湍流通量在确定西部边界流结构中的作用尚不清楚。关键过程包括：抑制强水深梯度、上梯度动量通量和下梯度浮力通量(或通过等效涡形式应力向下动量通量)的混合。西部边界流被认为是一个大量涡旋能量耗散的区域，这两者都是向西传播涡旋的墓地(翟田田等，2010，NAT。Geosi。)由于西边界流的斜压不稳定产生了大涡能量源。更好地理解这些涡流能量汇对于理解涡流-平均流相互作用的详细性质都是重要的(马歇尔等人，2017，GePhys，Res.，Lett；Mak等人，2018，提交给J.Phys。大洋洲。)边界层分离与粘性边界层外气流(“外流”)的减速有关(例如，马歇尔和坦斯利，2001，J.Phys)。海洋生物)。然而，在海洋中，这应该是沿倾斜的海底的底部边界层的减速。我们处理分离问题的方法需要有一个范式的转变，从从垂直侧壁分离转向从倾斜的底部边界分离表面强化的水流。对于分离问题来说，同样关键的是当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在通过时下降约800米。在边界流的地面混合层中有无数次中尺度过程在起作用。这些包括：边界流及其中尺度曲折/涡旋的对称和斜压不稳定，相对应力效应-与洋流切变有关的Ekman泵特别重要，Ekman输送使温暖的冷水输送导致边界流一侧的地面混合层不稳定。Bell(2018，Q.J.Roy.)最近的工作流星。SoC.)提供了一个分析西部边界流和涡旋内各种不稳定的统一框架。数值模式和计算资源的进步意味着，现在可以以极高的空间分辨率运行有限区域的西部边界流模式，分辨中尺度和(短时间窗)次中尺度。这些模型应该能够合理地、令人满意地表示陡峭的水深，并提供对墨西哥湾流绕大浅滩路径的动态的洞察，这是Met Office模型中SST的已知误差来源。(2011，J.Phys.大洋洲。)并建立了一个非常高分辨率的佛罗里达洋流和分离的墨西哥湾流(可能包括大浅滩)的模型。水深测量将采用理想化、半现实化和现实化的配置，回流通道和海绵将用于向赤道返回流体，并以计算有效的方式建立流入/流出边界条件(见Gelderlos等人)。以了解更多详细信息)。将使用各种涡度趋势和能量转换的诊断方法。

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.