Understanding and Improving the Path of the Gulf Stram extension in moderate resolution climate models

在大西洋，大部分热量从海洋转移到大气中是通过墨西哥湾流进行的，多年来，众所周知，大气层对墨西哥湾流延伸区的海洋表面温度非常敏感。政府间气候变化专门委员会报告的几乎所有气候模拟中都使用了具有10个网格间距的粗分辨率海洋模型，这些模型错误地描述了墨西哥湾流延伸的路径，因为墨西哥湾流太过带状且太偏南。高分辨率海洋模式(网格间距为1/12o)很好地再现了路径，最近用于季节预报的一些网格间距为1/4o的海洋模式也有改进的路径。斯凯夫等人。(2011)将其系统中冬季阻塞事件预测的改进在很大程度上归功于这一改进路径。气候模拟和季节预测可能在至少十年内使用这一分辨率的模型，它们表示墨西哥湾流延伸路径的准确性将是一个重要因素。这条路径在这个分辨率下的稳健性绝不是有保证的，所以重要的是更好地了解是什么决定了这条路径，并寻求能够在相对较粗的分辨率下为正确的原因提供良好模拟的技术。过去在理解决定这条路径的动力学方面取得了一些进展：特别是GreatBatch等人。(1991)，Zhang&Vallis(2007)。结果表明，与水深测量有关的涡旋伸展和底部压力力矩以及粘性力矩的贡献对确定深度积分流动的方向是重要的。已知中尺度不稳定性在加强水深流动方面起着重要作用，因此通过水深测量进行引导(Hurlburt&Hogan，2008)。用广义拉格朗日平均理论建立的动量方程的LANS--允许更好地表示中尺度的动能，当它只被部分分解时。Peterson，Hecht和Wingate在POP海洋模式中实现了LANS-a的简化形式，并发现在0.4°网格的模式中，通过水深测量来引导深度积分流动的能力得到了显著改善。Barnier等人。(2006)的研究还表明，在与水深测量有强烈相互作用的区域，NEMO内的动量平流公式的各种选择对ORCA025中的正压流动有重大影响。在这个配方中处理水深测量的问题最近被曝光，其中一名主管正在与巴尼耶和同事合作制定一项修订方案。因此，新的、修改的或改进的动力参数化和数值实施的组合可能会对墨西哥湾流延伸路径在粗分辨率下的稳健性产生重要的影响。PHD项目的第一步将是使用10度、1/4度和1/12度分辨率的模式来计算涡度趋势和墨西哥湾流路径的诊断。根据结果和学生的兴趣，PHD可以朝着以下方向发展：深入调查和解释涡度诊断的各个方面，在倾斜测深的情况下西部边界流的理论和动力学，评估动量平流或涡通量物理参数的不同表示对墨西哥湾流路径的影响。学生将被鼓励在海洋物理和动力学的数值、诊断和理论方面形成和实验新的想法。需要解决的研究问题包括：1)什么过程控制着湾流延伸区的涡度？2)西部边界流中尺度波动的作用是什么？2)中尺度波动如何加强与水深测量的相互作用

Much of the transfer of heat from the ocean to the atmosphere in the Atlantic takes place over the Gulf Stream and the atmosphere has been known for many years to be very sensitive to the ocean surface temperatures in the region of the Gulf Stream extension. The coarse resolution ocean models with 1o grid spacing that have been used in nearly all climate simulations reported by the IPCC misrepresent the path of the Gulf Stream extension, the flow being too zonal and too far to the south. High resolution ocean models (with 1/12o grid spacing) reproduce the path well and some ocean models with 1/4o grid spacing that have recently been used in seasonal prediction also have an improved path. Scaife et al. (2011) attribute much of the improvement in the prediction of winter-time blocking events in their system to this improved path. Climate simulations and seasonal predictions are likely to use models of this resolution for at least a decade and the accuracy of their representation of the path of the Gulf Stream extension will be an important factor. The robustness of the path at this resolution is by no means assured so it is important to understand better what determines the path and to seek techniques which can provide good simulations at relatively coarse resolutions for the right reasons.There have been a number of past contributions to progress in understanding the dynamics that determine this path: in particular Greatbatch et al. (1991), Zhang & Vallis (2007). They showed that the contributions from the vortex stretching and bottom pressure torques associated with the flow over bathymetry and the viscous torques are important in determining the direction of the depth integrated flow. Mesoscale instabilities are known to have an important role in strengthening the flow at depth and hence the steering by the bathymetry (Hurlburt & Hogan 2008). The LANS-a representation of the momentum equations, formulated using generalised Lagrangian mean theory allows, a better representation of the kinetic energy of the mesoscale when it is only partially resolved. Peterson, Hecht, and Wingate implemented a simplified form of LANS-a in the POP ocean model and found that the steering of the depth integrated flow by the bathymetry in a model with a 0.4o grid was significantly improved. Barnier et al. (2006) have also shown that the various options for the formulation of momentum advection within NEMO have a significant influence on the barotropic flow in ORCA025 in regions where there are strong interactions with bathymetry. Issues with the treatment of bathymetry in this formulation have recently come to light and one of the supervisors is developing a revised scheme in collaboration with Barnier and co-workers. Therefore new, modified, or improved combinations of parameterizations and numerical implementations of the dynamics may make an important difference to the robustness of the Gulf Stream extension's path at coarse resolution.The first step in the PhD project will be to calculate diagnostics of vorticity tendencies and the Gulf Stream path using models of 1o, 1/4o and 1/12o resolution. Depending on the results and the interests of the student the PhD could develop in the following directions: in-depth investigation and interpretation of aspects of the vorticity diagnostics, the theory and dynamics of western boundary currents in the presence of sloping bathymetry, assessment of the impact of alternative representations of momentum advection or physical parametrisations of eddy fluxes on the Gulf Stream path. The student will be encouraged to formulate and experiment with novel ideas in numerics, diagnostics and theory for ocean physics and dynamics. Research questions to be addressed include: 1) What processes control the vorticity in the Gulf Stream extension?2) What are the 1) roles of mesoscale fluctuations in western boundary currents?2) How do mesoscale fluctuations strengthen the interactions with bathymetry