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

在大西洋，大部分热量从海洋向大气的转移发生在墨西哥湾流上方，多年来人们一直知道，大气对墨西哥湾流延伸区的海洋表面温度非常敏感。在IPCC报告的几乎所有气候模拟中使用的网格间距为1 o的粗分辨率海洋模型歪曲了墨西哥湾流延伸的路径，气流过于纬向，过于向南。高分辨率海洋模式（1/12 o网格间距）很好地再现了路径，最近用于季节预测的一些1/4 o网格间距的海洋模式也有改进的路径。Scaife等人（2011）将冬季预测的大部分改进归因于气候模拟和季节预测可能使用这种分辨率的模式至少十年，它们对墨西哥湾流延伸路径的代表性将是一个重要因素。在这种分辨率下，路径的鲁棒性是无法保证的，因此，更好地理解是什么决定了路径，并寻求能够在相对粗糙的分辨率下提供良好模拟的技术是非常重要的。过去，在理解决定路径的动力学方面，有许多贡献：尤其是Greatbatch等人（1991），Zhang和Vallis（2007）。他们表明，与水深流和粘性力矩有关的涡流拉伸和底部压力力矩的贡献在确定深度综合流的方向方面很重要。已知中尺度不稳定性在加强深度流动方面具有重要作用，因此可通过水深测量进行引导（Hurlburt & Hogan，2008年）。的LANS-一个表示的动量方程，制定使用广义拉格朗日平均理论允许，一个更好的代表性的动能的中尺度时，它只是部分解决。Peterson、Hecht和Wingate在POP海洋模式中实施了一种简化形式的LANS-a，并发现在0.4o网格的模式中，水深测量对深度综合流的控制得到了显著改善。Barnier等人（2006年）还表明，在NEMO内形成动量平流的各种选择对ORCA 025中与水深有强烈相互作用的区域的正压流有重大影响。最近，在这一方案中处理水深测量的问题暴露出来，其中一名主管正在与巴尼耶及其同事合作制定一项订正方案。因此，新的，修改，或改进的参数化和数值实现的动态组合可能会使一个重要的差异的墨西哥湾流延伸的路径的鲁棒性在粗resolution.The博士项目的第一步将是计算诊断涡度趋势和墨西哥湾流路径使用模型的1 o，1/4 o和1/12 o的分辨率。根据结果和学生的兴趣，博士学位可以在以下方向发展：深入调查和解释涡度诊断、倾斜水深测量情况下西部边界流的理论和动力学，评估动量平流的替代表示或涡流通量的物理参数化对墨西哥湾流路径的影响。学生将受到鼓励在海洋物理学和动力学的数值、诊断和理论方面制定和试验新的想法。要解决的研究问题包括：1）什么过程控制墨西哥湾流延伸的涡度？2)西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