Testing a Role of Vertical Mixing in Abrupt Climate Change

测试垂直混合在气候突变中的作用Olivier Marchal海洋的经向翻转环流(MOC)因其伴随的极地热通量而被视为气候系统的重要组成部分。一种流行的理论将过去地质上的快速气候变化归因于北大西洋北部冰川融水输入造成的MOC的变化。然而，这一理论目前受到了基于理想化模式的研究的挑战，表明MOC对地面强迫的响应定性地取决于模式中垂直混合的表示：如果混合用固定的表观扩散系数来表示，则MOC随着地面赤道至极地密度对比度的减小而减小(通常的结果)，而如果扩散率取决于垂直密度层结(也许是一个更合理的假设)，则MOC随着密度对比度的减小而增加。在这里，我们将确定在一个真实的全球海洋模型中，环流对表面清新的依赖时间的反应是否适用于类似的结果。为了达到我们的目标，我们将利用海洋环流模式进行一系列数值实验，该模式依赖于不同的垂直混合假设，并将它们的结果与从沉积物获得的古海洋学数据进行比较。我们的工作计划包括三项主要任务。一是建立模型，以逼真的水深表示全球海洋。其次，将产生一系列假定垂直扩散系数恒定或依赖于稳定性的模型平衡态。第三，这些状态将被地表淡水通量中的异常所扰动。比较关于垂直混合的不同假设所得到的结果将对我们的假设进行检验。将模型结果与古海洋学数据进行比较，将使我们能够评估这些数据在多大程度上有助于物理海洋学中与垂直混合在MOC中的作用有关的一个悬而未决的问题。

Testing a Role of Vertical Mixing in Abrupt Climate ChangeOlivier MarchalThe meridional overturning circulation (MOC) of the ocean is viewed as an important component of the climate system owing to its attendant poleward heat flux. A popular theory ascribes rapid climate changes of the geological past to variations in the MOC caused by glacial meltwater input to the northern North Atlantic. The theory, however, is currently challenged by studies based on idealized models, showing that the response of the MOC to surface forcing depends qualitatively on the representation of vertical mixing in the models: if the mixing is represented with a fixed, apparent diffusivity, the MOC decreases with decreased equator-to-pole density contrast at the surface (the usual result), whereas if the diffusivity depends on vertical density stratification (perhaps a more plausible assumption), the MOC increases with decreased density contrast. Here we will determine whether a similar result holds for the time-dependent response of the circulation to surface freshening in a realistic model of the global ocean. To meet our goal we will conduct a set of numerical experiments with an ocean general circulation model, which rely on different assumptions about vertical mixing, and compare their results with paleoceanographic data obtained from sediments. Our work plan includes three major tasks.First, the model will be set up to represent the global ocean with realistic bathymetry. Second, a series of model equilibrium states assuming constant or stability-dependent vertical diffusivity will be generated. Third, these states will be perturbed by applying an anomaly in the surface freshwater flux. Comparing results obtained from different assumptions about vertical mixing will provide a test of our hypothesis. Comparing model results with paleoceanographic data will enable us to assess the extent to which these data could contribute to an outstanding question in physical oceanography, related to the role of vertical mixing in the MOC.