Testing a Role of Vertical Mixing in Abrupt Climate Change

垂直混合在气候突变中的作用的试验Olivier Marchal海洋的纬向翻转环流（MOC）由于其伴随的向极热通量而被视为气候系统的重要组成部分。一种流行的理论认为，地质学过去的快速气候变化是由于冰川融水输入北大西洋北方引起的MOC变化。然而，这一理论目前受到了基于理想模式的研究的挑战，这些研究表明，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：如果用一个固定的表观扩散率来表示混合，那么随着赤道到极点表面密度对比度的减小，最大主运行中心（MOC）也随之减小（通常的结果），而如果扩散率取决于垂直密度分层（也许是一个更合理的假设），MOC增加与密度对比度降低。在这里，我们将确定是否有一个类似的结果适用于与时间相关的响应的环流表面清新在一个现实的全球海洋模型。为了实现我们的目标，我们将进行一组数值实验的海洋环流模式，依赖于不同的假设垂直混合，并比较他们的结果与古海洋学数据从沉积物。我们的工作计划包括三个主要任务：第一，将建立模型，以真实的水深表示全球海洋。第二，一系列的模型平衡状态，假设恒定或稳定性依赖的垂直扩散将产生。第三，这些国家将受到扰动，在地表淡水通量的异常。通过比较不同垂直混合假设下的结果，可以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解决物理海洋学中一个悬而未决的问题，即垂直混合在MOC中的作用。

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.