A generic shortcoming of constant‐depth (or “z‐coordinate”) ocean models such as MOM5 and Nucleus for European Models of the Ocean (NEMO) is a tendency for the advection scheme to produce unphysical numerical diapycnal mixing, which may exceed the explicitly parameterized mixing based on observed physical processes. Megann (2018, https://doi.org/10.1016/j.ocemod.2017.11.001) estimated the effective diapycnal diffusivity in the Global Ocean Version 5.0 (GO5.0) 0.25° global implementation of the NEMO model and showed that this was up to 10 times the explicit diffusivity used in the model's mixing scheme and argued that this was at least partly caused by large transient vertical velocities on length scales comparable to the horizontal grid scale. The current UK global NEMO configuration GO6, as used in the Global Coupled Model version 3.1 (GC3.1) and UK Earth System Model (UKESM1), is integrated in forced mode at 0.25° resolution with a range of viscosity parameterizations. In the present study, the effective diffusivity is evaluated for each integration and compared with the explicit value from the model mixing scheme, as well as with that in the control (using the default viscosity). It is shown that there is a strong correspondence between lower viscosity and enhanced numerical mixing and that larger viscosities lead to a marked reduction in the unrealistic internal temperature drift seen in the control configuration, without incurring excessive damping of the large‐scale circulation, mixed layer depths, or sea ice cover. The results presented here will inform the choices made in global ocean configurations used in climate and Earth System models following the sixth Coupled Model Intercomparison Project (CMIP6).

中文翻译：

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在允许涡流的全球海洋模型中探索粘度空间：粘度是数字混合的有用控件吗？

恒定深度（或“ z“坐标”海洋模型，例如MOM5和欧洲海洋模型的核（NEMO），是对流方案产生非物理数值渗流混合的趋势，该混合可能超过基于观察到的物理过程的明确参数化混合。Megann（2018，https://doi.org/10.1016/j.ocemod.2017.11.001）估计了NEMO模型在全球海洋版本5.0（GO5.0）0.25°的全球实施中的有效对向扩散性高达模型混合方案中使用的显式扩散率的10倍，并认为这至少部分是由与水平网格尺度相当的长度尺度上的大瞬态垂直速度引起的。当前在全球耦合模型版本3.1（GC3.1）和英国地球系统模型（UKESM1）中使用的当前英国全球NEMO配置GO6，在0.25°分辨率下以强制模式集成了一系列粘度参数设置。在本研究中，对每个积分的有效扩散率进行了评估，并与模型混合方案中的显式值以及对照中的显式值（使用默认粘度）进行了比较。结果表明，在较低的粘度和增强的数值混合之间存在很强的对应关系，并且较大的粘度可显着降低在控制配置中看到的不切实际的内部温度漂移，而不会导致大规模循环混合层的过度阻尼深处，或海冰覆盖。此处提出的结果将为第六次耦合模型比对项目（CMIP6）之后的气候和地球系统模型所用的全球海洋配置做出选择提供依据。