Understanding the role of the ocean in non-flux-adjusted perturbed physics ensembles

这个项目将解决两个基本的科学问题：1.是什么决定了海洋对全球变暖速度的控制：是大气还是海洋？2.如果大气中二氧化碳的增加减缓了温盐环流(THC)，如果二氧化碳水平降低，它恢复的可能性有多大？它还将解决一个实际问题：3.海洋分辨率和通量调整的使用对使用扰动物理方法的概率气候预测有什么影响？该项目将通过使用以其固有的长期稳定性而闻名的大气-海洋耦合大气环流模式HadCM3进行两相集合气候变化模拟，从而为这些问题提供答案。许多不同的模型将以略有不同的初始条件和模型物理运行，这两者都包含很大的不确定性。这些结果将结合在一起，对影响全球变暖速度的海洋热量吸收和THC做出概率预测。它们将与我们目前的气候预测网络(Cpdn)实验的结果进行比较，该实验使用的是海洋分辨率较高的HadCM3版本和通量调整。通量调整指的是增加人工热量和淡水通量，以防止模型漂移到不现实的状态，并在气候变化实验中广泛使用。然而，通量调节没有物理基础，因此最好不要使用它。对目前的cpdn系综实验的分析发现，用通量调整的模式模拟THC存在很大困难。在第一个“自旋”阶段，海洋GCM在时间常数的强迫下运行，直到每次模拟都是稳定的。不同于目前的CPDN实验，我们使用通量调节来强制稳定的基本气候，这个项目不会使用通量调节，而是充分利用我们的分布式网络提供的大量计算资源来识别和选择稳定和现实的模式。这需要一种新的衍生方法，在这种方法中，我们使用数千个50到100年的模拟来识别具有在世纪时间尺度上稳定的真实基本气候的模型。这与通常采用的方法形成鲜明对比，即使用多个世纪的模拟来旋转单一模型，希望达到现实的均衡。在第二个“瞬变强迫”阶段，我们将运行1920年至2080年的历史强迫和未来强迫下的模式，就像目前的cpdn实验一样。其结果将是海洋气候和气候变化的关键敏感性。例如，尚不清楚海洋混合参数或水循环的表示在确定大规模海洋环流方面是否发挥更大作用。我们还将与哈德利中心的非NERC资助的研究人员合作，进行补充的“瞬变强迫”模拟，以检验THC强度变化的可逆性。从第一阶段选择的旋转模型开始，二氧化碳将快速增加，然后快速减少。二氧化碳强迫的这种瞬时变化预计将影响THC强度。哈德利中心将对结果进行分析，以量化THC在整个大系统中的人为变化，特别是结果是否随着恢复到工业化前的气候而可逆。计算将在数千台个人电脑上进行，这些个人电脑的空闲计算时间是由世界各地普通公众的志愿者捐赠的。实验将使用现已建立的cpdn分布式计算基础设施进行。一个由微软资助的高性能计算机集群将于明年在牛津电子研究中心上市，将用于在公开发行之前对模型进行测试。然而，需要计算支持才能引入新的模型版本、额外的诊断和新的实验设计。

This project will address two fundamental scientific questions: 1. What determines the ocean's control over the rate of global warming to externally driven climate change: the atmosphere or the ocean? 2. If the atmospheric CO2 increase slows down the thermohaline circulation (THC), what are the chances of it recovering if CO2 levels are reduced? It will also address one practical question: 3. What influence do ocean resolution and the use of flux adjustment have on probabilistic climate forecasts using a perturbed physics approach? The project will provide answers to them by conducting two-phase ensemble climate change simulations using the coupled atmosphere-ocean general circulation model (AOGCM), HadCM3, which is well known for its intrinsic long-term stability. Many different models will be run with slightly different initial conditions and model physics, both of which contain large uncertainties. The results will be combined to make probabilistic forecasts of the ocean's heat uptake, which impacts the rate of global warming, and the THC. They will be compared with the results from our current climateprediction.net (CPDN) experiment, which uses a coarse-oceanic-resolution version of HadCM3 and flux adjustment. Flux adjustment refers to the addition of artificial heat and freshwater fluxes to keep the model from drifting to unrealistic states, and is widely used in climate change experiments. However, flux adjustment has no physical basis, so it is desirable not to use it. Analysis of the current CPDN ensemble experiments has identified significant difficulties in simulating the THC with flux-adjusted models. In the first, 'spinup', phase, the ocean GCM is run with time-constant forcing until each simulation is stable. Unlike the current CPDN experiment, in which we employ flux adjustment to force a stable base climate, this project will not use flux adjustment but instead fully exploit the vast computing resources provided by our distributed network to identify and select stable and realistic models. This requires a novel approach to spinup in which we use thousands of 50 to 100 year simulations to identify models with realistic base climates that are stable on century timescales. This is in contrast to the commonly adopted method of using multi-century simulations to spin up a single model, hoping to reach a realistic equilibrium. In the second, 'transient forcing', phase, we will run the models under historical and future forcing from 1920 to 2080 as in the current CPDN experiment. The outcome will be the key sensitivities of the ocean climate and climate change. It is not yet known, for example, if ocean mixing parametrisation or the representation of the hydrological cycle play a larger role in determining large-scale ocean circulation. We will also conduct complementary 'transient forcing' simulations in collaboration with non-NERC-funded researchers at the Hadley Centre to examine the reversibility of the change in the THC strength. Starting from a selection of spun-up models from the first phase, CO2 will be rapidly increased, then rapidly decreased. This transient change in CO2 forcing is expected to impact the THC intensity. The Hadley Centre will analyse the results to quantify the anthropogenic changes to the THC in the large ensemble, in particular whether the results are reversible with a return to the pre-industrial climate. Computing will be performed on thousands of PCs, whose idle computing time are donated by volunteers from the general public worldwide. Experiments will be performed using the now well-established CPDN distributed-computing infrastructure. A Microsoft-funded high-performing-computer cluster, which will be available at the Oxford e-Research Centre in the next year, will be used to test the models before their public distribution. However, computing support will be required to introduce a new model version, additional diagnostics, and new experimental design.