Abstract. According to everyone’s experience, predicting the weather reliably over more than 8 days seems an impossible task
for our best weather agencies. At the same time, politicians and citizens are asking scientists for climate projections several
decades into the future to guide economic and environmental policies, especially regarding the maximum admissible emissions of CO₂. To what extent is this request scientifically admissible? In this lecture we will investigate this question, focusing on the topic of predictions of transitions between metastable states
of the atmospheric or oceanic circulations. Two relevant exemples are the switching between zonal and blocked atmospheric
circulation at midlatitudes and the alternance of El Niño and La Niña phases in the Pacific ocean. The main issue is whether
present climate models, that necessarily have a finite resolution and a smaller number of degrees of freedom than the actual
terrestrial system, are able to reproduce such spontaneous or forced transitions. To do so, we will draw an analogy between
climate observations and results obtained in our group on a laboratory-scale, turbulent, von Kármán flow, in which spontaneous
transitions between different states of the circulation take place. We will detail the analogy, and investigate the nature of the
transitions, the number of degrees of freedom that characterizes the latter and discuss the effect of reducing the number of
degrees of freedom in such systems. We will also discuss the role of fluctuations and their origin, and stress the importance of
describing very small scales to capture fluctuations of correct intensity and scale.

中文翻译：

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刘易斯·弗雷·理查森（Lewis fry Richardson）奖牌讲座–预测气候分叉需要几种模式？

摘要。根据每个人的经验，
对于我们最好的气象机构来说，可靠地预测超过8天的天气似乎是一项不可能的任务。同时，政界人士和公民正在要求科学家对
未来几十年的气候进行预测，以指导经济和环境政策，尤其是有关最大允许CO ₂排放量的政策。在科学上可以接受此要求的程度？在本讲座中，我们将研究这个问题，重点
是大气或海洋环流的亚稳态之间的转换预测。两个相关的例子是纬向和封闭大气之间的切换
太平洋中纬度的环流和厄尔尼诺和拉尼娜阶段的交替。主要问题是，
目前的气候模型是否必须
能够再现这种自发或强迫转变，这些气候模型必须具有一定的分辨率，并且自由度要少于实际的陆地系统。为此，我们将模拟
气候观测结果与在实验室规模的湍流vonKármán流中获得的研究结果之间的类比，在这种情况下
，不同循环状态之间会发生自发转变。我们将详细说明这一类比，并研究
过渡的性质，表征过渡的自由度的数量，并讨论减少过渡数量的影响。
这种系统的自由度。我们还将讨论波动的作用及其起因，并强调
描述非常小的比例尺以捕获正确强度和比例尺波动的重要性。