Abstract
Abstract. According to everyone's experience, predicting the weather reliably over more than 8 d 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 review we will investigate this question, focusing on the topic of predictions of transitions between metastable states of the atmospheric or oceanic circulations. Two relevant examples are the switching between zonal and blocked atmospheric circulation at mid-latitudes and the alternation of El Niño and La Niña phases in the Pacific Ocean. The main issue is whether present climate models, which 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, investigate the nature of the transitions and the number of degrees of freedom that characterize 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.
Highlights
The present review paper is based on the lecture delivered by Bérengère Dubrulle on the occasion of her reception of the Lewis Fry Richardson Medal 2021
They better represent the energy and water cycles, including more physical constraints on conservation laws (Irving et al, 2021). They are able to capture nowadays some non-linear interactions between the different components of the climate system, i.e. the El Niño–Southern Oscillation (ENSO) feedback to atmospheric motions (Bayr et al, 2020), monsoons (Yang et al, 2019), or stratospheric-totropospheric interactions (Olsen et al, 2007)
We seem to have understood why climate models work in the first place: the large-scale topology and externally forced transitions do not depend very much on the value of the viscosity and may be described with tools from statistical mechanics (Thalabard et al, 2015) involving only a few hundred to thousand modes that can be captured by present climate models
Summary
The present review paper is based on the lecture delivered by Bérengère Dubrulle on the occasion of her reception of the Lewis Fry Richardson Medal 2021. We have performed experiments in a second forcing mode in which we controlled the torque applied to the von Kármán flow by the impellers We can change the number of degrees of freedom in the von Kármán flow by considering more or less viscous fluids, going from N ∼ 1013 for a water-filled experiment at Re ∼ 106 down to N ∼ 104 for a glycerol-filled experiment at Re ∼ 102 This is an experimental equivalent of the reduction of the number of degrees of freedom that is explicitly done when using turbulent viscosities in present numerical simulations of atmospheric or oceanic circulation. From the point of view of a theoretical physicist, though, this difference only affects the detailed shape of the circulation
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