How many modes are needed to predict climate bifurcations? Lessons from an experiment

Type Article
Date 2022-02
Language English
Author(s) Dubrulle BérengèreORCID1, Daviaud François1, Faranda DavideORCID2, 3, 4, Marié Louis5, Saint-Michel Brice6
Affiliation(s) 1 : Université Paris-Saclay, CEA, CNRS, SPEC, CEA Saclay 91191 Gif-sur-Yvette CEDEX, France
2 : Laboratoire des Sciences du Climat et de l’Environnement, UMR 8212 CEA-CNRS-UVSQ, Université Paris-Saclay, IPSL, 91191 Gif-sur-Yvette CEDEX, France
3 : London Mathematical Laboratory, 8 Margravine Gardens, London, W6 8RH, UK
4 : Laboratoire de Météorologie Dynamique/Institut Pierre Simon Laplace, Ecole Normale Superieure, PSL research University, 75005 Paris, France
5 : LOPS, UMR6523, Univ. Brest, CNRS, IFREMER, IRD, 29280 Plouzané, France
6 : Department of Chemical Engineering, Delft University of Technology, Van der Maasweg 9, 2629HZ, Delft, the Netherlands
Source Nonlinear Processes In Geophysics (1023-5809) (Copernicus GmbH), 2022-02 , Vol. 29 , N. 1 , P. 17-35
DOI 10.5194/npg-29-17-2022
WOS© Times Cited 2
Note Centennial issue on nonlinear geophysics: accomplishments of the past, challenges of the future Editor(s): Roger Grimshaw, Ana M. Mancho, Daniel Schertzer, Olivier Talagrand, and Stéphane Vannitsem

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.

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