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26/5/2016 à 14h30: Séminaire de Freddy BOUCHET (LMD/ENS)

Large deviations in climate dynamics: the examples of abrupt climate changes and extreme heat waves
Quand ? Le 26/05/2016,
de 14:30 à 15:30
Où ? salle 235 B, 29 Rue d'Ulm
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For some aspects of climate dynamics, rare dynamical events may play a key role. A first class of problems are extreme events that have huge impacts, for instance extreme heat waves. A second class of problems are rare trajectories that suddenly drive the complex dynamical system from one attractor to a completely different one, for instance abrupt climate changes. In the recent past, new theoretical and numerical tools have been developed in the statistical mechanics community, in order to specifically study such rare events. Those approaches are based on large deviation theory for complex dynamical systems. We will present some of these tools and apply them to two paradigmatic examples in climate dynamics. First, the disappearance of one of Jupiter's jets during the period 1939-1940 is a simple example of a drastic climate change related to internal variability. We will demonstrate that quasi geostrophic turbulent models show this kind of ultra rare transitions where turbulent jets suddenly appear or disappear on times scales of tens of thousands times the typical dynamical time scale. We will explain how theoretical and numerical approaches, related to large deviation theory, predict transition rates, transition paths, and allow to sample transitions in a way that can not be achieved using direct numerical simulations. Second, we will study the probability of extreme heat waves over Europe, in a comprehensive climate model. We will demonstrate that large deviation algorithms allow to sample extremely efficiently extreme heat waves. The thousands of sampled extreme heat waves open the door to their dynamical studies, precursor and fluctuation paths, in a way that can not be foreseen using conventional tools based on model direct numerical simulations.

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