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Attribution et reconstruction du rôle de la variabilité interne et des forçages externes sur le climat passé récent et du dernier millénaire

Abstract : Using large ensembles of IPSLCM5A model simulations, we first investigate the roles of internal variability (and in particular the IPO) and external forcing in driving recent Peru-Chile regional cooling. The simulations reproduce the relative cooling, in response to an externally-forced southerly wind anomaly, which strengthens the upwelling off Chile in recent decades. This southerly wind anomaly results from the expansion of the Southern Hemisphere Hadley Cell in response to increasing greenhouse gases and stratospheric ozone depletion since ~1980. An oceanic heat budget confirms that the wind-forced upwelling dominates the cooling near the coast while a wind-forced deepening of the mixed layer drives the offshore cooling, irrespectively of the IPO phase, hence indicating the preeminent role of external forcing. Constraining the climate sensitivity from observations remains however fraught with uncertainties due to the limited instrumental window of observation. In a second part, a data assimilation method is developed to reconstruct past natural variability relying on a particles filter using CMIP-class climate models. Such method is confronted with a problem of degeneracy associated with the resolution of a large problem with a limited number of particles. This issue has been resolved using a statistical emulator of the IPSL model (LIM) as an integration model in a particle filter with resampling. The validation of this new method, called SIR-LIM, allows the reconstruction of the climate variability of the past centuries by assimilating observations and proxy records into a CMIP-class coupled model while preserving the physical coherence along the simulation.
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Submitted on : Monday, October 18, 2021 - 4:27:13 PM
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Beyrem Jebri. Attribution et reconstruction du rôle de la variabilité interne et des forçages externes sur le climat passé récent et du dernier millénaire. Milieux et Changements globaux. Sorbonne Université, 2020. Français. ⟨NNT : 2020SORUS162⟩. ⟨tel-03383697⟩

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