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9/1/2018 séminaire de Nicholas Lutsko

What Can the Internal Variability of CMIP5 Models Tell Us About Their Climate Sensitivity ?
Quand ? Le 09/01/2018,
de 10:30 à 12:30
Où ? Salle de réunion du LMD/Jussieu (T45-55, 3e étage)
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The relationship between the feedbacks controlling climate models’ internal variability and the feedbacks controlling the models’ sensitivity is investigated. Frequency-dependent regressions are performed between the outgoing top-of-atmosphere (TOA) energy fluxes and the global-mean surface temperature in the pre-industrial control simulations of the CMIP5 archive. Two distinct regimes are found. At sub-decadal frequencies the surface temperature and the outgoing short-wave flux are in quadrature, with the short-wave acting as a stochastic forcing on surface temperature. On longer time-scales the outgoing short-wave and long-wave fluxes are linearly related to temperature, and nearly cancel out. In addition to these different phase relationships, the two regimes can also be seen in estimates of the coherence and of the frequency-dependent regression co-efficients. The frequency-dependent regression co-efficients for the total cloudy-sky flux on time-scales of 2.5 to 3 years are found to be strongly (r 2 >0.6) related to the models’ equilibrium climate sensitivities (ECS’s), suggesting a potential “emergent constraint” for Earth’s ECS. A simple model for Earths surface temperature variability and its relationship to the TOA fluxes is used to provide a physical interpretation of these results.

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