SLS Seminar: Nick Lutsko (MIT)
What Can the Internal Variability of CMIP5 Models Tell Us About Their Climate Sensitivity?
The relationship between climate models' internal variability and their response to external forcings 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. On sub-decadal frequencies, the surface temperature and the outgoing short-wave flux are in quadrature, with the short-wave acting as a stochastic forcing of surface temperature. The long-wave flux is linearly related to temperature, and acts as a negative feedback on temperature perturbations. On longer time-scales the outgoing short-wave and long-wave fluxes are both linearly related to temperature, with the long-wave continuing to act as a negative feedback and the short-wave acting as a positive feedback on temperature variability. In addition to the different phase relationships, the two regimes can also be seen in estimates of the coherence and of the frequency-dependent regression coefficients. The frequency-dependent regression coefficients 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 (ECSs), suggesting a potential ``emergent constraint" for Earth's ECS. However, O(100) years of data are required for this relationship to become robust. A simple model for Earth’s surface temperature variability and its relationship to the TOA fluxes is used to provide a physical explanation of these results.
About the Speaker
Nick Lutsko is a postdoctoral associate in the Cronin Group. Before coming to EAPS he completed his PhD at Princeton University, in their Atmospheric and Oceanic Sciences program, supervised by Prof Isaac Held. Lutsko says the main goal of his research is to improve our understanding of the atmosphere by connecting results from idealized models to the behavior of comprehensive climate models and to the observed behavior of the atmosphere. He is also interested in climate science more generally, including the problem of climate sensitivity, climate model development and evaluation, ocean heat uptake and climate variations on all time-scales.
About the Series
The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm in 54-915. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.