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Will Hobbs, Matthew D. Palmer, and Didier Monselesan

Abstract

Climate model simulations of changes to Earth’s energy budget are fundamental to improve understanding of both historical and future climate change. However, coupled models are prone to “drift” (i.e., they contain spurious unforced trends in state variables) due to incomplete spinup or nonclosure of the energy budget. This work assesses the globally integrated energy budgets of 25 models in phase 5 of CMIP (CMIP5). It is shown that for many of the models there is a significant disagreement between ocean heat content changes and net top-of-atmosphere radiation. The disagreement is largely time-constant and independent of forcing scenario. Furthermore, most of the nonconservation seems to occur as a result of energy leaks external to the ocean model realm. After drift correction, the time-varying energy budget is consistent at decadal time scales, and model responses to climate forcing are not sensitive to the magnitude of their drift. This demonstrates that, although drift terms can be significant, model output can be corrected post hoc without biasing results.

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Damien Irving, Will Hobbs, John Church, and Jan Zika

Abstract

Coupled climate models are prone to “drift” (long-term unforced trends in state variables) due to incomplete spinup and nonclosure of the global mass and energy budgets. Here we assess model drift and the associated conservation of energy, mass, and salt in CMIP6 and CMIP5 models. For most models, drift in globally integrated ocean mass and heat content represents a small but nonnegligible fraction of recent historical trends, while drift in atmospheric water vapor is negligible. Model drift tends to be much larger in time-integrated ocean heat and freshwater flux, net top-of-the-atmosphere radiation (netTOA) and moisture flux into the atmosphere (evaporation minus precipitation), indicating a substantial leakage of mass and energy in the simulated climate system. Most models are able to achieve approximate energy budget closure after drift is removed, but ocean mass budget closure eludes a number of models even after dedrifting and none achieve closure of the atmospheric moisture budget. The magnitude of the drift in the CMIP6 ensemble represents an improvement over CMIP5 in some cases (salinity and time-integrated netTOA) but is worse (time-integrated ocean freshwater and atmospheric moisture fluxes) or little changed (ocean heat content, ocean mass, and time-integrated ocean heat flux) for others, while closure of the ocean mass and energy budgets after drift removal has improved.

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E. Povl Abrahamsen, Sandra Barreira, Cecilia M. Bitz, Amy Butler, Kyle R. Clem, Steve Colwell, Lawrence Coy, Jos de Laat, Marcel D. du Plessis, Ryan L. Fogt, Helen Amanda Fricker, John Fyfe, Alex S. Gardner, Sarah T. Gille, Tessa Gorte, L. Gregor, Will Hobbs, Bryan Johnson, Eric Keenan, Linda M. Keller, Natalya A. Kramarova, Matthew A. Lazzara, Jan T. M. Lenaerts, Jan L. Lieser, Hongxing Liu, Craig S. Long, Michelle Maclennan, Robert A. Massom, François Massonnet, Matthew R. Mazloff, David Mikolajczyk, A. Narayanan, Eric R. Nash, Paul A. Newman, Irina Petropavlovskikh, Michael Pitts, Bastien Y. Queste, Phillip Reid, F. Roquet, Michelle L. Santee, Susan Strahan, Sebastiann Swart, and Lei Wang
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