On the choice of ensemble mean for estimating the forced signal in the presence of internal variability

• Published in: Journal of Climate
• Main Authors: Frankcombe, LM, England, MH, Kajtar, JB, Mann, ME, Steinman, BA
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Meteorological Society 2018
• Subjects: Climate variability; Model evaluation/performance; Decadal variability; Interdecadal variability; Multidecadal variability; Sea ice
• Online Access: http://hdl.handle.net/10871/33880; https://doi.org/10.1175/JCLI-D-17-0662.1

This is the final version of the article. Available from American Meteorological Society via the DOI in this record. In this paper we examine various options for the calculation of the forced signal in climate model simulations, and the impact these choices have on the estimates of internal variability. We find that an ensemble mean of runs from a single climate model [a single model ensemble mean (SMEM)] provides a good estimate of the true forced signal even for models with very few ensemble members. In cases where only a single member is available for a given model, however, the SMEM from other models is in general out-performed by the scaled ensemble mean from all available climate model simulations [the multimodel ensemble mean (MMEM)]. The scaled MMEM may therefore be used as an estimate of the forced signal for observations. The MMEM method, however, leads to increasing errors further into the future, as the different rates of warming in the models causes their trajectories to diverge. We therefore apply the SMEM method to those models with a sufficient number of ensemble members to estimate the change in the amplitude of internal variability under a future forcing scenario. In line with previous results, we find that on average the surface air temperature variability decreases at higher latitudes, particularly over the ocean along the sea ice margins, while variability in precipitation increases on average, particularly at high latitudes. Variability in sea level pressure decreases on average in the Southern Hemisphere, while in the Northern Hemisphere there are regional differences. This work was supported by the Australian Research Council (ARC) through grants to L. M. F. (DE170100367) and to M. H. E. through the ARC Centre of Excellence in Climate System Science (CE110001028). J. B. K. is supported by the Natural Environment Research Council (Grant NE/N005783/1). B. A. S. was supported by the U.S. National Science Foundation (EAR-1447048).