09 Aug 2022
09 Aug 2022
Status: a revised version of this preprint is currently under review for the journal CP.

Quantifying the contribution of forcing and three prominent modes of variability on historical climate

Andrew P. Schurer1, Gabriele C. Hegerl1, Hugues Goosse2, Massimo A. Bollasina1, Matthew H. England3, Michael J. Mineter1, Doug M. Smith4, and Simon F. B. Tett1 Andrew P. Schurer et al.
  • 1School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, United Kingdom
  • 2Université Catholique de Louvain, Georges Lemaître Centre for Earth and Climate Research, Earth and Life Institute, Louvain-La-Neuve, B-1348, Belgium
  • 3Climate Change Research Centre and ARC Centre for Excellence in Antarctic Science, University of New South Wales, New South Wales 2052, Australia
  • 4Met Office Hadley Centre, Exeter, EX1 3PB, United Kingdom

Abstract. Climate models can produce accurate representations of the most important modes of climate variability, but they cannot be expected to follow their observed time-evolution. This makes direct comparison of simulated and observed variability difficult, and creates uncertainty in estimates of forced change. We investigate the role of three modes of climate variability, the North Atlantic Oscillation, El-Niño Southern Oscillation and the Southern Annular Mode, as pacemakers of climate variability since 1781, evaluating where their evolution masks or enhances forced climate trends. We use particle filter data assimilation to constrain the observed variability in a global climate model without nudging, producing a near free running model simulation with the time-evolution of these modes similar to those observed. Since the climate model also contains external forcings, these simulations, in combination with model experiments with identical forcing but no assimilation, can be used to compare the forced response to the effect of the three modes assimilated, and evaluate to what extent these are confounded with the forced response. The assimilated model is significantly closer than the “forcing only” simulations to annual temperature and precipitation observations over many regions, in particular the tropics, the north Atlantic and Europe. The results indicate where initialized simulations that track these modes could be expected to show additional skill. Assimilating the three modes cannot explain the large discrepancy previously found between observed and modelled variability in the southern extra-tropics but constraining the El-Niño Southern Oscillation reconciles simulated global cooling with that observed after volcanic eruptions.

Andrew P. Schurer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2022-55', Anonymous Referee #1, 01 Nov 2022
  • RC2: 'Comment on cp-2022-55', Anonymous Referee #2, 07 Nov 2022

Andrew P. Schurer et al.

Andrew P. Schurer et al.


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Short summary
We adopt an existing data assimilation technique to constrain a model simulation to follow three important modes of variability, the North Atlantic Oscillation, El-Niño Southern Oscillation and the Southern Annular Model. How it compares to the observed climate is evaluated, with improvements over simulations without data-assimilation found over many regions, in particular the tropics, north Atlantic and Europe and discrepancies with global cooling following volcanic eruptions are reconciled.