13 Oct 2023
 | 13 Oct 2023

Early warnings of the transition to a superrotating atmospheric state

Mark S. Williamson and Timothy M. Lenton

Abstract. Several general circulation models (GCMs) have showed bifurcations of their atmospheric state under a broad range of warm climates. These include some of the more extreme global warming scenarios. This bifurcation can cause the transition to a superrotating state, a state where its angular momentum exceeds the solid body rotation of the planet. Here we use an idealized GCM to simulate this transition by altering a single non-dimensional control parameter, the thermal Rossby number. For a bifurcation induced transition there is potential for early warnings and we look for these here. Typically used early warning indicators, variance and lag 1 autocorrelation, calculated for the mean zonal equatorial wind speed, increase and peak just before the transition. The full autocorrelation function taken at multiple lags is also oscillatory, with a period of 25 days preceding the transition. This oscillatory behaviour is reminiscent of a Hopf bifurcation. Motivated by this extra structure, we use a generalised early warning vector technique to diagnose the dominant spatial modes of the horizontal windfield fluctuations. We find a zonal wavenumber zero pattern we call the `precursor' mode, that appears shortly before and disappears soon after the transition. We attribute the increase in the early warning indicators to this spatial precursor mode.

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Mark S. Williamson and Timothy M. Lenton

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2036', Anonymous Referee #1, 17 Nov 2023
  • RC2: 'Comment on egusphere-2023-2036', Anonymous Referee #2, 21 Nov 2023
  • RC3: 'Comment on egusphere-2023-2036', Anonymous Referee #3, 23 Nov 2023
  • RC4: 'Comment on egusphere-2023-2036', Anonymous Referee #4, 27 Nov 2023
Mark S. Williamson and Timothy M. Lenton
Mark S. Williamson and Timothy M. Lenton


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Short summary
Climate models have transitioned to a superrotating atmospheric state under a broad range of warm climates. Such a transition would change global weather patterns should it occur. Here we simulate this transition using an idealized climate model and look for any early warnings of the superrotating state before it happens. We find several early warning indicators that we attribute to an oscillating pattern in the windfield fluctuations.