Symmetric and Antisymmetric Components of Polar-Amplified Warming

CO2-forced surface warming in general circulation models (GCMs) is initially polar amplified in the Arctic but not in the Antarctic-a largely hemispherically antisymmetric signal. Nevertheless, we show in CESM1 and 11 LongRunMIP GCMs that the hemispherically symmetric component of global-mean-normalized, zonal-mean warming (T-sym*) under 4 x CO2 changes weakly or becomes modestly more polar amplified from the first decade to near-equilibrium. Conversely, the antisymmetric warming component (T-asym*) weakens with time in all models, modestly in some including FAMOUS, but effectively vanishing in others including CESM1. We explore mechanisms underlying the robust T-sym* behavior with a diffusive moist energy balance model (MEBM), which given radiative feedback parameter (lambda) and ocean heat uptake (O) fields diagnosed from CESM1 adequately reproduces the CESM1 T-sym* and T-asym* fields. In further MEBM simulations perturbing lambda and O, T-sym* is sensitive to their symmetric components only, and more to that of lambda. A three-box, two-time-scale model fitted to FAMOUS and CESM1 reveals a curiously short Antarctic fast-response time scale in FAMOUS. In additional CESM1 simulations spanning a broader range of forcings, T-sym* changes modestly across 2-16 x CO2, and T-sym* in a Pliocene-like simulation is more polar amplified but likewise approximately time invariant. Determining the real-world relevance of these behaviors-which imply that a surprising amount of information about near-equilibrium polar amplification emerges within decades-merits further study.

Automated summary

Increasing atmospheric CO2 concentration leads to surface temperature warming. The warming is generally amplified in the Arctic compared to the Antarctic. However, the symmetric component of global-mean-normalized, zonal-mean warming weakly amplifies in the Arctic over time, while the antisymmetric component weakens in all models. The study also found that the symmetric component is more sensitive to the symmetric components of radiative feedback parameter and ocean heat uptake.