Symmetric and antisymmetric components of polar-amplified warming
CO$_2$-forced surface warming in general circulation models (GCMs) is initially polar-amplified in the Arctic but not Antarctic -- a largely hemispherically antisymmetric signal. Nevertheless, we show in CESM1 and eleven LongRunMIP GCMs that the hemispherically symmetric component of global-mean-nor...
| Main Authors: | , , , |
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| Format: | Report |
| Language: | unknown |
| Published: |
arXiv
2020
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| Subjects: | |
| Online Access: | https://dx.doi.org/10.48550/arxiv.2012.09228 https://arxiv.org/abs/2012.09228 |
| Summary: | CO$_2$-forced surface warming in general circulation models (GCMs) is initially polar-amplified in the Arctic but not Antarctic -- a largely hemispherically antisymmetric signal. Nevertheless, we show in CESM1 and eleven LongRunMIP GCMs that the hemispherically symmetric component of global-mean-normalized, zonal-mean warming ($T^*_\mathrm{sym}$) under 4\(\times\)CO$_2$ changes weakly or becomes moderately more polar-amplified from the first decade to near-equilibrium. Conversely, the antisymmetric warming component ($T^*_\mathrm{asym}$) weakens with time in all models, moderately in some including FAMOUS but effectively vanishing in others including CESM1. We explore mechanisms underlying the robust $T^*_\mathrm{sym}$ behavior with a diffusive moist energy balance model (MEBM), which given radiative feedback parameter ($λ$) and ocean heat uptake ($\mathcal{O}$) fields diagnosed from CESM1 adequately reproduces the CESM1 $T^*_\mathrm{sym}$ and $T^*_\mathrm{asym}$ fields. In further MEBM simulations perturbing $λ$ and $\mathcal{O}$, $T^*_\mathrm{sym}$ is sensitive to their symmetric components only, and more to that of $λ$. A three-box, two-timescale model fitted to FAMOUS and CESM1 reveals a curiously short Antarctic fast-response timescale in FAMOUS. In additional CESM1 simulations spanning a broader range of forcings, $T^*_\mathrm{sym}$ changes modestly across 2-16\(\times\)CO$_2$, and $T^*_\mathrm{sym}$ 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. : 18 pages, 9 figures, revised for Journal of Climate |
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