Understanding the drivers of Arctic amplification through an idealized radiative-advective equilibrium model

We construct a radiative-advective model to investigate the drivers of Arctic amplification. The Rapid Radiative Transfer Model for GCMs (RRTMG) is utilized to calculate radiative heating rates, while the atmospheric horizontal energy transport (AHT) from JRA-55 reanalysis data is used as boundary conditions. We perturb individual factors in the model to assess the warming contributions from radiative forcing by different greenhouse gases, poleward energy transport at different vertical levels, and clouds.

We first examine the Arctic climate sensitivity to CO₂, CH₄, and O₃. The climate sensitivity is defined as the surface temperature change per unit of TOA flux perturbation. The Arctic climate sensitivity to CO₂ is 3.46 K/W/m². When CO₂ is doubled, the instantaneous radiative forcing at TOA is 1.68 W/m², resulting in 5.7 K surface warming. For CH₄, the Arctic climate sensitivity is 1.65 K/W/m², and doubling CH₄ leads to a TOA perturbation of 0.46 W/m², leading to merely 0.76 K surface warming. The sensitivity to O₃ is 0.21 K/W/m², with a doubling of O₃ causing a 3.51 W/m² perturbation and 0.75 K surface warming.

The sensitivity to AHT is strongly dependent on its vertical structure, with greater sensitivity at lower levels. At 975 hPa level, the climate sensitivity reaches its peak value of 3.15 K/W/m², comparable to that of CO₂. At the 900 hPa level where climatological AHT peaks, the climate sensitivity dramatically drops to 0.73 K/W/m². At higher altitude, the sensitivity continues to decrease: 0.64 K/W/m² at 850 hPa, 0.62 K/W/m² at 700 hPa, and 0.30 K/W/m² at 500 hPa. The climate sensitivity to clouds is 1.62 K/W/m². The climatological cloud fraction in the Arctic is 15%, with radiative effect of 2.09 W/m² at the TOA, resulting 3.41 K surface warming.

In summary, the Arctic region shows highest climate sensitive to CO₂, followed by sensitivity to AHT at 975 hPa and CH₄, although CH₄ increase does not induce significant flux perturbations at the TOA. Clouds also play an important role. The sensitivities to AHT above 900 hPa and O₃ are relatively smaller.