Polar amplification in a coupled climate model with locked albedo

Abstract

In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling of the atmospheric CO₂ content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature amplification.

Appendix

An approximate estimation of the greenhouse effect from an increase of water vapour in the atmosphere is given here. The method is valid for clear-sky conditions. Raval and Ramanathan (1989; hereafter referred to as RR) studied the role of the water vapour for the greenhouse effect and found that for a given surface temperature, the greenhouse effect increases linearly (as a first order polynomial) with the natural logarithm of water-vapour content.

Let $\hat{G}$ represent the greenhouse effect, normalized by the upward surface LWR, $\hat{G}=G/L_s=(L_s-L_t)/L_s.$ It is shown by RR that $\hat{G}$ depends on the lapse rate and emissivity, where the dependence on the lapse rate is small compared to that on the emissivity. The emissivity, in turn, depends on the surface temperature. For fixed H₂O concentrations, the dependence is due to changes in spectroscopic parameters with temperature. However, the dependence on temperature is much larger when the H₂O concentration is free to increase with temperature. Hence to a good approximation:

$$ {\frac{{\rm{d}}\hat{G}}{{\rm{d}}T_s}} = {\frac{\partial\hat{G}}{\partial \ln W}} {\frac{{\rm{d}} \ln W}{{\rm{d}} T_s}}, $$

(2)

where T _s is surface temperature, and W is the total water-vapour content in a unit vertical column of air. It was shown by RR that $\partial \hat{G} / {\partial \ln W}$ is approximately constant over the temperature range of the Earth’s surface, except for temperatures over 298 K reached at low latitudes. Integration of Eq. 2 from T ₁ to T ₂, which represent the equilibrium surface temperature before and after a doubling of CO₂, yields:

$$ \Updelta \hat{G} = \hat{G_2}-\hat{G_1} = c \ln \left({\frac{W_2} {W_1}} \right), $$

where $c = \partial \hat{G} / {\partial \ln W}$ is assumed constant. Hence, the water-vapour greenhouse effect for a doubling of CO₂ is given by:

$$ \Updelta G \simeq c L_s \ln \left({\frac{W_2}{W_1}} \right), $$

(3)

neglecting the change of L _s associated with a CO₂ doubling. This is the water-vapour feedback for clear-sky conditions. For cloudy conditions this feedback is smaller. When water vapour is increased in or above clouds, it absorbs less LWR from below than it would have done had the clouds not been present. It should further be mentioned that water vapour absorbs short-wave radiation (Held and Soden 2000). On a global average, ∼15% of the absorbed radiation by water vapour is of the short-wave type. Even though Eq. 3 does not give an exact estimate of the water-vapour feedback, especially in cloudy areas, zonal-mean quantities as those shown in Fig. 9 provide a hint about the latitudinal differences of this feedback.