The relation between global warming and poleward heat transport of climate models

The largest warming over the last several decades has been observed in high latitudes, which is well captured by CGCM simulations. It is known that there exists a range of global warming projections under the same amount of radiative forcing. This study will attempt to explore whether part of the differences in 14 climate models' global warming projections can be explained by the models' poleward heat transport strengths. The radiation budget at the top of the atmosphere (TOA) using IPCC AR4 climate model simulations forced by anthropogenic radiative forcings made at different climate centers were analyzed. The results indicate that both the radiation energy surplus in low latitudes and deficit in high latitudes at the TOA have been strengthened over the last several decades suggesting an enhanced polewar heat transport. Furthermore, the analysis of the net radiation budget between the surface and the TOA confirms that the change in the TOA energy imbalance is indeed due to the upward trend in the poleward heat transport. Such an enhanced poleward heat transport seems to suggest that part of the large surface warming in high latitudes in the models' simulations is due to the dynamical amplifier feedback (Cai, 2005). There is a large model-to-model variability of the poleward heat transport in the mean state among the 14 climate model simulations. It is found that the warming amount in CGCMs which varies from 1.5K to 4K for the same amount of CO2 increase strongly depends on the strength of the poleward heat transport in the unperturbed mean state, in consistence with the dynamical amplifier theory. 37% of the global warming projection uncertainties can indeed be attributed to the strength of the polward heat transport. There is also a large variation of the intensification of the poleward heat transport among the models. It explains about 59% of the global warming projection uncertainties and about 71% and 49% in the northern and southern hemispheres high latitudes. Therefore, it can be concluded that a large part of the uncertainties in the CGCM's global warming projections can be explained by the differences in the climate models' heat transport and change in heat transport.