Some Aspects of Climate Response to Black Carbon in Quasi-equilibrium Experiments with the GFDL GCM

It is anticipated that impact of inhomogeneous constituents such as black carbon on regional climate could be much larger than that implied by global mean radiative forcing. Nevertheless, current understanding of the effect of non-greenhouse-gases on climate change is relatively poor. A new GFDL atmospheric GCM coupled with a mixed layer ocean model (SM2.1z) is used to study the equilibrium response to various radiative forcing agents. The model incorporates only the direct effect of aerosols. Some aspects of model responses to changes in black carbon concentrations are presented, and comparisons to other forcing agents are also shown. Three idealized perturbation experiments are conducted involving changes in black carbon concentration: 1) the difference between years 2000 and 1860 which is linearly amplified to a global mean radiative forcing of about 2 W m-2; 2) the same amount of global mean radiative forcing as in 1) but with the perturbation concentrated only in the lower troposphere; and 3) same as in 1) but with a radiative forcing of about 1 W m-2. The "efficacy" of black carbon, defined here as the rate of near-surface warming per unit radiative forcing, is about 57-68% of CO2. The global mean precipitation rate decreases in all black carbon experiments even though the global mean temperature increases. The experiment with more black carbon in the lower troposphere than the upper troposphere exhibits larger surface warming and smaller reduction in precipitation. In boreal summer, the largest warming is simulated in interior North America even though the radiative forcing is largest in eastern Asia. This warming is strongly related to a weakening of the regional hydrological cycle, and accompanied by low-level cloud feedback. It also shows a nonlinear relation between forcing and response. Furthermore, a poleward shift of the storm track is seen in the three experiments, which is consistent with an increased and a decreased baroclinicity in mid-to-high latitudes and mid-to-low latitudes, respectively.