Impact of Interactive Ozone on Climate Reconstruction in an Earth System Model: the Case of Antarctica in mid-Holocene

Stratospheric ozone change can influence on tropospheric climate. For example, Sigmond and Fyfe (2010) pointed out the Antarctic sea ice could increase due to the Antarctic ozone hole: Stratospheric cooling due to the ozone hole increases a westerly anomaly of the polar night jet by satisfying the thermal wind balance, and annular mode response increases westerly anomaly near the surface. The increase of the surface westerly intensifies the northward component of the Ekman transport in the ocean and promotes the sea ice transport to lower latitudes. However, the impact of ozone change in paleoclimate has not been investigated in detail. In most of paleoclimate experiments of CMIP5/PMIP3, the distribution of ozone is fixed to the estimated value of 1850 AD despite the ozone distribution depends on the solar radiation distribution as a function of latitude and time (season). This treatment may cause some bias to the simulation results. In this study, therefore, we examine the impacts of forecasted ozone distribution in paleoclimate experiments with an Earth system model. In this presentation, we focus on the Antarctic region in the mid-Holocene (6k year before present, hereafter MH) experiment, where significant impact is obtained.

We utilize Japan Meteorological Agency Meteorological Research Institute (JMA-MRI) Earth system model, which is a coupled model of the atmosphere-ocean-aerosol general circulation model of MRI-CGCM3 (Yukimoto et al., 2012) which was used in CMIP5 and the chemistry model of MRI-CCM2 (Deushi and Shibata, 2011). We examine the MH experiment and the preindustrial (hereafter PI) control experiment under the condition of 1850 AD, both of which are corresponding to CMIP5. Boundary conditions in the both experiments are almost same except for the orbital parameters. Solar radiation in the Antarctic region in the MH experiment has positive anomaly about 50 W/m² in October and negative anomaly about 20 W/m² in February if compared with its distribution in the PI experiment. The period of integration is 100 years in each experiment. Contribution of the chemical processes on the climate change by the difference of orbital elements between MH and PI can be diagnosed by the following difference:

(MH_active - PI_active) - (MH_fix - PI_fix),

where the subscript “active” denotes an experiment whose ozone is forecasted, and “fix” denotes an experiment whose ozone is fixed to the estimate value of 1850 AD (seasonal variation is included). Here the results of “fix” are the same as those of MRI-CGCM3 in CMIP5.

The contribution of the chemical processes on the difference between PI and MH shows positive anomaly up to about 1 K in both polar regions for the annual mean zonal mean temperature at 2 m from the surface, whereas it is small in low and mid latitudes. Here we focus on the Antarctic region, because opposite trend is found to the relationship of sea ice and the Antarctic ozone hole in these decades as described above. Positive anomaly of the ozone is observed in the Antarctic stratosphere in February, in which the negative anomaly of solar radiation has a maximum. The relationship is interpreted as that the deceleration of ozone depletion reaction due to temperature decrease exceeds the deceleration of ozone production by the decrease of the amount of ultraviolet radiation in the stratosphere. This positive ozone anomaly descends in the polar vortex through autumn and winter. In spring, the temperature in the lower stratosphere and the troposphere has positive anomaly corresponding to the positive ozone anomaly, and the zonal mean zonal wind has easterly anomaly in the troposphere, satisfying the thermal wind balance. The easterly wind anomaly at the surface causes southward anomaly of the Ekman transport in the ocean, which reduces the sea ice transport to low latitudes. The reduction of the sea ice is consistent with that of meridional transport. As a result, surface albedo decreases, upward energy fluxes at the surface increase, and the surface air temperature shows positive anomaly.

We made mid-Holocene and preindustrial control experiments by using the MRI Earth system model that contains chemical processes, and investigated the impact of the ozone variations to climate by the change of orbital parameters of the Earth. The Antarctic sea ice in spring decreases through the deceleration of the westerly jet due to the increase of stratospheric ozone. As a result, temperature anomaly is up to about 1 K near the surface. This result suggests that the ozone distribution consistent with the solar insolation in the targeted era should be used in paleoclimate simulations in order to improve the accuracy of the climate reconstruction in the polar regions.