Comments on “Current GCMs' Unrealistic Negative Feedback in the Arctic”

In contrast to prior studies showing a positive lapse-rate feedback associated with the Arctic inversion, Boé et al. reported that strong present-day Arctic temperature inversions are associated with stronger negative longwave feedbacks and thus reduced Arctic amplification in the model ensemble from phase 3 of the Coupled Model Intercomparison Project (CMIP3). A permutation test reveals that the relation between longwave feedbacks and inversion strength is an artifact of statistical self-correlation and that shortwave feedbacks have a stronger correlation with intermodel spread. The present comment concludes that the conventional understanding of a positive lapse-rate feedback associated with the Arctic inversion is consistent with the CMIP3 model ensemble.

Seasonal Changes in Solar Radiation and Relative Humidity in Europe in Response to Global Warming*

Future seasonal changes in surface incident solar radiation and relative humidity (RH) over Europe and adjacent ocean areas were assessed based on phase 3 of the Coupled Model Intercomparison Project (CMIP3) model ensemble. Under the A1B scenario, by 2070–99, summertime solar radiation is projected to increase by 5%–10% in central and southern Europe. In winter, radiation decreases in most of northern and eastern Europe by 5%–15%. RH drops in summer in the southern European inland by 8%–12%, whereas in winter a small increase of 2%–3% is projected for northeastern Europe. In spring, the change is an intermediate between those in the extreme seasons, while in autumn the patterns resemble summer. Over the northern Atlantic Ocean, RH increases in all seasons by 1%–2%. The intermodel agreement on the sign of all these shifts is good, and the patterns recur in the responses to the A2 and B1 scenarios. Substantial changes are already simulated to occur before the midcentury, for example, in summer RH decreases by more than 5% in the inner Balkan Peninsula. Projected changes in these two variables agree well and are also mainly consistent with precipitation responses both in the multimodel mean and in individual models. According to all indicators, southern European summers become more arid, while winters, in the north particularly, become moister and darker. The increasing radiation and declining RH exacerbate summertime drought in southern Europe, whereas excessive humidity in the north may, for example, inflict moisture damages in constructions.

Storm-Track Activity in IPCC AR4/CMIP3 Model Simulations

The climatological storm-track activity simulated by 17 Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/phase 3 of the Coupled Model Intercomparison Project (CMIP3) models is compared to that in the interim ECMWF Re-Analysis (ERA-Interim). Nearly half of the models show significant biases in storm-track amplitude: four models simulate storm tracks that are either significantly (>20%) too strong or too weak in both hemispheres, while four other models have interhemispheric storm-track ratios that are biased by over 10%. Consistent with previous studies, storm-track amplitude is found to be negatively correlated with grid spacing. The interhemispheric ratio of storm-track activity is highly correlated with the interhemispheric ratio of mean available potential energy, and this ratio is biased in some model simulations due to biases in the midlatitude temperature gradients. In terms of geographical pattern, the storm tracks in most CMIP3 models exhibit an equatorward bias in both hemispheres. For the seasonal cycle, most models can capture the equatorward migration and strengthening of the storm tracks during the cool season, but some models exhibit biases in the amplitude of the seasonal cycle. Possible implications of model biases in storm-track climatology have been investigated. For both hemispheres, models with weak storm tracks tend to have larger percentage changes in storm-track amplitudes over the seasonal cycle. Under global warming, for the NH, models with weak storm tracks tend to project larger percentage changes in storm-track amplitude whereas, for the SH, models with large equatorward biases in storm-track latitude tend to project larger poleward shifts. Preliminary results suggest that CMIP5 model projections also share these behaviors.

Comment on "Tropospheric temperature response to stratospheric ozone recovery in the 21st century" by Hu et al. (2011)

In a recent paper Hu et al. (2011) suggest that the recovery of stratospheric ozone during the first half of this century will significantly enhance free tropospheric and surface warming caused by the anthropogenic increase of greenhouse gases, with the effects being most pronounced in Northern Hemisphere middle and high latitudes. These surprising results are based on a multi-model analysis of CMIP3 model simulations with and without prescribed stratospheric ozone recovery. Hu et al. suggest that in order to properly quantify the tropospheric and surface temperature response to stratospheric ozone recovery, it is necessary to run coupled atmosphere-ocean climate models with stratospheric ozone chemistry. The results of such an experiment are presented here, using a state-of-the-art chemistry-climate model coupled to a three-dimensional ocean model. In contrast to Hu et al., we find a much smaller Northern Hemisphere tropospheric temperature response to ozone recovery, which is of opposite sign. We suggest that their result is an artifact of the incomplete removal of the large effect of greenhouse gas warming between the two different sets of models.