Fig. 16: Sea ice thickness in the historical simulation during the frozen season in the Arctic (averaged over December to March for each year) and the Antarctic (averaged over May to September for each year). The blue lines show the ensemble mean of the 5 members indicated by different colors. Satellite estimates for the Arctic from merged CryoSat-2 and SMOS data (CS2SMOS product) are shown by black squares.

5.3 Large scale circulation response

Similar to other climate models and as stated before, large scale circulation exhibits biases of the same order of magnitude as the simulated response to anthropogenic forcing affecting the reliability of the projections. Nevertheless, a few features are worth mentioning:
The mean sea level pressure (MSLP) response to increasing greenhouse gas concentrations (Fig. 17) is generally characterized by low anomalies over the polar regions and high anomalies in the southern mid-latitudes. Considering the geostrophic balance, this leads to an increase of the westerly flow in the northern and southern mid-latitudes mostly around 60° latitude. Over the Northern Hemisphere this increase is most pronounced in boreal autumn (SON) and winter (DJF). In the North Atlantic region the increase of the westerly flow is located further to the north compared to the CMIP5 ensemble mean as can be seen from Zappa and Shepherd (2017, their Fig. 1), while in the North Pacific region the location of the increase of the westerly flow is comparable. An intensified Aleutian low in boreal winter leads to a shift of the increased westerly flow over the North Pacific sector towards lower latitudes with a maximum around 45°N. Over the Southern Hemisphere the increased westerly flow is equally present in all seasons with a shift in the African sector towards lower latitudes in austral winter (JJA) and spring (SON).
(a) (b)