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)