4.2 Atmospheric circulation
AWI-CM shows a too strong westerly flow above the Southern Ocean
especially in austral summer, indicated by too low mean sea level
pressure (MSLP) over the southern high latitudes and too high MSLP over
the southern mid-latitudes (Fig. 3). In the Euro-Atlantic sector there
is evidence for a southward shift of the jet stream, resulting in a too
strong westerly flow over Southern Europe and a too weak westerly flow
over Northern Europe in boreal winter and spring. This bias has been
found in numerous CMIP5 models (Zappa et al., 2013), and it can be
associated with an underestimation of Euro-Atlantic blocking (Jung et
al. 2012). Especially in boreal winter, the Aleutian low is too weak.
This feature was observed in previous ECHAM6 simulations as well
(Stevens et al., 2013). The MSLP biases are not negligible and amount to
up to 7 hPa. In the regions they occur, these biases are comparable to
the climate change signal indicating that the confidence in projections
of circulation changes is low.
The MSLP bias is dependent on the season as shown in Fig. 3 (a) to (d).
However, in the following we will also consider the annual mean sea
level pressure biases (Fig. 3 (e)) to make our results more comparable
with previous studies of CMIP6 models such as Müller et al. (2018) for
MPI-ESM (their Fig. 7d), using ECHAM6 as the atmosphere component like
AWI-CM. In the annual mean, biases are smaller than 1 hPa over large
areas of the tropics, subtropics, and southern mid-latitudes. . This is
consistent with results from Müller et al. (2018). However, differences
to Müller et al. (2018) exist over the South Atlantic gyre where AWI-CM
shows stronger high pressure biases (2-3 hPa) compared to MPI-ESM (1-2
hPa), and in the south-east Pacific north of West Antarctica where
AWI-CM shows negative biases of 1-2 hPa and MPI-ESM positive biases of
2-5 hPa. A thorough comparison between MPI-ESM and AWI-CM, which goes
beyond the scope of this study, is planned in collaboration with MPIM.
(a) (b)