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)