Generally, a pronounced interannual variability can be seen both in the simulations and in ERA5. With increasing greenhouse gas concentrations, there is a tendency towards a more zonal flow in boreal summer and autumn while in winter and spring there is no robust change. This is consistent with the proposed tug-of-war (e.g. Barnes and Polvani, 2015; Blackport and Kushner, 2017; Chen et al. 2020): the upper tropospheric warming in the tropics leads to an increased meridional temperature gradient, stronger mean westerly flow and decreased waviness. In contrast, in boreal winter the effect of Arctic Amplification leads to a reduced meridional temperature gradient, weaker mean westerly flow and increased waviness offsetting the impact of upper tropospheric warming in the tropics. However, the impact on the waviness is very much under debate and shows very little robustness. Due to the lack of Arctic Amplification in boreal summer, the upper tropospheric warming in the tropics (Fig. 18a) may lead to a stronger zonal and less wavy flow. However, even in boreal summer differences are small compared to the strong interannual variability. Averaged over the year, the zonal mean zonal wind mainly increases in the stratosphere and only to some extent in the upper troposphere in the northern mid-latitudes while in the southern mid-latitudes zonal mean zonal wind increases are present throughout the troposphere, possibly due to the relative lack of Antarctic Amplification (Fig. 18a).
Fig. 19: Monthly sinuosity index (unity) in the Northern Hemisphere according to Cattiaux et al. (2016) from control, historical, scenario simulations, and from ERA5 reanalysis data (Copernicus Climate Change Service (C3S), 2017; Hersbach et al., 2020). Historical and scenario simulations are taken for the 30-year periods 1985–2014 and 2071–2100, respectively. From the control simulation all 30-year periods corresponding to the different ensemble members of historical and scenario simulations are considered resulting in multiple curves. The shaded areas represent the standard deviations of the 30 monthly sinuosity values for each simulation.

5.4 Ocean response

The Atlantic Meridional Overturning Circulation (AMOC) is an important element of the global ocean circulation. Transporting heat from the tropics to the northern North Atlantic, it has profound implications not only for the climate of north-western Europe but for the whole Northern Hemisphere. It is also associated with ocean heat transport from the South Atlantic to the tropics (Weijer et al., 2019). Figs. 20 and 21 show the maximum AMOC strength at 26°N for piControl, historical, scenario simulations, and RAPID observations (Smeed et al., 2019) as well as for piControl, 1pctCO2 and abrupt-4xCO2 simulations, respectively. For the 15-year record of the RAPID observations, our model agrees well both in terms of the mean value and in terms of the range of interannual variability with the observations. The historical simulation is indistinguishable from the control simulation, i.e. agrees within a standard deviation with the control simulation, even though other parameters such as the Arctic sea ice and near-surface temperature show substantial changes towards the end of the historical period. Furthermore, the development of the AMOC strength according to the weakest scenario SSP126 is indistinguishable from the control simulation until the end of this century. For the three other emission scenarios the signal starts to emerge from the noise later than 2050, i.e. values are continuously lower than the piControl value minus one standard deviation.
In the case of a transient increase of the greenhouse gas forcing (historical, scenario, and 1pctCO2 simulations), the AMOC strength at 26°N gradually decreases by around 20% until the end of the 21st century with the high emission scenario SSP585 and by around 25% within 150 years in the idealized 1pctCO2 simulation. In the abrupt-4xCO2 simulation, the maximum AMOC strength decreases markedly by around 30% over the first 20 to 30 years. Over the remaining 120–130 years of the simulation, it slightly increases again by about 5% and thus reaches values of about 13Sv, which amounts to about 75% of the original AMOC strength.