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.