Fig. 18: (a) Zonal mean temperature response (°C), (b) zonal mean zonal
wind (m/s) response SSP370 scenario simulations (2071-2100, annual
means) compared to historical simulations (1985-2014) (shaded contours).
For both the scenario and the historical simulations the 5 member
ensemble means have been computed. The solid black lines represent
positive values from the historical simulations and the dashed black
lines negative values.
There is an ongoing discussion on how the waviness of the atmospheric
flow in mid-latitudes will change in the future as a result of changes
in the Arctic, through Arctic Amplification, and in the tropics, through
upper tropospheric warming. The contrasting driving from the Arctic vs
the tropics has been termed a tug of war in the mid-latitudes (e.g.
Barnes and Polvani, 2015; Blackport and Kushner, 2017; Chen et al.,
2020) Will there be a more zonal flow with a decrease in the intensity
of atmospheric waves implying less extreme warm and cold events or will
the meridionality of the flow get stronger implying more extreme warm
and cold events in the mid-latitudes or will there be no change? To
answer this question, various different objective indices have been
defined. Cattiaux et al. (2016) defined the sinuosity index (SI) as the
length of an isohypse of a specific value divided by the length of the
50°N latitude circle. If due to features such as cut-off lows there are
separated isohypses of the specific value, the sum of the lengths of
these isohypses is taken. The value of the isohypse is chosen as the
area average of z500 over 30 to 70°N to accomodate for seasonal
differences and climate change signals. If the SI equals to 1 the flow
is zonal since the chosen isohypse is a straight line. The higher the
SI, the stronger the meridional component of the atmospheric flow.
Fig. 19 shows the SIs computed for the piControl, historical, scenario
simulations, and the ERA5 reanalysis. Overall the differences between
the different simulations are smaller than differences between the model
and reanalysis data. In all simulations, the waviness of the flow is
more pronounced in boreal winter and spring compared to summer and
autumn. The annual cycle is shifted compared to the ERA5 reanalysis.
While the simulations show the maximum of waviness around February,
according to the ERA5 reanalysis it is around May. The minimum of
waviness occurs around August in the simulations and around October
according to the ERA5 reanalysis. While the amount of the maximum
waviness is well captured in the model compared to the reanalysis, the
minimum is too pronounced in the simulations indicating a too zonal flow
in late summer.