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.