The Response of Extreme Jet Stream Waviness on Large-Scale Spatial Warming on an Aquaplanet

Key characteristics of anthropogenic climate change are polar amplification and upper tropospheric tropical warming. These large-scale spatial warming patterns alter the equator-to-pole temperature gradient in the lower and upper troposphere. The modified meridional temperature gradients affect the tropospheric jet streams. How the future jet streams will be affected is not fully understood. We perform four aquaplanet simulations with different sea surface temperature (SST) distributions to mimic large scale spatial warming patterns. Compared to the control run the SSTs of the SST4 simulations are increased with 4 K. In the Reduced Temperature Gradient (RTG) simulation the SSTs are gradually warmed from the equator with the maximum temperature increase of 5 K occurring poleward of 60° latitude. The Polar Amplification (PA) simulation uses the SST distribution of control between 45°S and 45°N, with SSTs set to 5 K poleward of these latitudes. We quantify the impact of these sea surface temperature distributions on jet stream strength, wave amplitudes and jet stream  waviness, quantified by a modified Sinuosity Index.

Large-scale spatial warming strengthens the jet stream by a uniform warming scenario SST4 and weakens the jet stream in the two scenarios RTG and PA where the meridional temperature gradient is reduced. However, all scenarios indicate substantial decreases in the magnitude of large wave amplitudes, extreme jet stream waviness and reduced variability of these diagnostics. Our results contradict the earlier proposed mechanism that low-level  polar warming weakens the jet stream and increases wave amplitudes and jet stream waviness. We conclude that a weaker jet stream does not become necessarily wavier.