Aquaplanet simulations with winter and summer hemispheres: The setup and circulation response to warming
Authors: Schemm Sebastian , Röthlisberger Matthias 
Published: June 2023

Abstract. To support further understanding of circulation changes in a warming climate, an idealised aquaplanet model setup containing summer and winter hemispheres is presented and the results of circulation changes under warming are discussed. First, a setup is introduced that allows aquaplanet simulations with a warmer and a colder hemisphere, with realistic looking summer and winter jet streams, storm tracks and precipitation patterns that are similar as in observations, with a more intense and more equatorward storm track in the winter compared to the summer hemisphere. The sea surface temperature (SST) distribution used is inspired by the June-July-August zonal mean SST found in reanalysis data, and is flexible to allow control of the occurrence of a single or double Inter-Tropical Convergence Zone (ITCZ). The setup is then used to investigate circulation changes under uniform warming, motivated by recently discussed research questions. First, we show that jet stream waviness decreases under warming when compared on isentropes with maximum wind speed or on isentropes at similar height in pressure space. Jet stream waviness increases under warming when compared at similar-valued isentropes, but solely because the corresponding isentrope is closer to the surface in the warmer climate and waviness increases downward in the atmosphere. However, we also observe a waviness increase at isentropes at very high levels (e.g., 350 K) in the colder hemisphere, which does not appear to be due to a change in height. A detailed analysis of the changes in wave amplitude for different wave numbers confirms that the amplitude of large waves increases with warming, while that of short waves decreases. The reduction in wave amplitude of short synoptic waves is found to dominate in the jet core region, where jet waviness also decreases, and is more pronounced on the equatorward side of the jet. Long waves increase in amplitude on the poleward side of the jet and at upper stratospheric levels, consistent with increased jet waviness at these levels. The projected increased amplitude of planetary waves and the reduced amplitude of synoptic waves are thus clearly apparent in our aquaplanet simulations and thus do not require zonal asymmetries or regional warming patterns. During so-called high-amplitude wave events, there is no evidence for a preferential phase of Rossby waves of wavenumbers five or seven, indicating the crucial role of stationary waves forced by orography or land-sea contrast in setting previously reported occurrences of preferential phases. Finally, we confirm that feature-based block detection requires significant tuning to the warmer climate to avoid the occurrence of spurious trends. After adjustment for changes in tropopause height, the block detection used here shows no trend in the summer hemisphere and an increase in blocking in the colder hemisphere. We also confirm previous findings that the number of surface cyclones tends to decrease globally under warming and the cyclone lifetimes become shorter, except for very long-lived cyclones.