Numerical dissipation strongly affects the equatorial jet speed in simulations of hot Jupiter atmospheres

Abstract

Hot Jupiters are tidally locked gaseous exoplanets with atmospheric circulations dominated by a super-rotating equatorial jet. Their global circulation is often studied with simulations in 3D general circulation models (GCMs). Energy builds up at the smallest scales in these models and must be dissipated. Many models use 'hyperdiffusion' for this, which applies a tendency to the prognostic variables based on a high-order Laplacian operator. This removes the unrealistic and unstable build-up of energy at small scales, and ideally does not affect the large-scale circulation. In this study, we show that hyperdiffusion can in fact affect the large-scale circulation of simulations of hot Jupiters. These planets have large velocity gradients, so hyperdiffusion can produce a momentum tendency that may affect the largest scales. We analyse four simulations with different hyperdiffusion parameters in the GCM THOR. These show that hyperdiffusion can affect the atmospheric zonal momentum budget as strongly as the physical forcing. The hyperdiffusion slows down and spreads out the jet, reducing its speed by more than 50% at some levels. We analyse simulations from the GCMs MITgcm and Exo-FMS and compare the effects of their different dissipation methods. The drag on the jet due to hyperdiffusion can be reduced by using a weaker hyperdiffusion coefficient, a higher resolution, or higher order diffusion. We aim to provide a basis for a study to investigate the 'real' momentum budget and jet speed of hot Jupiters. This study shows the need to examine long-held modelling assumptions when studying novel exoplanets.