Mechanisms of Superrotation in Slowly Rotating and Tidally Locked Planets

Abstract Superrotation is a common feature of quickly rotating gas giants (e.g., Jupiter), slowly rotating planetary bodies (e.g., Titan), and tidally locked planets. In this paper we compare and contrast the mechanisms of superrotation in slow rotators and tidally locked planets. We cover a wide range of planetary properties, in particular varying the thermal Rossby number Ro T (controlled by planetary size, rotation rate, and instellation) and a radiative relaxation timescale T rad (which parameterizes atmospheric optical thickness). We use a two-level primitive equation model that contains the principal mechanisms for superrotation in both regimes yet remains analytically tractable. Linearizations of the model elucidate the behavior of superrotation-inducing eddies. In tidally locked planets a baroclinic Matsuno–Gill-like structure arises in response to the zonal heating asymmetry but only produces superrotation when low-level drag is present. Nonlinear integrations further explore the superrotating regimes and exhibit significant time variability even in statistical equilibrium. Not all tidally locked regimes superrotate: subrotation arises at high T rad (optically thick atmospheres) and weak low-level drag. On axisymmetrically forced slow rotators, superrotation is ubiquitously linked to a previously identified Rossby–Kelvin instability. The instability itself is also linked to the spin-up of superrotation in some tidally locked regimes. Finally, we explore the continuous transition in the mechanisms of superrotation from axisymmetrically forced to tidally locked planets by applying a progressively stronger zonally asymmetric equatorial forcing. The Matsuno–Gill pattern quickly dominates over traveling planetary Rossby–Kelvin waves in forcing superrotation, although both mechanisms can coexist. These results provide a unified view of superrotation mechanisms across a wide range of planetary bodies.