A mechanism for super- and sub-rotation on the gas and ice giants

The giant planets exhibit a fundamental contrast in their equatorial circulation: Jupiter and Saturn are characterized by broad eastward equatorial jets, or superrotation, whereas Uranus and Neptune exhibit strong westward equatorial flow, or subrotation. This dichotomy has often been interpreted as evidence for distinct dynamical regimes on the gas and ice giants. However, here we show that these opposing equatorial states may instead arise from a common dynamical mechanism, with the direction of the equatorial jet determined by the way convection, rotation, and eddy momentum transport interact within the planetary interior and atmosphere.

Here we present a unified perspective on the formation of super- and sub-rotating equatorial jets across the giant planets. In deep rotating convection models, columnar convective motions can organize angular momentum in a way that produces either eastward or westward equatorial flow. The two outcomes emerge as separate equilibrated branches of the same dynamical system, implying that the transition from superrotation to subrotation may be understood as a bifurcation rather than as a change in the underlying forcing. Importantly, both regimes are maintained by comparable wave and eddy processes, suggesting that the gas and ice giants need not require fundamentally different explanations for their equatorial jets.

This picture connects naturally with idealized atmospheric 3D hydrodynamical simulations of the ice giants, which show that Uranus- and Neptune-like subrotation can be sustained when the modeled circulation extends sufficiently deep and allows eddy momentum fluxes to converge angular momentum away from the equator. Taken together, the two approaches suggest a continuous dynamical pathway linking the deep convective interiors of the gas giants with the deep atmospheric circulation of the ice giants. The observed contrast between superrotation and subrotation may therefore reflect different equilibrated states of a shared convective-eddy system, controlled by planetary parameters such as convective forcing, stratification, rotation rate, and effective dynamical depth.

This unified framework provides a basis for interpreting the diversity of zonal winds among the giant planets and offers testable predictions for future observations of Uranus and Neptune, where the depth and maintenance of the equatorial jets remain key open questions.