Off-equatorial Jets in Giant Planet Atmospheres

Jupiter and Saturn have similar radii, rotation rates, and atmospheres. Yet their off-equatorial jets differ markedly: Jupiter has 15--20 off-equatorial jets, with speeds at the level of the visible clouds around 20 m/s; Saturn has only 5--10 wider off-equatorial jets, with speeds around 100 m/s. Here it is shown that the differences between the off-equatorial jets can be accounted for by differences in the magnetohydrodynamic (MHD) drag the jets experience in the planetary interiors. The relation between jet characteristics and drag strength is examined systematically through simulations with a general circulation model (GCM). The GCM domain is a thin spherical shell in the upper atmosphere of a giant planet, with flow parameters relevant for Jupiter. Rayleigh drag at an artificial lower boundary (with mean pressure of 3~bar) is used as a simple representation of the MHD drag the flow on giant planets experiences at depth. The drag coefficient is varied to investigate how it affects characteristics of off-equatorial jets. As the drag coefficient decreases, the eddy length scale and eddy kinetic energy increase. Jets become wider and stronger, with increased interjet spacing. Coherent vortices also become more prevalent. Generally, the jet widths scale with the Rhines scale, which is of similar magnitude as the Rossby radius in the simulations. The jet strengths increases primarily through strengthening of the barotropic component, which increases as the drag coefficient decreases because the overall kinetic energy dissipation remains roughly constant: an increasing mean flow strength in the drag layer roughly balances a decreasing drag coefficient to lead to the same kinetic energy dissipation, which is dominated by the drag on the mean flow. The overall kinetic energy dissipation remains roughly constant presumably because it is controlled by baroclinic conversion of potential to kinetic energy in the upper troposphere, where differential solar heating has an influence; this baroclinic conversion is only weakly dependent on bottom drag and barotropic flow variations. For Jupiter and Saturn, these results imply that the wider and stronger jets on Saturn may arise because the MHD drag on Saturn is weaker than on Jupiter.