Influence of jet depth on the equatorial jet direction on giant planets

It remains puzzling why, despite their similar nature, Jupiter and Saturn possess a prograde equatorial jet, whereas Uranus and Neptune have a retrograde one. To understand this discrepancy, we use a 2-D quasi-geostrophic model to explore how the penetration depth of the jet, regulated by Ohmic dissipation, influences jet patterns. When jets penetrate deeply into the planet's interior, the Taylor column height decreases equatorward in the equatorial jet region, yielding a negative gradient of planetary potential vorticity (i.e., a negative beta effect). The tendency to eliminate such a gradient leads to a prograde equatorial jet, as observed on Jupiter and Saturn. In contrast, a relatively shallow system favors retrograde jets along the equator due to the positive beta effect of geometry. If the proposed mechanism applies to Uranus and Neptune, the observed jet structure may suggest the presence of a stratified layer or Ohmic dissipation layer near their surfaces.