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Numerical simulations of the shallow water equations on rotating spheresproduce mixtures of robust vortices and alternating zonal jets, as seen in theatmospheres of the gas giant planets. However, simulations that includeRayleigh friction invariably produce a sub-rotating (retrograde) equatorial jetfor Jovian parameter regimes, whilst observations of Jupiter show asuper-rotating (prograde) equatorial jet that has persisted over severaldecades. Super-rotating equatorial jets have recently been obtained in shallowwater simulations that include a Newtonian relaxation of perturbations to thelayer thickness to model radiative cooling to space, and in simulations of thethermal shallow water equations that include a similar relaxation term in theirtemperature equation. Simulations of global quasigeostrophic forms of thesedifferent models produce equatorial jets in the same directions as the parentmodels, suggesting that the mechanism responsible for setting the directionlies within quasigeostrophic theory. We provide such a mechanism by calculatingthe effective force acting on the thickness-weighted zonal mean flow due to thedecay of an equatorially trapped Rossby wave. Decay due to Newtonian coolingcreates an eastward zonal mean flow at the equator, consistent with theformation of a super-rotating equatorial jet, while decay due to Rayleighfriction leads to a westward zonal mean flow at the equator, consistent withthe formation of a sub-rotating equatorial jet. In both cases the meridionallyintegrated zonal mean of the absolute zonal momentum is westward, consistentwith the standard result that Rossby waves carry westward pseudomomentum, butthis does not preclude the zonal mean flow being eastward on and close to theequator.

Super- and sub-rotating equatorial jets: Newtonian cooling versus Rayleigh friction