Formation of zonal jets by large-scale latent heating on the giant planets

The question of what causes the persistent east-west jets on Jupiter, Saturn, Uranus, and Neptune remains a major unsolved problem in planetary science. On Jupiter and Saturn, there exist ~20-30 jets, whereas Uranus and Neptune exhibit only ~3 jets each. Moreover, the equatorial jet is eastward on Jupiter and Saturn but westward on Uranus and Neptune. This observation presents a formidable puzzle that has proved difficult to explain. Existing models for jet formation generally do not predict the correct equatorial jet direction for all four planets and thereby fail to provide a single coherent explanation that encompasses both the gas giants (Jupiter/Saturn) and ice giants (Uranus/Neptune). Here, we test the hypothesis that large-scale latent heating associated with condensation of water vapor can pump the jets on the giant planets. Specifically, we investigate whether we can explain (i) the approximate number and speed of jets, and (ii) the direction of the equatorial jet on all four planets within the context of a single mechanism.

To do so, we present 3D numerical simulations using the MITgcm that include advection of water vapor. Condensation, and the associated latent heating, occurs whenever the relative humidity exceeds 100%. A slow cooling, representing radiation to space, is evenly applied throughout the model's upper troposphere. At the present time, we neglect particle microphysics and simply assume that all condensate is rained out, although we are working to include ammonia and water clouds in the simulations.

In our simulations, the circulation leads to spatially variable latent heating and the consequent development of localized eddies. These eddies induce accelerations that pump numerous zonal jets -- about 20 in our Jupiter-like cases and 3-5 in our Uranus/Neptune cases. Interestingly, the Jupiter-like cases develop strong equatorial superrotation. The Uranus/Neptune cases, however, show strong westward flow at the equator at most altitudes, with an eastward jet in the high-latitudes of each hemisphere. This provides a possible explanation for the existence of superrotation on Jupiter and Saturn and the lack of superrotation on Uranus and Neptune. More generally, the approximate agreement with the observed zonal-wind profiles supports the idea that latent heating plays an important role in generating the jets on giant planets.