Linking zonal winds and gravity – II. Explaining the equatorially antisymmetric gravity moments of Jupiter

Abstract

ABSTRACT The recent gravity field measurements of Jupiter (Juno) and Saturn (Cassini) confirm the existence of deep zonal flows reaching to a depth of 5 per cent and 15 per cent of the respective radius. Relating the zonal wind-induced density perturbations to the gravity moments has become a major tool to characterize the interior dynamics of gas giants. Previous studies differ with respect to the assumptions made on how the wind velocity relates to density anomalies, on the functional form of its decay with depth, and on the continuity of antisymmetric winds across the equatorial plane. For the case of Jupiter, most of the suggested vertical structures exhibit a rather smooth radial decay of the zonal wind, which seems at odds with the observed secular variation of the magnetic field and the prevailing barotropy of the zonal winds. Moreover, the results relied on modifications of the surface zonal flows, an artificial equatorial regularization or ignored the equatorial discontinuity altogether. We favour an alternative structure, where the equatorially antisymmetric zonal wind in an equatorial latitude belt between ±21° remains so shallow that it does not contribute to the gravity signal. The winds at higher latitudes suffice to convincingly explain the measured gravity moments. Our results indicate that the winds are barotropic, i.e. constant along cylinders, in the outer $3000$ km and decay rapidly below. The preferred wind structure is 50 per cent deeper than previously thought, agrees with the measured odd gravity moments, is compliant with the requirement of an adiabatic atmosphere and unbiased by the treatment of the equatorial discontinuity. We discuss possible implications for the interpretation of the secular variation of Jupiter’s magnetic field.