Del Genio, A.D., Barbara, J.M., Ferrier, J., Ingersoll, A.P., West, R.A., Vasavada, A.R., Spitale, J., Porco, C. (2007). "Eddies, convection, and maintenance of the Saturn general circulation: Results from Cassini imaging." Eos Trans. AGU, 88(52) Fall Meet. Suppl., Abstract P23C-04..


Abstract
Cassini ISS imaging of Saturn's southern hemisphere during approach and in the months after orbit insertion have shown the mean zonal wind pattern to be relatively invariant with time outside the equatorial region. A major objective of the Cassini imaging investigation is to deduce the physical processes responsible for maintaining the momentum balance of the Saturn jets and to thereby constrain theories of jovian planet circulation. A related question is whether the mean meridional overturning circulation on Saturn more closely resembles Earth's tropical Hadley cell, with convective heating driving a thermally direct circulation, or its midlatitude Ferrel cell, with larger-scale eddies mechanically driving a thermally indirect overturning. Voyager images provided too small a sample at too low resolution to address this issue. For Cassini, we applied an automated cloud tracking algorithm to pairs of images over 4 Saturn rotations to estimate eddy fluxes of zonal momentum. The resulting dataset produces an order of magnitude more wind vectors than were the case for Voyager, with relatively unbiased sampling. The results show clear patterns of eddy momentum flux that are positively correlated with the latitudinal shear of the mean zonal wind, indicating that eddies are supplying the kinetic energy of the mean flow. We also detect moist convective storms by monitoring the evolution of anomalous bright features in continuum images, with supporting evidence of high cloud tops in weak methane band images when available. These show a tendency for convection to occur preferentially in cyclonic shear zones and not at all in anti-cyclonic shear regions. This implies that air rises in the cyclonic regions, drifts equatorward (poleward) across eastward (westward) jets, and sinks in the anti-cyclonic regions. The Coriolis force on the implied mean meridional flow decelerates (accelerates) these jets, potentially balancing the acceleration (deceleration) due to the eddy fluxes. This picture is consistent with recent Cassini and Galileo inferences about Jupiter and supports the Ferrel cell picture. Recent models that simulate baroclinic instability on jovian planets produce similar momentum balances. Latent heating due to moist convection in this picture may create the temperature contrasts that drive the baroclinic eddies that perform most of the eddy momentum transport. Recent images providing our first look at Saturn's northern hemisphere during the Cassini era show that the westward (eastward) jets at 34 (42) degrees North planetocentric latitude are still present, and that eddy fluxes are positively correlated with the mean zonal wind shear for this jet pair as well.