Stratospheric and uppertropospheric circulation

Temperatures in the upper tropospheres of the giant planets may be estimated from thermal-infrared observations at approximately 18 ^m (550 cm-1), both by spacecraft and by ground-based observations. Zonal contrast is clearly seen, and applying the thermal wind equation the zonal wind structure is predicted to decay to zero at approximately 3-4 scale heights above the cloud tops. The source of the friction implied is probably due to eddy motions or gravity-wave breaking (as was mentioned earlier). The temperatures in the stratosphere at ~20mbar may be estimated from observations in the methane vibration-rotation band at 7.7 ^m (1,300 cm-1). A number of two-dimensional radiative-dynamical models have been constructed to estimate how the atmosphere responds to solar irradiation and thermal cooling to space. These models have been used to calculate the long-term meridional flow structures in the stratospheres of giant planets, which match the estimated stratospheric temperatures. Conrath et al. (1990) considered direct heating of the stratosphere through absorption of visible and near-infrared solar irradiation by methane gas alone and predicted that the residual mean circulation (or diabatic circulation) in the stratospheres of all giant planets had air rising near the subsolar latitude (where the solar flux is highest) and descending near the poles. A similar residual circulation is observed in the Earth's stratosphere and is known as "Brewer-Dobson circulation''. Air rising at the subsolar latitude means that Conrath et al.'s calculations are seasonally dependent for Saturn, Uranus, and Neptune, but less so for Jupiter whose obliquity is close to zero. West et al. (1992) challenged Conrath et al.'s findings for the case of Jupiter since the absorption of UV sunlight by stratospheric hazes near the pole had been neglected in Conrath's model. In West's model, air rose over the poles above the ~10mbar level and descended at the equator! However, at lower altitudes, air descended over both poles as in Conrath's model. Hence, air drifted equatorwards above the 10 mbar level and polewards below it. However, these models are highly dependent on the assumed gas and haze absorption coefficients, and a later study of the Jovian atmosphere by Moreno and Sedano (1997), based upon West's model, but with revised haze and methane absorption characteristics, has a meridional flow structure closer to Conrath et al.'s calculations. For Saturn, the role of UV-absorbing polar stratospheric hazes may also affect the calculations of Conrath et al., but for Uranus and Neptune, which do not have UV-absorbing polar stratospheric hazes, Conrath et al.'s model would seem to be reliable.

These residual mean calculations are useful in understanding mean stratospheric meridional flow, but they represent time averages over long periods and do not necessarily model how tracers are actually transported in the stratosphere. In par ticular, they neglect horizontal eddy diffusion processes that can transport material meridionally in much shorter time periods. This was well demonstrated by the collision of Comet Shoemaker-Levy 9 with Jupiter's atmosphere in 1994 (Figure 5.11, see color section). All the models of Jovian stratospheric circulation mentioned predict that air moves poleward between the lOOmbar and 10mbar pressure levels and thus the sooty debris of the impact deposited at these altitudes at 45° S was expected to drift towards the South Pole (Friedson et al., 1999). Instead, the debris (observed at 230 nm by HST) drifted towards the equator and had reached a latitude of 20°S by 1997. In addition, trace constituents introduced by the comet such as HCN and C02 were observed to cross the equator into the northern hemisphere in the four years after the Comet Shoemaker-Levy 9 impact (Lellouch et al., 2002; Moreno et al., 2003). Observations by Cassini CIRS in 2000 (Kunde et al., 2004) found that both C02 and HCN were more concentrated in the southern hemisphere than the northern hemisphere, but that the latitude of maximum HCN abundance was 45°S, while that of C02 was 60°S. The abundance of HCN was found to drop at the South Pole and also fall away polewards of 40°N.

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