Meteorology Of Neptune General circulation and zonal structure

The atmosphere of Neptune is powered by extremely low energy fluxes. Internal heat energy flux is estimated to be 0.45 Wm-2 (Table 3.2) while the absorbed solar flux is estimated to be 0.27Wm-2, compared with values of 0Wm-2 and 205.5Wm-2, respectively, for the Earth. However, the cloud top zonal winds on Neptune are found to be very high with an extremely fast westward retrograde equatorial jet reaching speeds of 400 ms-1, gradually decreasing in the poleward direction and becoming eastward and prograde at latitudes poleward of 50°. It has been postulated that such high winds are allowed because the atmosphere of Neptune has low turbulence and thus low eddy viscosity. The general zonal wind structure is similar to that of Uranus, as we saw in Section 5.2.2, and why the equatorial jets of both planets should be blowing in the opposite direction to that of Jupiter and Saturn is unclear. In some ways it is easier to see how a retrograde equatorial jet is driven than a prograde, super-rotating jet. Air rising from deep levels at the equator, and initially rotating at the internal rotation rate would be expected to slow at higher levels (and thus greater distance from the center) due simply to conservation of angular momentum, giving rise to a westward-blowing air stream. Similarly, fast-flowing air at the equator which moves polewards would be expected to acquire additional eastward momentum via the same mechanism, giving prograde jets near the poles. Alternatively, air rising at midlatitudes and then traveling both polewards and equatorwards would give rise to a similar wind structure. This view is supported by Voyager 2 thermal-IR measurements (Conrath et al., 1989), which found that in the 0.3 bar to 1 bar pressure region, midlatitudes were K cooler than the equator and poles, which had roughly equal temperatures. This temperature difference was found to increase to ~5K in the 30 mbar to 120 mbar pressure region. While Voyager 2 found the equator and poles to have similar temperatures, VLT observations in 2006 (Orton et al., 2007c) found that at the 100 mbar level the temperature polewards of 70°S was significantly greater than the temperature at both the equator and midlatitudes. This observation is suggestive of Neptune possessing a warm cyclonic vortex at its South Pole in 2006, similar to the cyclonic vortices found at both poles in Saturn's atmosphere.

Thermal wind shears calculated from Voyager 2 thermal-IR measurements (Conrath et al., 1989) show that, like all the other giant planets, zonal winds decay with height. It is also of great interest, as we have seen, to determine how deep the zonal winds extend into the interior. This may be answered unequivocally for both Neptune and Uranus by determination of the J4 gravitational constant. If the zonal currents were superficial such that virtually all the planetary mass rotates with the same deep period, then calculations suggest that J4 should be negative and have a value close to that given in Table 2.4. If, however, the zonal currents are surface expressions of internal differentially rotating cylinders then J4 is predicted to be positive and have an absolute value roughly twice as big as that observed (Hubbard, 1997d). Hence, at least for the case of Uranus and Neptune, the zonal winds do appear to be restricted to the upper levels of the atmosphere, although the thermal wind equation suggests they must still be reasonably deep given the small latitudinal variation in temperature seen at pressures greater than approximately 1 bar.

Another curious feature of Neptune's atmosphere is that, while some banding is observed (more so than Uranus) the relationship between visible albedo and vorticity appeared from Voyager observations at some latitudes to be the opposite of that observed for Jupiter and Saturn with the anticyclonic, midlatitude regions appearing generally darker, and the equator and poles appearing generally bright (as was mentioned in Section 4.4.4). Unfortunately the flux at 5 ^m is too low to determine if these albedo variations are due to total cloud opacity changes or to some other effect. In addition to these general banded features, transitory white clouds are often observed and these clouds remain bright in the near-IR methane absorption bands, indicating high cloud tops. The most plausible explanation of these features is that they are convectively produced methane clouds, although how they are initiated is unclear since the zonal wind flow is everywhere apparently stable to baroclinic and barotropic instabilities according to the Charney-Stern criterion. These clouds are observed at a number of latitudes, but they usually appear at midlatitudes (i.e., in anticyclonic vorticity regions just as they do for all of the other giant planets). This interpretation was strengthened by Voyager 2 IRIS observations that the coolest tropopause temperatures, indicating divergence at the tropopause, and thus convection from below, are found at midlatitudes. The rapid overturning of Neptune's atmosphere is also indicated by the detection of disequilibrium species such as CO and the modeled possible presence of N2. The background banded appearance of Neptune is thus probably due to variations in either the depth or color of the lower H2S cloud and it would appear that this cloud deck responds counter-intuitively to the convective motion of the atmosphere above by appearing darker or being depressed to deeper levels where upper-level convection occurs. This picture is remarkably similar to the model proposed by Ingersoll et al. (2000) to account for the dynamics and cloud formation in Jupiter's atmosphere (Section 5.2.2 and Figure 5.6). Since the Voyager 2 flyby in 1989, Neptune has been observed with HST and from the ground. The disk-averaged visible albedo of Neptune has been seen to increase steadily since the 1950s (Section 4.4.4) and may possibly be correlated with the latitude of the subsolar point. This increase appears to be due, in part, to an increase in convective activity in the two main "cloud belts'' seen from 25°S to 60°S and 20°N to 40°N.

On an historical note, the high equatorial wind speeds on Neptune led to early confusion about its internal structure. As we discussed in Chapter 2, for a body in hydrostatic equilibrium rotating with a single period P, the oblateness, J2, and P are uniquely related. Prior to the arrival of Voyager 2 at Neptune in 1989, the rotational period was estimated from observation of the variation of Neptune's disk-averaged albedo caused by the transition of distinct cloud features which occur at midlatitudes and equatorial latitudes. Since these clouds lie in a strong retrograde zonal flow, the rotational period was estimated to be 18 hours which was found to be incompatible with the observed oblateness and J2. However, Voyager 2 measured the rotational period of the magnetic field (and thus presumably the interior) to be just over 16 hours, which is consistent with the other data.

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