Storms and vortices

The atmosphere of Jupiter contains numerous examples of large, long-lived ovals, of which almost 90% of are anticyclonic (as was discussed earlier in Section 5.3.3). Stable cyclones do exist, however, and the most prominent are the brown barges which appear at the northern edge of the NEB at 16°N (Figure 5.17). Brown barges were very prominent during the Voyager epoch, but have become less prominent during the Galileo and Cassini epochs. The strong cyclonic shear found at this latitude may help to stabilize the brown barges, which are observed to be dark at visible wavelengths, but bright at 5 ^m, suggesting that they are regions of reduced cloud cover and subsidence.

In addition to brown barges, features called "equatorial plumes'' appear at the southern edge of the NEB and appear (at visible wavelengths) as a sequence of bright regions, separated by darker, hook-shaped features, which extend from the NEB into the EZ. These dark features coincide with 5 ^m "hotspots", regions of very low total

Figure 5.17. Voyager I image of a brown barge on Jupiter. Courtesy of NASA.

cloud cover allowing thermal radiation from the 3 bar to 8 bar region to be observed at 5 ^m. Like brown barges, equatorial plumes were more prominent in the Voyager epoch than in more recent observations.

Analysis of cloud top wind vectors observed by Voyager (noted earlier) suggested that small-scale eddies, in general, pump energy into the mean zonal flow and large-scale eddies (Beebe et al., 1980; Ingersoll et al., 1981), and analysis of Cassini observations by Salyk et al. (2006) seems to confirm this scenario, although with 2-4 x smaller eddy-zonal kinetic energy conversion than previously estimated. Salyk et al. (2006) estimated that the power transfer between eddies and jets is 48% of the total thermal energy emitted by Jupiter.

The largest visible Jovian oval is the GRS, which is a huge anticyclonic vortex centered at 22.4° ± 0.5°S (planetographic) and has a constant latitudinal extent of 11° or 12,000 km, and a longitudinal extent of 17° or 20,000 km in 2002. Winds in the vortex rise to over 100 m s_1 in the outer annulus, but the center is found to be quiescent and also roughly 8K cooler than the surroundings at the cloud tops. Applying the thermal wind equation, this implies that the wind speed should decrease with depth into the atmosphere and thus that the GRS is probably only 200 km thick, which is tiny compared with its horizontal dimensions. Clouds in the center of the spot are found to be very thick and very high and also tilt slightly from the north to south and more subtly from east to west. The GRS thus resembles a "tilted pancake'' (Simon-Miller et al., 2002; West, 1999) and is quite unlike a terrestrial hurricane with which it is often compared (West, 1999), whose breadths are only 20-30 x their heights. The thick clouds and low upper-tropospheric temperatures imply upwelling in the center of the GRS, and at the edges, high 5 ^m emissions indicate low cloud opacity and thus subsidence, again unlike a hurricane where subsidence takes place in the central eye. The GRS appears to be very long-lived, although the visibility of the spot changes greatly with time and was particularly clear during the Pioneer encounters. However, at other times, such as during SEB disturbances the spot almost disappears. A large spot at the current GRS latitude was first observed in 1665 by Robert Hooke, and a year later by Jean-Dominique Cassini (Rogers, 1995; Simon-Miller et al., 2002). However, it is not clear whether the current GRS is actually the same spot since continuous observations can only be traced back to 1830, 120 years after the last sighting of Hooke's spot. Today's GRS is observed to be gradually shrinking in the longitudinal direction at a rate of 0.193° per year (Simon-Miller et al., 2002), which translates to approximately 4,000 km between the time of the Voyager and Cassini flybys. In addition, the winds in the collar have increased since the Voyager flyby. If the current rate of shrinking continues then by around the year 2040 the GRS will be perfectly circular. A circular aspect ratio is believed to be an unstable configuration for such a large anticyclone and hence it is possible that the GRS may actually disappear 30 years from now! Rogers (1995) has proposed that this may be what happened to Hooke's spot (which was reported to be roughly circular) in around 1700 and that the current GRS formed from a belt-wide disturbance (rather like the formation of the STBs white ovals in 1939) at about the same time and has been continuously shrinking in the longitudinal direction ever since. Its formation may even have been fed by its predecessor, Hooke's spot! The quiescent conditions at the center of the GRS may mean that air is trapped inside it for substantial periods of time, which may lead to the production of the characteristic red chromophore that gives the GRS its apparent reddish color. An alternative explanation is that the clouds seen in the center of the GRS have much higher cloud tops than anywhere else on the planet, suggesting vigorous convection. Hence, high levels of gases such as phosphine may be present whose photolysis may lead to the production of triclinic red phosphorous P4(s).

The STBs white ovals are the most prominent storm systems after the GRS and the current oval system first appeared in 1939 when a wavy disturbance appeared in the STB, although similar ovals had previously been seen at this latitude. This disturbance developed into six pinched regions that were labeled A to F which eventually coalesced into the three white ovals labeled BC, DE, FA sandwiched between the STZ and STB and which were observed during the Voyager flybys in

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