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B.M. Fisher, G.S. Orton, P. Yanamandra-Fisher, M. Ressler (JPL/Caltech), P. Fukamura-Sawata, W. Golisch, D. Greip (Inst. For Astronomy, IRTF), J. Spencer (Lowell Obs.), L. Stromovsky, F. Fry (U. Wisconsin)
Two examples of discrete cyclonic regions that were at least metastable were observed in 1998 and 1999. Each shows characteristics of a forced convergence and downwelling. One is the cyclonic region that appeared between two white ovals during their closest approach to one another in June of 1999. The other is the most recent example of small spots with unprecedented low albedo and high 5-\mum radiance.
In the first case, white ovals FA and ``BE'' converged toward one another starting late in the summer of 1998 at a rate near 0.09\circ/day. Then their relative motion stopped, and from early June 1999, their positions remained stationary. Their positions were detected using the L' filter (effective wavelength 3.78 \mum) in the NASA Infrared Telescope Facility (IRTF) near-infrared camera, NSFCAM. On June 30, 1999, and July 2, near their closest proximity, a third feature was observed between and slightly north of their centers. This feature appeared to have fewer particulates above the 1-bar level than the surrounding regions to the north and south; it also appeared \underbar{warmer} than its surroundings by +1.0 K, as opposed to the \underbar{cooler} temperatures of FA and ``BE'' (-1.3 K and -2.6 K, respectively).
The second feature is a small dark spot, also near 30\circS planetocentric latitude. This feature developed into a very low albedo spot and evolved into a dark ring that was bright at 5 \mum. The feature was observed to be warmer than its surroundings in kinetic temperature near the 200-mbar level, by about 1.7 K. It appears to be solitary in nature. The dark nature of the visible cloud material suggests that dark albedo clouds may be characteristic of the atmosphere at several bars pressure, although the color and albedo are different from 5-\mum hot spots.