[10]  NEBn:  The enigmatic encounter with white spot Z (Report,  2006 July 30)

Here is a preliminary report on that encounter of two anticyclonic white ovals (AWOs or 'portholes') in the NEBn. White spot Z (WSZ) is the brilliant, long-lived, rapidly-moving AWO  at 19 deg.N,  White spot Y was another one p. it, which at first was almost as big and bright.  The encounter did not go at all according to expectations!  In brief, they converged rapidly until 11 deg apart, but then almost stopped for 12 days.  Then, with both spots shrinking, WSY suddenly began to swing round the S side of WSZ (or vice versa -- June 27); but 2 days later it had emerged on the Sf. side of WSZ without any definite merger (June 29).  Thereafter, observations were scanty, but it appeared that the shrunken remnant of WSY moved on f. WSZ and disappeared, while WSZ itself slowly split into 2 halves!  By July 21 it seems to be almost back to normal.

Observations in more detail:

1)  Approach:  From Feb. to May, WSZ was moving at DL2 = -19 deg/mth, which I think must be its fastest speed ever in its 9-year history.  It then converged rapidly on WSY, which had rebounded from another AWO further p. in late May, and was almost stationary in L2.  From late May to early June, WSY and WSZ were converging at 0.7 deg/day. By June 9 they were centred only 11 deg. apart (less than the sum of their diameters) and a merger appeared imminent.  More background was given in the bulletin which I sent out on June 9.  But on that date, they almost stopped!  They were still 8.5—10 deg. apart in all the images on June 17-21.

2)  Encounter:  Then, on June 22, they started moving again: as Christophe Pellier reported, that evening they were clearly closer in longitude and WSZ was further N than WSY – just as expected if they were to start orbiting each other anticyclonically. This trajectory continued until June 27 when WSY was in close contact with the Sp. edge of WSZ. Unfortunately there were no images on June 28, the date of conjunction. But on June 29, WSY appeared to be on the Sf. edge of WSZ, and over the next few days it continued to move f., at least until July 2, possibly for several days longer.  Throughout this time (June 29 on) it was a tiny pale yellowish spot, much inferior to white spot Z (which was still bright, though now small).

Some observers took good images in the I-band (indeed European observers found this near-infrared waveband gave sharper images in poor seeing). During their approach (as the observers pointed out) the white spots seemed more elongated towards each other in I-band images.  But during their close encounter on June 27-July 1, the appearance seemed to be the same as in visible images.

3) Aftermath:  What happened next is tantalisingly unclear, partly because terrible weather afflicted Australia and the Philippines (3 typhoons!) for the next 3 weeks, and partly because the few hi-res images that were obtained were difficult to interpret.  It appears that WSZ began to split into E and W halves, starting on July 1 and gradually becoming more separate. On July 6 there was a third tiny white spot Np. WSZ (confirmed in 3 images by Daniel Chang). On July 11 WSZ appeared to be triple (confirmed in 3 images by Fabio Carvalho): I think the two southern spots were the two halves seen earlier, and the northern one correspond-ed to a light strip that has sometimes been seen previously, and again on July 21. [Two images on July 13, received late from Japan and Brazil, show WSZ was still irregular and changing then.]  By July 21, WSZ seemed to be fairly normal again although quite elongated.

Discussion: 

It is unfortunate that we could not follow the final scene in detail, but it is clear that the encounter of WSY and WSZ did not lead smoothly to a merger. Theory predicts that such AWOs will merge if there is no cyclonic circulation to keep them apart (Youssef & Marcus). And that is what was observed for AWOs in the S. Temperate region (2000) and S. S. Temperate region (2002), when the space between the two AWOs was less than the diameter of one of them (Rogers et al., in press in Icarus).

Here are some preliminary ideas about what happened with WSZ.  It may become clearer when the measurements have been completed by the JUPOS team.

1) Approach: Why did the AWOs suddenly stop converging when they were so close, and remained apart for 12 days? There was definitely no cyclonic circulation between them, as: (i) any cyclonic circulation in the NEB would be a visible dark barge, as seen in Voyager and Cassini maps; (ii) there were small diagonal rifts passing immediately S of the pair which were incompatible with a cyclonic circulation there – as confirmed by a blink pair of images 2 rotations apart on June 19-20.  It looks as WSY and WSZ hit a 'buffer zone' around each one which prevented them getting any closer.

2) Encounter:   The simplest interpretation is that WSY did not merge with WSZ: it squeezed around the S side of WSZ and survived for about a week, although it was smaller and its cloud-cap was reduced (dull yellowish instead of white), and then it disappeared.  Measurements of the latitudes on July 1 (from 3 images by Miyazaki, Akutsu, and Go) give 20.8 deg.N for WSZ and 19.7 deg.N for the remnant of WSY, all +/- 1 deg.  This confirms the visual impression that it was WSZ which moved N, rather than WSY which moved S!  And the remnant of WSY was still in the anticyclonic domain (N of the retrograding jet which is at 17.6 deg.N), so it was not a cyclonic eddy created in the interaction.

Perhaps AWOs in the NEBn, or WSZ in particular, behave differently from those in temperate domains.  Such white ovals in the NEBn have never previously been observed to merge – sometimes one like WSY has disappeared before reaching WSZ (though we have never previously watched two of them approach so close at such speed). There are several features of the wind patterns in the N. Tropical domain which differ from those in temperate domains, and might lead to different behaviour when AWOs interact:

a) The domain is not symmetrical, because of the long gradient of speed towards the great equatorial jet.

b) The vorticity may be comparatively low in the NEBn AWOs. These AWOs, unlike most temperate ones, do not have high cloud-caps to make them bright in methane images.  Legarreta & Sanchez-Lavega (2005) measured vorticities of anticyclonic ovals all over the planet, from Voyager and Galileo images, including one of these NEBn portholes in 1997.  Its maximum wind speed (38 +/- 11 m/s) and mean vorticity (4.2 +/- 1.2 x10^-5 /s) were among the lowest for any oval measured, similar to a NTBs jet spot, and much weaker than a NNTBs jet spot or the S.Temp. or S.S.Temp. AWOs.

c) WSY may have been weaker than WSZ (which is always bright and persistent), so perhaps the interaction was unequal, more resembling the interactions of SEBs jetstream spots with the GRS.  These sometimes hesitate p. the GRS (though often they do not).  And when they do swing round the RSH, the Voyager movies showed they sometimes pause at the f. end of the GRS, and sometimes split with a part persisting f. the GRS.  This behaviour is quite similar to the interaction of WSY with WSZ, but again it emphasises that mergers of anticyclonic ovals do not happen very easily.

On the one hand, we cannot rule out that some energy or material was transferred from WSY to WSZ: certainly WSY was fatally weakened by the encounter.

On the other hand, perhaps we should acknowledge the observational evidence that anticyclonic spots do not readily merge, and the events in the S. Temp. and S.S. Temp. domains are the exception rather than the rule!

3) Aftermath:  What happened to WSZ from July 1 to 11?  It looked as if it split into two, but there was no evidence for the components orbiting each other nor converging; and sometimes extra tiny bright components were present, at the limit of resolution.  Was a part of WSY really orbiting within WSZ? Or did dark streaks get mixed up in WSZ?  So far this is just another unsolved riddle to add to the list of the dynamics of this amazing storm.

Perhaps our best hope is for another AWO to collide with WSZ next year and repeat the event, so that we can follow it fully next time!