Dynamics and Interaction between a Large-Scale Vortex and the Great Red Spot in Jupiter

Transcription

1 ICARUS 136, (1998) ARTICLE NO. IS Dynamics and Interaction between a Large-Scale Vortex and the Great Red Spot in Jupiter A. Sanchez-Lavega and R. Hueso Departamento Física Aplicada I, E.T.S. Ingenieros, Universidad del País Vasco, Bilbao, Spain wupsalaa@bi.ehu.es J. Lecacheux Department Recherches Spatiales, Observatoire Paris-Meudon, France F. Colas Bureau des Longitudes, Paris, France J. F. Rojas Departamento Física Aplicada I, E.U.I.T.I., Universidad del Pais Vasco, Bilbao, Spain J. M. Gomez Grup d Estudis Astronomics, Barcelona, Spain I. Miyazaki Okinawa, Japan and D. Parker Coral Gables, Florida Received November 24, 1997; revised April 2, 1998 tude (graphic) 21.5 (extremes 20.5 to 23.5 ); zonal A unique large-scale vortex, the White Tropical Oval velocity (relative to System III) 4 m/s (extremes 2 to 7 (WTrO), was first observed in the South Tropical Zone of Jupisouth) 5100 km. Its average zonal velocity showed a signifi- m/s); major axis (east west) 8100 km; minor axis (north ter, at the latitude of the Great Red Spot (GRS) in Its origin is probably related to a period of intense formation of cant departure relative to the ambient flow velocity of 35 eddies in the Southern edge of the South Equatorial Belt at ms 1. The tangential velocity along the southern flank of the vortex was 8 to 40 ms 1, giving an area averaged anticyclonic latitude 20. The WTrO survived many changes in the cloud vorticity s 1. This value is close to that of the structure of the South Equatorial Belt. However, in mid-may ambient flow indicating that the WTrO was a weak vortex. 1997, the WTrO was entrained by the GRS peripheral flow. Most of the time the WTrO showed a white oval form sur- Because of its large size, the WTrO did not circulate around rounded by a darker ring, although during some months in the GRS s collar, as other smaller eddies do, but instead, after 1993 the southern part turned redder, with a color similar to travelling one-quarter of the GRS ellipse it was expelled and that of the GRS. The relative spectral reflectivity from 230 nm finally destroyed when it became advected by the GRS s sur- to 2.3 m suggests that the WTrO had a cloud structure similar rounding zonal flow. The GRS responded to this interaction to other well-known jovian anticyclones Academic Press by exhibiting small latitude and longitude displacements ( 3 ). Key Words: Jupiter; atmosphere; dynamics; vortices; Great The main properties of the WTrO based on our prolonged Red Spot. imaging program (14 years) were the following: Average lati /98 $25.00 Copyright 1998 by Academic Press All rights of reproduction in any form reserved. 14

2 A JOVIAN TROPICAL VORTEX INTRODUCTION (Vasavada et al. 1997, Simon et al. 1997). However, the WTrO s life ended in a different way when it became The archetypes of large-scale and long-lived anticyclonic entrained and expelled by the GRS. Thus, it is important vortices in the jovian atmosphere are the Great Red Spot to characterize under which conditions merging (total or (GRS), observed probably since Cassini s time in 1665 partial), collision, and repulsion between anticyclones ocat latitude 22, and the three White Temperate Ovals cur. This point can be crucial to test models of giant planet (WOSs) named BC, DE, and FA that formed from 1938 vortices and their maintenance against dissipation (Ingerto 1940 at latitude 33 (see, e.g., Peek 1954, Rogers 1995). soll 1990). Most models of large-scale vortices are based on observa- In this work we present an exhaustive analysis of the tions of the most representative prototype, the GRS. Its WTrO history, morphology, and dynamical properties and long lifetime, huge size, and longitude isolation represent describe its final interaction with the GRS. It is based on a some of its characteristic properties for Earth-based ob- long-term, multi-wavelength continuous imaging program servers. The arrival of the Voyager spacecrafts in 1979 and with very good temporal sampling, spanning the 14-year 1980 permitted detailed measurements of its dynamical lifetime of the WTrO ( ). properties (Mitchell et al. 1981, Flasar et al. 1981, Sada et al. 1994) and resulted in new theories on its nature (Ingersoll and Cuong 1981, Williams and Yamagata 1984, 2. THE OBSERVATIONS Williams and Wilson 1988, Marcus 1993, Dowling and Ingersoll 1989, Achterberg and Ingersoll 1994, Nezlin and A long-term monitoring survey of changes in Jupiter s Sutyrin 1994, Williams 1996, 1997). cloud morphology has been performed by some of the The isolated character of the GRS in the South Tropical authors since the 1970s using images obtained with differ- Zone of Jupiter (i.e., the nonexistence of a companion at ent instruments and detectors. During this study of the its latitude) is one of its most notorious properties, and WTrO ( ) we employed the following techniques: thus most models try to reproduce this singularity (Marcus (1) Period: We used photographs ob- 1993). This isolation was in some way broken in the 1980s tained with the 1.23-m telescope at Calar Alto Observatory when a new, large-scale vortex about one third the size of (CAHA-MPIA, Spain), with the 1-m planetary dedicated the GRS (i.e., about the current size of the WOSs) formed telescope at Pic-du-Midi Observatory (France), and with at the GRS latitude (Sanchez-Lavega et al. 1994, Hueso a series of 40-cm telescopes in Japan, Spain, and the United et al. 1997). We will call this vortex the White Tropical States. We used broad-band filters with effective central Oval (WTrO). The historical records of jovian observa- wavelengths eff 450 nm (blue), eff 560 nm (yellow) tions during the past century reveal no previous reports and eff 650 nm (red). of a large and long-lived vortex in this region (Peek 1954, Rogers 1995). The WTrO was a rare, remarkable vortex (2) Period: We employed CCD images that merits a detailed study by itself. For instance it is taken with the 1-m Pic-du-Midi telescope (since 1987) and interesting to compare its origin, interactions, and dynamiwith the 40- to 60-cm telescopes in Japan, Spain, and the cal properties to those of the GRS because both lie in the United States (starting in 1993). The wavelength coverage same latitude and anticyclonic domain. Furthermore, the ranged from blue ( eff 400 nm) to near-infrared ( eff WTrO became important when it interacted with the GRS 900 nm), using a variety of broadband filters and narrow in May 1997, fourteen years after its formation. Until now, interference filters centered on the methane bands at 619, the only reported interactions between vortices and the 725, and 890 nm and on their adjacent continuums. During GRS were those of smaller-size anticyclonic eddies (one 1994 and 1995 we also obtained images in the 1 to 2.3 sixth GRS s scale) that moved with the westward jetstream micrometer spectral range using near-infrared cameras in the southern edge of the South Equatorial Belt (SEB) with the 1-m Pic-du-Midi and the 3.5-m Calar Alto Obser- (Sanchez-Lavega et al. 1996). These eddies penetrated and vatory telescopes. A list of filters, their central wavelengths, and their bandwidths appears in Table I. circulated around the GRS s periphery, entering its interior and merging with the GRS clouds (Smith et al. 1979a). (3) Period May August We used Hubble Space Although vortex merging between small anticyclones was Telescope archived images obtained with the Wide Fieldthe common result of such interactions (Mac Low and Planetary Camera during the Comet SL9 Jupiter collision Ingersoll 1986), there are also observations of collisions observing campaign (May August 1994). The wavelength and repulsion (backward motion) with no merging (Sato coverage spanned from the ultraviolet (336 nm) to the near 1974). This occurred most recently during the approach of infrared (953 nm), including the methane band filter at 890 the WOSs BC and DE, and later on of FA, resulting in nm (see Table I). These images were used to analyze the an alternating pattern of six cyclonic anticyclonic cloud structure and dynamics of the vortex at different wavelengths systems as observed in detail by the Galileo spacecraft and at high resolution.

3 16 SANCHEZ-LAVEGA ET AL. TABLE I Photometric Properties of the WTrO and Other Anticyclonic Vortices WTrO WTrO WTrO- GRS A-41 Filter Observatory ctr (nm) (nm) (1993) (1994) West ( ) (1994) 255W HST a W HST M HST Gunn 2 Pic b W HST B Pic Gunn 3 Pic M HST V Pic W HST cnt Pic CH 4 Pic R Pic Gunn 4 Pic N HST CH 4 Pic cont Pic cont Pic Gunn 5 Pic CH 4 Pic d 0.43 e QCH 4 N HST f CH 4 Calar Alto c /1.0 g Notes. Typical errors associated to these measurements are of the order of 10% except for the 2.29 micron images where errors are 20%. a The HST measurements are from May 18 and July 15, b The Pic data are from the following dates. (WTrO): November 25 27, 1988; November 10 and 30, 1989; January 8, 1993; July 20, 1994; and June 26 27, (GRS): November 25 and ; November 17 and 30, 1990; January 8 and June 16, 1993; July 20 and 21, 1994; April 14 and May 21, 1995; and June 26 27, c The Calar Alto dates are August 10 11, d 890-nm methane band contrast of the WTrO showed changes from 0.04 to 0.11; Pic data integrate the dark core. e 890-nm methane band contrast of the GRS showed changes from 0.16 to f HST WFPC2 high resolution images ( arcsec pixel 1 in WF mode and arcsec pixel 1 in PC mode) permitted measurement of the core. This value is for the bright area. The core contrast was g At 2.29 micrometers the GRS showed two areas of different brightness (extremes ) A total of about 2000 images were selected, processed, Voyager 1 and 2 in 1979 and 1980 (Smith et al. 1979a, and analyzed for this study. Position measurements were 1979b). This activity was characterized by the development performed directly on some positive copies of the photo- of series of small-scale anticyclonic eddies (zonal length graphs during the initial period ( ). However, 4000 km, separation between edges 8000 km) along the most of this work is based on the CCD images (1987 and southern edge of the South Equatorial Belt at a planetoafterward) that were used for spectrophotometric and posi- graphic latitude 20 (all latitudes in this paper are planettioning measurements employing the LAIA software de- ographic). At this latitude a westward jetstream resides in veloped for planetary analysis using a PC environment the visible cloud level, with an averaged zonal velocity u (Cano 1998). 55 ms 1 (Limaye 1986). These eddies, observed in visual wavelengths at ground-based resolution as dark 3. THE WHITE TROPICAL OVAL GENESIS AND spots, moved rapidly to encounter the GRS s eastern ex- MORPHOLOGY tremity, circulating and penetrating into its interior, or 3.1. Genesis being deflected backward if a South Tropical Disturbance was present (Smith et al. 1979a, 1979b). Between these Unfortunately we have very few photographs of the inipearance eddies a slightly higher albedo area, which had the aptial stages and formation of a WTrO. However the images of a bay in the SEB s dark belt, was noted suggest that the vortex formed in 1983 during a period of during April July 1983 and was observed from 1983 to1986 intense activity in the SEB similar to that observed by (Fig. 1). However, with the advent of CCD imaging, the

4 A JOVIAN TROPICAL VORTEX 17 However, we note that although the WTrO was similar in size and properties to the temperate WOSs, its formation responded to a different mechanism. The WOSs grew inside the band enclosed between the two opposed jets at 32.6 (u 21 ms 1 ) and 36.5 (u 31.6 ms 1 ), when darker clouds formed between both jets, breaking the band into three large and white separate sectors. These white areas shrunk independently and probably acquired the anticyclonic vorticity of the ambient flow when the opposing jets formed curved streamlines along the eastern and western extremities of each region, generating the closed circulation pattern (see Peek 1954 for details on WOS s origin). This mechanism most likely acts when a disturbance exists that occupies the whole latitude width of the band between the opposed jets. Perhaps the GRS formed in a similar way to the WOSs during a South Tropical Disturbance event (Sanchez-Lavega and Rodrigo 1985) which exhibits these types of curved flows (see also Rogers 1995). This genesis mechanism was not observed in the case of the WTrO and can be ruled out. FIG. 1. Early photographic images of the WTrO ( bay aspect ): (A) May 22, 1983; (B) August 7, South is up and West to the right in all the images Structure Figure 2 shows a selected set of ground-based images of the WTrO from 1983 to The morphology of the WTrO during its 14-year lifetime was dependent on the state of the SEB. During most of the time when the SEB was a dark belt, the WTrO s visual aspect ( nm) was that of an oval with a reflectivity slightly higher than that of the background South Tropical Zone (details on the spectral reflectivity are presented in Table I and Section 5). In the 890-nm methane absorption band, sensitive to cloud top altitudes, the WtrO was as bright as other jovian anticyclones (Fig. 2A). During periods of intense SEB ac- tivity (SEBD1 phase), as in late 1990, the core of the oval appeared fully surrounded by the darker SEB material (Fig. 2B). During the fading periods of the SEB (SEBF), the contrast decreased, and the oval s visibility was possible because of the presence of dark material along its periphery and western extremity, as in early 1990 (Fig. 2C). However, its aspect changed dramatically during 1993, following a fade of the SEB. During that time a red spot, much like the GRS (Fig. 2D), was observed in its position. It is not evident from our images whether this red spot contained the whole oval or represented only part of it, since a bay was visible in the SEB southern edge (denoting the pres- ence of circulation northward of the red spot). This change in coloration was similar to that occurring in the GRS during an SEB fade. Later on, in 1994, following the SEB Disturbance that started in April 1993 (Sanchez-Lavega et al. 1996), the WTrO became again the classical white oval. High resolution images obtained at Pic-du-Midi in Feb- ruary 1992, showed the vortex surrounded by a narrow feature was usually detected as a distinctive white spot, oval in shape. This difference in appearance was probably caused by the lower resolution and contrast provided by the photographs when compared with CCD images. The longitudinal location of both features (bay and spot) in 1986 and 1987 supports our view that they were the same object. In any case, we have no doubt of its identification as a unique object since Our photographs of the WTrO suggest two possibilities for its genesis and rules out a third one. One is that it formed as a single, independent structure in the southern flank of the westward jet at 20. For instance, Williams (1996) recently presented numerical models of vortex gen- eration from baroclinically unstable currents which could represent the case of the 20 westward jet. A second possibility is that the WTrO grew from the merging of the smaller-size eddies present in 1983 in the SEB southern edge (at 20 ). Numerical experiments by Ingersoll and Cuong (1981), Marcus (1988), Dowling and Ingersoll (1989), and Williams (1996) among others, have shown that coalescence of vortices can occur in the jovian shear flow environment under a large variety of models (shallow water and quasi-geostrophic).

5 18 SANCHEZ-LAVEGA ET AL. FIG. 2. Ground-based CCD images of Jupiter showing different aspects of the WTrO: (A) September 25, 1988 (890-nm methane band filter; SEB normal aspect); (B) November 30, 1990 (SEB disturbed, white oval aspect); (C) August 11, 1995 (1.7- m methane band filter; SEB normal aspect); (D) May 15, 1993 (WTrO shows a red spot aspect). dark ring ( the collar ), which contained small dark spots that, coming from the east, circulated anticyclonically around the WTrO. This aspect was confirmed by inspection of the higher resolution archived images obtained with the repaired WFPC2 of the HST in 1994 in a broad spectral range from UV to near infrared (Fig. 3). At this time the vortex center was at latitude , extending from 26.3 to Moreover, at 890 nm, a dark eye in the center of the vortex could be observed. Belt-like features (thin narrow belt segments) emerged from its east- ern and western extremities, similar to the patterns observed in the GRS. Table II summarizes our measurements of the dynamical characteristics of the WTrO Vorticity During June and July 1996, we detected some dark spots moving with the westward jet at 20, entering the collar of the WTrO. The spots circulated anticyclonically, and their tracking allowed us to determine the tangential velocity and estimate the vorticity of the WTrO. From June 26

6 A JOVIAN TROPICAL VORTEX 19 TABLE II Dynamical Properties of the White Tropical Oval Property Average latitude (graphic) Zonal velocity (relative to System III) Zonal velocity (relative to background flow, Limaye s 1986 profile) Tangential velocity Vorticity average (vortex) ambient planetary Lifetime Size ( 336 nm 953 nm): Outer ellipse (major axis) (minor axis) eccentricity Inner Ellipse (major axis) (minor axis) eccentricity Collar width Elliptical eye at 890 nm Value g 21.5 (extremes 20 to 23.5 ) u 4ms 1 (extremes 2 to 7ms 1 ) c 35 ms 1 (extremes 51 ms 1 at 20, 6 ms 1 at 23.3 ) V T (max) 40 ms 1 at 16 V T (min) 8ms 1 at s 1 u/ y s 1 f s 1 14 years (destroyed by collision with the GRS) 2a ( km) 2b ( km) a ( km) 2b ( km) e 0.5 (630 km) 2a ( km) 2b ( km) From the parabolic fit we calculate V T as a function of the position angle, obtaining extreme values of V T ( 16 ) 40 8ms 1 and V T ( 150 ) 8 8ms 1. In order to estimate the average vorticity of the WTrO we assume that it has the same tangential velocity in the southern and northern flanks. This occurs in the WOS (Mitchell et al. 1981) but not in the GRS, which has a lower tangential velocity in its northern perimeter. Under this assumption, the average vorticity is calculated as V T dl t1 V T (t)r(t) d Area t r 2 d dt dt s 1. This value compares well with the averaged vorticity of FIG. 3. Hubble Space Telescope images of the WTrO and surthe ambient flow u/ y s 1 eff 410 nm; (B) eff 890 as derived from rounding area taken on July 15, 1994: (A) nm (methane band filter). the Limaye s profile (the planetary vorticity, i.e., the Cori- olis parameter, at the center of the WTrO is f 2 sin ). to July 8, one feature was very well tracked in the southern half of the WTrO perimeter (Fig. 4). For the different 4. MOTIONS dates observed, we have calculated its polar coordinates, r(t) and (t), relative to the WTrO center, fitting the measurements to a parabolic curve as shown in Fig. 5. The the System II reference frame (angular velocity The detailed drift in longitude of the WTrO relative to tangential velocity along the southern half perimeter of degrees/day) during the period is shown in Fig. the oval is given simply by: 6. We selected System II because the GRS was nearly stationary in it during this period, so the relative motion between both vortices is clearly seen. The initial longitudi- V T (t) r(t) d dt (t). nal wandering of the WTrO in this System stabilized

7 20 SANCHEZ-LAVEGA ET AL. description of the SEB phenomenology). Globally, the following SEB phases were present during this period (details on the dates are given in Sanchez-Lavega et al. 1996): (1) SEB normal stage (dark belt); (2) SEBF (fading belt); (3) SEBD0 (outbreak of a Disturbance); (4) SEBF (fading belt); (5) SEBD0 (outbreak); (6) SEBD (end of the SEBD). The largest accelerations took place during the periods of changing albedo in the SEB. For instance, this occurred between early 1992 (initiation of an SEBF, point 4 in Fig. 7) and late 1993 following the SEBD outburst of FIG. 4. Pic-du-Midi Observatory images ( eff m) showing a small dark spot rotating anticyclonically around the periphery of the WTrO: (A) June 26, 1996; (B) June 30, FIG. 5. Polar coordinates (r, ) of different spots rotating anticyclon- ically around the WTrO. The line is a polynomial fit to the measurements and is used to calculate the WTrO vorticity. around 1987, initiating then a steady approach to the GRS. Both vortices finally interacted in mid-may Because of this wander in longitude, we have calculated the averaged velocity relative to the System III internal rotation period (angular velocity degrees/day) for some selected temporal intervals following the inflection points in System II shown in Fig. 6. This is presented in Fig. 7A together with the corresponding averaged latitudes in Fig. 7B. The major velocity changes are marked by a number and are related to the changes that occurred in the cloud structure of the South Equatorial Belt during this period (see Sanchez-Lavega and Gomez 1996, for a

8 A JOVIAN TROPICAL VORTEX 21 ences c u obs u Lim 6 and 51 ms 1 (average value c 35 ms 1 ). This means that the WTrO propagated to the east with respect to the mean flow. This eastward drift has also been observed in the GRS and WOS BC with values of 21.5 and 15 ms 1, respectively (Achterberg and Ingersoll 1994). The determination of the vortex drift relative to background flow is a very important parameter for constraint models of the vortices and of the flow beneath the observed cloud layer (Achterberg and Ingersoll 1994). For example our measurements contradict predictions of vortices moving westward (relative to the mean flow) at the maximum local Rossby long-wave speed: c L 2 R FIG. 6. Drift in System II Longitude reference frame of the WTrO and the GRS from 1983 to Year and month are indicated in particular points of the track. April 1993 (point 6 in Fig. 7; see Sanchez-Lavega et al. 1996). The eastward acceleration of the WTrO during this period was 5.3 ms 1 in 22 months. This acceleration was coincident with the epoch of predominant red color in the spot. These accelerations and decelerations of the WTrO might have been produced as a result of the momentum transferred to (or lost by) the vortex due to the dynamical mechanism involved in the SEB changes. An additional effect that could have contributed is a latitude migration of the WTrO and the corresponding different motion in the zonal shear flow. This can be appreciated in Fig. 8A, which shows the zonal velocity of the oval as a function of latitude (this is an enlargement of part of Fig. 8B). Although latitude errors are high, there is some tendency for the oval to move fast when close to the equator, in agreement with the ambient shear flow sign (see below). It is also important to note that the WTrO survived these dramatic changes in the SEB cloud structure. This is a signature of the oval s robustness and coherence. In Fig. 8B we have plotted the averaged velocity of the WTrO (u obs ) in the background zonal wind measured by Limaye in 1986 (u Lim ). We have also added our own measurements of the zonal wind velocity obtained by tracking FIG. 7. (A) Temporal changes in the averaged zonal velocity of small features during the period The the WTrO relative to System III; and (B) Temporal changes in the agreement between both sets of data for the background corresponding averaged latitude. The numbers in (A) indicate the phase flow is very good, confirming the well-known stability of of the SEB lifecycle in the corresponding epoch (see Sanchez-Lavega and Gomez 1996): (1) SEB normal stage (dark belt); (2) SEBF (fading the jovian zonal winds. belt); (3) SEBD0 (outbreak of the 1991 Disturbance); (4) SEBF (fading From Fig. 8B it is evident that the WTrO had a different belt); (5) SEBD0 (outbreak of the 1993 Disturbance); (6) SEBD (end velocity from that of the ambient flow, with extreme differ- of the disturbed phase).

9 22 SANCHEZ-LAVEGA ET AL. 5. SPECTRAL REFLECTIVITY As indicated above, during most of its lifetime the WTrO was seen at visual wavelengths as an oval brighter than the background clouds and surrounded by a dark ring. During 1993 the vortex became red, following an SEB fade that took place in mid-1992 (Sanchez-Lavega et al. 1996). To quantify the color changes, we have performed measurements of the spectral reflectivity of the central part and western area of the WTrO relative to a nearby patch in the South Tropical Zone. To make comparisons with other vortices, we have performed similar measurements on the center of the Great Red Spot and on one anticyclone at 41 (named A-41) relative to the STrZ and vortex background, respectively. These background areas were always the same for each feature and were selected for their homogeneous albedo and for their locations close enough to the vortices to prevent the effect of the geometrical dependence of the reflectivity. We have measured the mean data number (DN) of these features using a 2 arc-second diameter diaphragm on the Pic-du-Midi images and a 1 arc-second diaphragm on the HST images. The wavelength contrast is simply defined as C feat ( ) DN feat DN bkgnd DN bkgnd FIG. 8. (A) Zonal velocity of the WTrO plotted against its latitude location during its whole lifetime period; (B) Comparison of the WTrO zonal velocity with the averaged ambient flow (dark squares) measured by Limaye (1986), and by the authors on selected features during the period (circles). FIG. 9. Comparison of the spectral reflectivity contrast for different features relative to their backgrounds: The WTrO interior in 1993 ( red spot period) and 1994 (classical white spot ); the WTrO westward dark region in 1994; the GRS (average period), and an anti- ciclonic eddy at latitude 41 in 1994 (A-41). The characteristics of the filters employed are given in Table I. where L R is the local Rossby deformation radius, being L R (gh) 1/2 /f (g is the gravity acceleration and H the depth of the fluid). If the GRS, WOSs, and WTrO are some type of Rossby vortex (Williams 1985, Williams and Wilson 1988, Nezlin and Sutyrin 1994, Williams 1996), a forcing term should be added to the momentum equation to fit the observed eastward drift relative to background flow (Williams and Wilson 1988).

10 A JOVIAN TROPICAL VORTEX 23 and was computed for each available filter ( is the central WTrO had, like the GRS, a central elliptical area, an wavelength). Results for two different periods (1993 and eye, darker in the 890-nm band than the rest of the vortex. 1994) appear in Table I and in Fig. 9. The following conclu- Finally, the western area of the WTrO showed a low, nearly sions can be drawn from these data. First, during 1993 the neutral, spectral albedo, indicating a different cloud struc- spectral reflectivity of the WTrO ( red aspect ) was similar ture there. The fact that the 41 anticyclone (A-41) had to that of the GRS (period ). This red aspect higher contrast than the WTrO is due to the lower reflecti- was acquired during an SEBF stage, a phenomena that vity of the area surrounding the 41 feature than the also occurs with the GRS. Second, the WTrO normally had region surrounding the WTrO. a spectral reflectivity similar to other anticyclonic vortices (e.g., WOSs and spots at 41 ). Third, the reflectivity of 6. THE WTrO GRS INTERACTION the WTrO in the methane bands (890 nm, 1.7 m, 2.29 m) was always higher than its surroundings, similar to The steady approach of the WTrO to the GRS s eastern the GRS and the other anticyclones. However, its reflectivity extremity can be observed in Fig. 6. It ended in mid-may was lower relative to these other anticyclones. This 1997 when the WTrO entered the GRS peripheral flow. This was a singular event since no previous observation of such a large closed anticyclone interaction with the GRS had been reported in the historical records. In ground- means that its cloud tops were higher than the surrounding STrZ, but at a lower altitude (or having a lower optical depth) than those of the GRS and WOSs. Fourth, the FIG. 10. Ground-based images showing the final interaction between the WTrO and the GRS in 1997: (A) May 15 (the WTrO is entering the GRS outer periphery; (B) May 22 (WTrO inside the GRS); (C) June 3 (the WTrO clouds overflow the GRS northern extremity and are shed apart by the mid-seb eastward flow); (D) June 11 (the WTrO white clouds are dispersed zonally by this eastward flow).

11 24 SANCHEZ-LAVEGA ET AL. based photographs (Reese and Smith 1968) and in detailed Voyager images (e.g., Smith et al. 1979a), smaller, low albedo spots were observed to penetrate into and circulate within the GRS periphery. Figure 10 presents a time-lapse series of images showing the interaction between both vortices. The WTrO was entrained by the GRS flow around May 15, reaching the northern extremity of the GRS on May 22 (the centers of both features were then on the same meridian). Figures 11A and 11B show the latitude and longitude position of both features during the interaction period: a simultaneous change in the latitude of the WTrO s center and a strong acceleration in its longitude position can be seen. The latitude of the WTrO changed 2.5 between May 15 and 22. Its drift rate in longitude changed in System II from 0.16 /day (May 1 15) to 1.73 /day (May 15 22). In addition, the GRS responded to the interaction by showing a drift in its longitude position of 3.5 in System II and of 2 in latitude, both occurring in about 7 days. While the initial longitudinal position of the GRS recovered in early June, the GRS showed erratic changes in latitude during June and July. Most probably this was caused by a momentum and rotational energy transfer from the WTrO to the GRS. Although the quality of our images during this period was not very good, we can confirm that the WTrO as a whole did not circulate around the GRS perimeter. It seems that the main body of the clouds forming the WTrO stopped in the northern half of the GRS, after having gone one fourth of the circuit around the GRS perimeter. It is, however, possible that a small part of the WTrO clouds, undetectable in ground-based images, moved around the periphery. In any case, no changes were noted in the red color of the dark central ellipse of the GRS, suggesting that mixing did not occur. In early June (see Figs. 10C 10D) we observed the white material from the vortex overflowing the perimeter of the GRS at a latitude of These clouds were then advected eastward by the winds. Between May 24 and 29 the advection took place with a velocity u ms 1, but between May 29 and June 24 we measured u ms 1. These values are within the errors given in Limaye s profile at this latitude. The difference in velocity between both dates could be an effect produced by the evaporation or by mixing of the preceding edge of the white clouds with background clouds. It seems evident that the vortex was so large that the GRS could not entrain it completely, so the WTrO was expelled from the GRS. horizontal structure), and methane band reflectivity. The reflectivity differences between them can be explained by a smaller number density of particles (optical depth) and lower altitude of the upper clouds of the WTrO compared to the GRS and WOSs. This correspondence is also noted in their dynamical properties, except for the vorticity of the WTrO that seems to be lower than that of the GRS and WOSs. Apparently, the WTrO was a less energetic weak version of the WOSs and GRS. Another similarity is that the WTrO formed in a latitude very close to that of the GRS and, like it, survived the drastic changes in the 7. DISCUSSION AND CONCLUSION The WTrO closely resembled the other well-known large-scale and long-lived anticyclones (GRS and WOSs) FIG. 11. Interaction between the WTrO and the GRS as measured in their broadband color, cloud morphology (shape and by their positions in latitude (A) and System II longitude (B).

12 A JOVIAN TROPICAL VORTEX 25 their mutual size ratio L (large)/l (small) 6, as, for example, occurs between the 20 eddies and the GRS. Partial entrainment and expulsion of the mid-scale anticyclone by the larger one. This occurs for ratios 3, as, for example, between the WTrO and the GRS. Close approach with no merging, resulting in a cyclonic vortex (pre-existent or no) between both anticyclones. This occurs when the ratio is 1 and the ovals occupy the whole latitude domain between opposed jets, as occurred recently between WOSs DE and BC. ACKNOWLEDGMENTS This work is partially based on observations made with the NASA/ ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA Contract NAS The 3.5-m telescope in Calar Alto (Spain) is operated by the Max-Planck-Institut für Astronomie (Heidelberg, Germany) and by the Comisión Nacional de Astronomía (Spain). The Spanish team was supported by Universidad del Pais Vasco research grant EA 150/96 and the French team by the Programme National de Planetologie. Both teams collaborated during this project through Accion Integrada HF from the Ministere des Affaires Etrangères (France) and Ministerio de Educacion y Cultura (Spain). Note added in proof. During the 1998 Sun Jupiter conjunction, the White Ovals BC and DE interacted, changing into a single white oval (Lecacheux, J., P. Drossart, F. Colas, G. Orton, B. Fisher, A. Sanchez- Lavega, R. Hueso, and J. F. Rojas IAU Circ. No. 6942). The new oval BE resulting from the merge is about 20% larger than the former BC or DE. REFERENCES Achterberg, R. K., and A. P. Ingersoll Numerical simulation of FIG. 12. Conceptual scheme showing the observed pair interactions baroclinic jovian vortices. J. Atmos. Sci. 51, between major vortices in the jovian atmosphere: GRS and SEBs anticyclonic eddies (top), WTrO and the GRS (middle), and WOS BC and Cano, J. A L.A.I.A.: Laboratorio de Análisis de Imágenes Astro- DE (bottom). nómicas. Grup d Estudis Astronomics, Barcelona, Spain. Dowling, T. E., and A. P. Ingersoll Jupiter s Great Red Spot as a shallow water system. J. Atmos. Sci. 46, Flasar, F. M., B. J. Conrath, J. A. Pirraglia, P. C. Clark, R. G. French, cloud morphology and dynamics that took place in the and P. J. Gierasch Thermal structure and dynamics of the jovian South Equatorial Belt. Its genesis occurred during a period atmosphere. I. The Great Red Spot. J. Geophys. Res. 86, of intense activity (in the form of smaller-scale anticyclonic Hueso, R., A. Sanchez-Lavega, J, Lecacheux, F. Colas, J. M. Gomez, I. eddies) in the SEB southern edge, where an unstable west- Miyazaki, and D. Parker The history of the long-lived South Tropical Oval that interacted with the GRS in May Bull. Amer. ward jet resides. This origin differed from the observed Astron. Soc. 29, generation mechanism of the WOSs and most probably Ingersoll, A. P Atmospheric dynamics of the outer planets. Science that of the GRS. Its existence ended when it collided with 248, the GRS, being partially entrained by the GRS peripheral Ingersoll, A. P., and P. G. Cuong Numerical model of long-lived flow, and finally expelled from the GRS and destroyed jovian vortices. J. Atmos. Sci. 38, when its clouds became dispersed by the ambient flow. Limaye, S. S Jupiter: New estimates of the mean zonal flow at the Accordingly, we propose that anticyclonic interactions between vortices can be of at least three different types de- Mac Low, M. M., and A. P. Ingersoll Merging of vortices in the cloud level. Icarus 65, pending on their relative size (see Fig. 12): atmosphere of Jupiter: an analysis of Voyager images. Icarus 65, Entrainment, peripheral circulation, and merging of Marcus, P. S Numerical simulation of Jupiter s Great Red Spot. the small anticyclone by the larger one. This occurs when Nature 331,

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