Dynamics and Interaction between a Large-Scale Vortex ... - AstroSurf

ICARUS

136, 14–26 (1998) IS986006

ARTICLE NO.

Dynamics and Interaction between a Large-Scale Vortex and the Great Red Spot in Jupiter A. Sanchez-Lavega and R. Hueso Departamento Fı´sica Aplicada I, E.T.S. Ingenieros, Universidad del Paı´s Vasco, Bilbao, Spain E-mail: [email protected]

J. Lecacheux Department Recherches Spatiales, Observatoire Paris-Meudon, France

F. Colas Bureau des Longitudes, Paris, France

J. F. Rojas Departamento Fı´sica 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) 5 221.58 (extremes 220.58 to 223.58); zonal velocity (relative to System III) 5 24 m/s (extremes 22 to 27 m/s); major axis (east–west) 5 8100 km; minor axis (north– south) 5 5100 km. Its average zonal velocity showed a significant departure relative to the ambient flow velocity of 235 ms21. The tangential velocity along the southern flank of the vortex was 8 to 40 ms21, giving an area 2 averaged anticyclonic vorticity 5 1.35 3 1025 s21. This value is close to that of the ambient flow indicating that the WTrO was a weak vortex. Most of the time the WTrO showed a ‘‘white’’ oval form surrounded by a darker ring, although during some months in 1993 the southern part turned redder, with a color similar to that of the GRS. The relative spectral reflectivity from 230 nm to 2.3 mm suggests that the WTrO had a cloud structure similar to other well-known jovian anticyclones.  1998 Academic Press Key Words: Jupiter; atmosphere; dynamics; vortices; Great Red Spot.

A unique large-scale vortex, the White Tropical Oval (WTrO), was first observed in the South Tropical Zone of Jupiter, at the latitude of the Great Red Spot (GRS) in 1983. Its origin is probably related to a period of intense formation of eddies in the Southern edge of the South Equatorial Belt at latitude 2208. The WTrO survived many changes in the cloud structure of the South Equatorial Belt. However, in mid-May 1997, the WTrO was entrained by the GRS peripheral flow. Because of its large size, the WTrO did not circulate around the GRS’s collar, as other smaller eddies do, but instead, after travelling one-quarter of the GRS ellipse it was expelled and finally destroyed when it became advected by the GRS’s surrounding zonal flow. The GRS responded to this interaction by exhibiting small latitude and longitude displacements (P38). The main properties of the WTrO based on our prolonged imaging program (14 years) were the following: Average lati14 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.

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1. INTRODUCTION

The archetypes of large-scale and long-lived anticyclonic vortices in the jovian atmosphere are the Great Red Spot (GRS), observed probably since Cassini’s time in 1665 at latitude 2228, and the three White Temperate Ovals (WOSs) named BC, DE, and FA that formed from 1938 to 1940 at latitude 2338 (see, e.g., Peek 1954, Rogers 1995). Most models of large-scale vortices are based on observations of the most representative prototype, the GRS. Its long lifetime, huge size, and longitude isolation represent some of its characteristic properties for Earth-based observers. The arrival of the Voyager spacecrafts in 1979 and 1980 permitted detailed measurements of its dynamical 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, Williams and Wilson 1988, Marcus 1993, Dowling and Ingersoll 1989, Achterberg and Ingersoll 1994, Nezlin and Sutyrin 1994, Williams 1996, 1997). The isolated character of the GRS in the South Tropical Zone of Jupiter (i.e., the nonexistence of a companion at its latitude) is one of its most notorious properties, and thus most models try to reproduce this singularity (Marcus 1993). This isolation was in some way broken in the 1980s when a new, large-scale vortex about one third the size of the GRS (i.e., about the current size of the WOSs) formed at the GRS latitude (Sanchez-Lavega et al. 1994, Hueso et al. 1997). We will call this vortex the White Tropical Oval (WTrO). The historical records of jovian observations during the past century reveal no previous reports of a large and long-lived vortex in this region (Peek 1954, Rogers 1995). The WTrO was a rare, remarkable vortex that merits a detailed study by itself. For instance it is interesting to compare its origin, interactions, and dynamical properties to those of the GRS because both lie in the same latitude and anticyclonic domain. Furthermore, the WTrO became important when it interacted with the GRS in May 1997, fourteen years after its formation. Until now, the only reported interactions between vortices and the GRS were those of smaller-size anticyclonic eddies (one sixth GRS’s scale) that moved with the westward jetstream in the southern edge of the South Equatorial Belt (SEB) (Sanchez-Lavega et al. 1996). These eddies penetrated and circulated around the GRS’s periphery, entering its interior and merging with the GRS clouds (Smith et al. 1979a). Although vortex merging between small anticyclones was the common result of such interactions (Mac Low and Ingersoll 1986), there are also observations of collisions and ‘‘repulsion’’ (backward motion) with no merging (Sato 1974). This occurred most recently during the approach of the WOSs BC and DE, and later on of FA, resulting in an alternating pattern of six cyclonic–anticyclonic cloud systems as observed in detail by the Galileo spacecraft

(Vasavada et al. 1997, Simon et al. 1997). However, the WTrO’s life ended in a different way when it became entrained and expelled by the GRS. Thus, it is important to characterize under which conditions merging (total or partial), collision, and repulsion between anticyclones occur. This point can be crucial to test models of giant planet vortices and their maintenance against dissipation (Ingersoll 1990). In this work we present an exhaustive analysis of the WTrO history, morphology, and dynamical properties and describe its final interaction with the GRS. It is based on a long-term, multi-wavelength continuous imaging program with very good temporal sampling, spanning the 14-year lifetime of the WTrO (1983–1997). 2. THE OBSERVATIONS

A long-term monitoring survey of changes in Jupiter’s cloud morphology has been performed by some of the authors since the 1970s using images obtained with different instruments and detectors. During this study of the WTrO (1983–1997) we employed the following techniques: (1) Period: 1983–1987. We used photographs obtained with the 1.23-m telescope at Calar Alto Observatory (CAHA-MPIA, Spain), with the 1-m planetary dedicated telescope at Pic-du-Midi Observatory (France), and with a series of 40-cm telescopes in Japan, Spain, and the United States. We used broad-band filters with effective central wavelengths leff p 450 nm (blue), leff p 560 nm (yellow) and leff p 650 nm (red). (2) Period: 1987–1997. We employed CCD images taken with the 1-m Pic-du-Midi telescope (since 1987) and with the 40- to 60-cm telescopes in Japan, Spain, and the United States (starting in 1993). The wavelength coverage ranged from blue (leff p 400 nm) to near-infrared (leff p 900 nm), using a variety of broadband filters and narrow interference filters centered on the methane bands at 619, 725, and 890 nm and on their adjacent continuums. During 1994 and 1995 we also obtained images in the 1 to 2.3 micrometer spectral range using near-infrared cameras with the 1-m Pic-du-Midi and the 3.5-m Calar Alto Observatory telescopes. A list of filters, their central wavelengths, and their bandwidths appears in Table I. (3) Period May–August 1994. We used Hubble Space Telescope archived images obtained with the Wide FieldPlanetary Camera during the Comet SL9–Jupiter collision observing campaign (May–August 1994). The wavelength coverage spanned from the ultraviolet (336 nm) to the near infrared (953 nm), including the methane band filter at 890 nm (see Table I). These images were used to analyze the structure and dynamics of the vortex at different wavelengths and at high resolution.

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TABLE I Photometric Properties of the WTrO and Other Anticyclonic Vortices Filter

Observatory

lctr (nm)

Dl (nm)

WTrO (1993)

WTrO (1994)

WTrOWest

GRS (1993–1996)

A-41 (1994)

255W 336W 410M Gunn 2 439W B Gunn 3 547M V 588W 600 cnt 619 CH4 R Gunn 4 673N 725 CH4 750 cont 830 cont Gunn 5 890 CH4 QCH4N 2.3 CH4

HSTa HST HST Picb HST Pic Pic HST Pic HST Pic Pic Pic Pic HST Pic Pic Pic Pic Pic HST Calar Altoc

261 332 409 430 435 480 500 548 580 588 600 619 650 660 673 725 750 830 840 890 888 2290

42 38 15 30 51 80 50 49 140 44 4 2 150 50 15 2 5 4.5 65 4.5 15 200

— — — — — 20.13 — — 20.11 — — — 20.09 — — — — — — — — —

20.049 0.145 0.093 0.08 0.082 — 0.05 0.003 — 0.023 0.035 0.04 — 0.03 0.023 0.05 0.06 0.03 20.01 0.07 d 0.018 f 0.29

— 20.148 20.089 — 20.12 — 20.13 20.14 — 20.11 — — — 20.07 20.11 — — 20.05 20.09 — 20.044 —

— — — — — 20.12 — — 20.03 — — — — — — 0.06 0.03 0.03 — 0.43e — 0.38/1.0 g

20.014 0.38 0.22 — 0.27 — — 0.22 — 0.13 — — — — 0.084 — — — — — 0.093 —

Notes. Typical errors associated to these measurements are of the order of 10% except for the 2.29 micron images where errors are P20%. a The HST measurements are from May 18 and July 15, 1994. 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, 1996. (GRS): November 25 and 27 1988; 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, 1996. c The Calar Alto dates are August 10–11, 1995. 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 0.53. f HST WFPC2 high resolution images (0.09953 arcsec pixel21 in WF mode and 0.0455 arcsec pixel21 in PC mode) permitted measurement of the core. This value is for the bright area. The core contrast was 20.013. g At 2.29 micrometers the GRS showed two areas of different brightness (extremes 0.38–1.00)

A total of about 2000 images were selected, processed, and analyzed for this study. Position measurements were performed directly on some positive copies of the photographs during the initial period (1983–1987). However, most of this work is based on the CCD images (1987 and afterward) that were used for spectrophotometric and positioning measurements employing the LAIA software developed for planetary analysis using a PC environment (Cano 1998). 3. THE WHITE TROPICAL OVAL GENESIS AND MORPHOLOGY

3.1. Genesis Unfortunately we have very few photographs of the initial stages and formation of a WTrO. However the images suggest that the vortex formed in 1983 during a period of intense activity in the SEB similar to that observed by

Voyager 1 and 2 in 1979 and 1980 (Smith et al. 1979a, 1979b). This activity was characterized by the development of series of small-scale anticyclonic eddies (zonal length p 4000 km, separation between edges P 8000 km) along the southern edge of the South Equatorial Belt at a planetographic latitude 2208 (all latitudes in this paper are planetographic). At this latitude a westward jetstream resides in the visible cloud level, with an averaged zonal velocity kul P 255 ms21 (Limaye 1986). These eddies, observed in visual wavelengths at ground-based resolution as dark spots, moved rapidly to encounter the GRS’s eastern extremity, circulating and penetrating into its interior, or being deflected backward if a South Tropical Disturbance was present (Smith et al. 1979a, 1979b). Between these eddies a slightly ‘‘higher albedo area,’’ which had the appearance of a ‘‘bay’’ in the SEB’s dark belt, was noted during April–July 1983 and was observed from 1983 to1986 (Fig. 1). However, with the advent of CCD imaging, the

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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 232.68 (u 5 221 ms21) and 236.58 (u 5 131.6 ms21), 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. 3.2. Structure

FIG. 1. Early photographic images of the WTrO (‘‘bay aspect’’): (A) May 22, 1983; (B) August 7, 1985. South is up and West to the right in all the images.

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 1987. 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 2208. For instance, Williams (1996) recently presented numerical models of vortex generation from baroclinically unstable currents which could represent the case of the 2208 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 2208). 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).

Figure 2 shows a selected set of ground-based images of the WTrO from 1983 to 1997. 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 (400–900 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 activity (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 presence 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 February 1992, showed the vortex surrounded by a narrow

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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-em 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 223.38 6 0.168, extending from 226.38 to 220.68. 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. 3.3. Vorticity During June and July 1996, we detected some dark spots moving with the westward jet at 2208, 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

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TABLE II Dynamical Properties of the White Tropical Oval Property

Value

wg 5 221.58 (extremes 2208 to 223.58) u 5 24 ms21 (extremes 22 to 27 ms21) kcl 5 235 ms21 (extremes 251 ms21 at 2208, 26 ms21 at 223.38) VT (max) 5 40 ms21 at u 5 168 VT (min) 5 8 ms21 at u 5 1508

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

kz l 5 1.35 3 1025 s21 ­u/­y 5 1.4 3 1025 s21 f 5 1.29 3 1024 s21 14 years (destroyed by collision with the GRS)

Size (l 5 336 nm 2 953 nm): —Outer ellipse (major axis) (minor axis) eccentricity —Inner Ellipse (major axis) (minor axis) eccentricity —Collar width —Elliptical ‘‘eye’’ at 890 nm

2a1 5 7.0 6 0.68 (8070 6 696 km) 2b1 5 4.1 6 0.38 (5109 6 375 km) « 5 0.774 2a2 5 6.0 6 0.68 (6910 6 696 km) 2b2 5 3.1 6 0.48 (3863 6 464 km) « 5 0.829 e 5 0.58 (630 km) 2a3 5 1.5 6 0.28 (1762 6 232 km) 2b3 5 0.9 6 0.28 (1084 6 224 km)

From the parabolic fit we calculate VT as a function of the position angle, obtaining extreme values of VT (u 5 168) 5 40 6 8 ms21 and VT (u 5 1508) 5 8 6 8 ms21. 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

RV k jl 5

T

dl

Area

E

t1

5

t0

VT (t)r(t)

E

u1 u0

FIG. 3. Hubble Space Telescope images of the WTrO and surrounding area taken on July 15, 1994: (A) leff 5 410 nm; (B) leff 5 890 nm (methane band filter).

to July 8, one feature was very well tracked in the southern half of the WTrO perimeter (Fig. 4). For the different dates observed, we have calculated its polar coordinates, r(t) and u(t), relative to the WTrO center, fitting the measurements to a parabolic curve as shown in Fig. 5. The tangential velocity along the southern half perimeter of the oval is given simply by: VT (t) 5 r(t)

du (t). dt

du dt dt

r 2 du

5 1.35 3 1025 6 0.3 s21.

This value compares well with the averaged vorticity of the ambient flow k­u/­yl 5 1.4 3 1025 s21 as derived from the Limaye’s profile (the planetary vorticity, i.e., the Coriolis parameter, at the center of the WTrO is f 5 2V sinw 5 1.29 3 1024). 4. MOTIONS

The detailed drift in longitude of the WTrO relative to the System II reference frame (angular velocity 5 870.270 degrees/day) during the period 1983–1997 is shown in Fig. 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 longitudinal wandering of the WTrO in this System stabilized

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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 (leff P 0.7 2 1 em) showing a small dark spot rotating anticyclonically around the periphery of the WTrO: (A) June 26, 1996; (B) June 30, 1996.

around 1987, initiating then a steady approach to the GRS. Both vortices finally interacted in mid-May 1997. Because of this wander in longitude, we have calculated the averaged velocity relative to the System III internal rotation period (angular velocity 5 870.5360 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

FIG. 5. Polar coordinates (r, u) of different spots rotating anticyclonically around the WTrO. The line is a polynomial fit to the measurements and is used to calculate the WTrO vorticity.

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ences c 5 uobs 2 uLim 5 6 and 51 ms21 (average value kcl 5 35 ms21). 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 ms21, 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: cb 5 2bL2R

FIG. 6. Drift in System II Longitude reference frame of the WTrO and the GRS from 1983 to 1997. 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 P5.3 ms21 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 (uobs) in the background zonal wind measured by Limaye in 1986 (uLim). We have also added our own measurements of the zonal wind velocity obtained by tracking small features during the period 1984–1993. The agreement between both sets of data for the background flow is very good, confirming the well-known stability of the jovian zonal winds. From Fig. 8B it is evident that the WTrO had a different velocity from that of the ambient flow, with extreme differ-

FIG. 7. (A) Temporal changes in the averaged zonal velocity of the WTrO relative to System III; and (B) Temporal changes in the corresponding averaged latitude. The numbers in (A) indicate the phase of the SEB lifecycle in the corresponding epoch (see Sanchez-Lavega and Gomez 1996): (1) SEB normal stage (dark belt); (2) SEBF (fading belt); (3) SEBD0 (outbreak of the 1991 Disturbance); (4) SEBF (fading belt); (5) SEBD0 (outbreak of the 1993 Disturbance); (6) SEBD (end of the disturbed phase).

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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 2418 (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 Cfeat (l) 5

DNfeat 2 DNbkgnd DNbkgnd

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 1983–1997 period (circles).

where LR is the local Rossby deformation radius, being LR 5 (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).

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 1993–1996 period), and an anticiclonic eddy at latitude 2418 in 1994 (A-41). The characteristics of the filters employed are given in Table I.

A JOVIAN TROPICAL VORTEX

and was computed for each available filter (l is the central wavelength). Results for two different periods (1993 and 1994) appear in Table I and in Fig. 9. The following conclusions can be drawn from these data. First, during 1993 the spectral reflectivity of the WTrO (‘‘red aspect’’) was similar to that of the GRS (period 1993–1996). This red aspect was acquired during an SEBF stage, a phenomena that also occurs with the GRS. Second, the WTrO normally had a spectral reflectivity similar to other anticyclonic vortices (e.g., WOSs and spots at 2418). Third, the reflectivity of the WTrO in the methane bands (890 nm, 1.7 em, 2.29 em) was always higher than its surroundings, similar to the GRS and the other anticyclones. However, its reflectivity was lower relative to these other anticyclones. This 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

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WTrO had, like the GRS, a central elliptical area, ‘‘an eye,’’ darker in the 890-nm band than the rest of the vortex. Finally, the western area of the WTrO showed a low, nearly neutral, spectral albedo, indicating a different cloud structure there. The fact that the 2418 anticyclone (A-41) had higher contrast than the WTrO is due to the lower reflectivity of the area surrounding the 2418 feature than the region surrounding the WTrO. 6. THE WTrO–GRS INTERACTION

The steady approach of the WTrO to the GRS’s eastern extremity can be observed in Fig. 6. It ended in mid-May 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-

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).

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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 2.58 between May 15 and 22. Its drift rate in longitude changed in System II from 10.168/day (May 1–15) to 11.738/day (May 15–22). In addition, the GRS responded to the interaction by showing a drift in its longitude position of 3.58 in System II and of 28 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 15.48 6 1.18. These clouds were then advected eastward by the winds. Between May 24 and 29 the advection took place with a velocity u 5 16.7 6 4 ms21, but between May 29 and June 24 we measured u 5 3.3 6 1.8 ms21. 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) in their broadband color, cloud morphology (shape and

FIG. 11. Interaction between the WTrO and the GRS as measured by their positions in latitude (A) and System II longitude (B).

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their mutual size ratio L (large)/L (small) $ 6, as, for example, occurs between the 2208 eddies and the GRS. • Partial entrainment and expulsion of the mid-scale anticyclone by the larger one. This occurs for ratios P3, 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 P1 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 5–26555. The 3.5-m telescope in Calar Alto (Spain) is operated by the Max-Planck-Institut fu¨r Astronomie (Heidelberg, Germany) and by the Comisio´n Nacional de Astronomı´a (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 HF1996–0070 from the Ministere des Affaires Etrange`res (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. SanchezLavega, R. Hueso, and J. F. Rojas 1998. IAU Circ. No. 6942). The new oval ‘‘BE’’ resulting from the merge is about 20% larger than the former BC or DE.

REFERENCES FIG. 12. Conceptual scheme showing the observed pair interactions between major vortices in the jovian atmosphere: GRS and SEBs anticyclonic eddies (top), WTrO and the GRS (middle), and WOS BC and DE (bottom).

cloud morphology and dynamics that took place in the South Equatorial Belt. Its genesis occurred during a period of intense activity (in the form of smaller-scale anticyclonic eddies) in the SEB southern edge, where an unstable westward jet resides. This origin differed from the observed generation mechanism of the WOSs and most probably that of the GRS. Its existence ended when it collided with the GRS, being partially entrained by the GRS peripheral flow, and finally expelled from the GRS and destroyed when its clouds became dispersed by the ambient flow. Accordingly, we propose that anticyclonic interactions between vortices can be of at least three different types depending on their relative size (see Fig. 12): • Entrainment, peripheral circulation, and merging of the small anticyclone by the larger one. This occurs when

Achterberg, R. K., and A. P. Ingersoll 1994. Numerical simulation of baroclinic jovian vortices. J. Atmos. Sci. 51, 541–562. Cano, J. A. 1998. L.A.I.A.: Laboratorio de Ana´lisis de Ima´genes Astrono´micas. Grup d’Estudis Astronomics, Barcelona, Spain. Dowling, T. E., and A. P. Ingersoll 1989. Jupiter’s Great Red Spot as a shallow water system. J. Atmos. Sci. 46, 3256–3278. Flasar, F. M., B. J. Conrath, J. A. Pirraglia, P. C. Clark, R. G. French, and P. J. Gierasch 1981. Thermal structure and dynamics of the jovian atmosphere. I. The Great Red Spot. J. Geophys. Res. 86, 8759–8767. Hueso, R., A. Sanchez-Lavega, J, Lecacheux, F. Colas, J. M. Gomez, I. Miyazaki, and D. Parker 1997. The history of the long-lived South Tropical Oval that interacted with the GRS in May 1997. Bull. Amer. Astron. Soc. 29, 1019. Ingersoll, A. P. 1990. Atmospheric dynamics of the outer planets. Science 248, 308–315. Ingersoll, A. P., and P. G. Cuong 1981. Numerical model of long-lived jovian vortices. J. Atmos. Sci. 38, 2067–2076. Limaye, S. S. 1986. Jupiter: New estimates of the mean zonal flow at the cloud level. Icarus 65, 335–352. Mac Low, M. M., and A. P. Ingersoll 1986. Merging of vortices in the atmosphere of Jupiter: an analysis of Voyager images. Icarus 65, 353–369. Marcus, P. S. 1988. Numerical simulation of Jupiter’s Great Red Spot. Nature 331, 693–696.

26

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Marcus, P. S. 1993. Jupiter’s Great Red Spot and other vortices. Annu. Rev. Astron. Astrophys. 31, 523–573. Mitchell, J. L., R. J. Terrile, B. A. Smith, J. P. Muller, and A. P. Ingersoll 1981. Flow fields within Jupiter’s Great Red Spot and Oval BC. J. Geophys. Res. 86, 8751–8757. Nezlin, M. V., and G. G. Sutyrin 1994. Problems of simulation of large, long-lived vortices in the atmospheres of the giant planets (Jupiter, Saturn, Neptune). Surv. Geophys. 15, 69–99. Peek, B. M. 1954. The Planet Jupiter. Faber and Faber, London. Reese, E. J., and B. A. Smith 1968. Evidence of vorticity in the Great Red Spot of Jupiter. Icarus 9, 474–486. Rogers, J. H. 1995. The Giant Planet Jupiter. Cambridge Univ. Press, Cambridge, U.K. Sada, P. V., R. F. Beebe, and B. J. Conrath 1996. Comparison of the structure and dynamics of Jupiter’s Great Red Spot between the Voyager 1 and 2 encounters. Icarus 119, 311–335. Sanchez-Lavega, A., and J. M. Gomez 1996. The South Equatorial Belt of Jupiter, I: Its life cycle. Icarus 121, 1–17. Sanchez-Lavega, A., and R. Rodrigo 1985. Ground-based observations of synoptic cloud systems in southern equatorial to temperate latitudes of Jupiter from 1975 to 1983. Astron. Astrophys. 148, 67–78. Sanchez-Lavega, A., J. M. Gomez, J. Lecacheux, F. Colas, I. Miyazaki, D. Parker, and J. Guarro 1996. The South Equatorial Belt of Jupiter, II: The onset and development of the 1993 disturbance. Icarus 121, 18–29. Sanchez-Lavega, A. , J. Lecacheux, F. Colas, D. Parker, I. Miyazaki, and

J. M. Gomez 1994. A new long-lived anticyclonic vortex in the South Tropical Zone of Jupiter. Bull. Amer. Astron. Soc. 26, 1102. Sato, T. 1974. Statistical establishment of the repulsive force between the long enduring white ovals in the South Temperate Zone of Jupiter. J. British Astron. Assoc. 84, 439–442. Simon, A. A., R. F. Beebe, P. J. Gierasch, and the Galileo SSI Team 1997. Global context of the Galileo-E6 observations of Jupiter’s White Ovals. Bull. Amer. Astron. Soc. 29, 1008–1009. Smith, B. A. and 21 colleagues 1979a. The Jupiter system through the eyes of Voyager 1. Science 204, 951–972. Smith, B. A. and 21 colleagues 1979b. The galilean satellites and Jupiter: Voyager 2 imaging science results. Science 206, 927–950. Vasavada A. R. and 13 colleagues (1998). Galileo imaging of Jupiter’s atmosphere: The Great Red Spot, equatorial region and White Ovals, Icarus 135, 265–275. Williams, G. P. 1985. Jovian and comparative atmospheric modeling. Adv. Geophys. A 28, 381–429. Wiiliams, G. P. 1996. Jovian dynamics. Part I: Vortex stability, structure and genesis. J. Atmos. Sci. 53, 2685–2734. Williams, G. P. 1997. Planetary vortices and Jupiter’s vertical structure. J. Geophys. Res. 102, 9303–9308. Williams, G. P., and R. J. Wilson 1988. The stability an genesis of Rossby vortices. J. Atmos. Sci. 45, 207–241. Williams, G. P., and T. Yamagata 1984. Geostrophic regimes, intermediate solitary vortices and jovian eddies. J. Atmos. Sci. 41, 453–478.