⦿ Barrado-Izagirre and colleagues studied a color changing anticyclone in Jupiter from 2012 to 2019.
⦿ They referred this anticyclone as NTrZ-Oval. It is the third largest and long lived oval after great red spot (GRS) and BA oval.
⦿ It’s tangential velocity and vorticity are below half the value of GRS and BA.
⦿ Despite being a weak vortex it has survived for years after mergers and disturbances.
The ubiquitous presence of vortices at cloud level is one of the most important characteristics of the meteorology of the giant planets along with the zonal winds organized in a multiple jet system. Jupiter is the most prolific planet in showing a great variety of closed circulation vortices. Vortices with sizes above 2,000 km are observed at all latitudes of the Jovian disk except very close to the Equator. Recent observations obtained by the Juno mission show also stable cyclones close to both poles. The vortices can be visually distinguished by their reflectivity contrast with respect to adjacent clouds and by their shape, showing an oval form that encircles a region of closed or nearly-closed vorticity. The surrounding cloud patterns make them appear as “bright” or “dark” ovals with sizes ranging from one hundred kilometers to 40,000 km, the maximum size measured at the end of the XIX century for the largest oval in Jupiter, the well-known Great Red Spot (GRS).
Vortices are classified according to their relative vorticity as cyclones or anticyclones. In the Northern Hemisphere, cyclones show anti-clockwise rotation while anticyclones rotate clockwise. Anticyclones appear in a great number and a variety of sizes in the anticyclonic domains of the Jovian zonal wind profile, being more stable than cyclones, except at polar latitudes where cyclones are the stable vortices. The most apparent and well known vortices, due to their longevity and size, are the Great Red Spot (GRS, planetographic mean latitude 22ºS) and oval BA (planetographic mean latitude 33ºS) (Figure 1), both anticyclones.
The essential properties to understand a vortex are its vorticity distribution and its relation with the environment flow shear. Other aspects that can be important in Jovian vortices are their color, possible changes in time, and interactions with other vortices, which may include mergers (constructive interactions) or destructive interactions with eddies. In the giant planets, where the mechanisms powering the zonal winds are mostly unknown, the interaction between vortices, eddies and jets are one of the mechanisms proposed to have an important role in forcing and maintaining the zonal jets.
The first vorticity measurements in Jupiter’s atmosphere were obtained for the GRS using ground-based photographic observations. These were later much improved with precise measurements from the Voyager 1 and 2 flybys including accurate measurements of the detailed flow field. The Voyagers also obtained precise measurements of the local vorticity for White Oval BC (at 33ºS) and for a cyclone “barge” at 16ºN. Images obtained by the Galileo obiter allowed to measure the wind field of the GRS and a few smaller vortices. Currently, HST images can also be used to retrieve the internal flow field of the largest vortices such as the GRS or oval BA and JunoCam has provided data with enough spatial resolution and temporal separation to measure the wind field of the GRS and polar vortices.
When two vortices of the same vorticity type closely interact they can merge (when they are of similar sizes), or if they have very different sizes the smaller one might get absorbed totally or partially. Vortex mergers occur in different areas of the planet with the best well-known example being the chain of large vortex mergers in 1998 and 2000 that resulted in oval BA. The new white oval BA turned red in August 2005 with a very similar shade to that of the GRS. Extensive dynamic studies did not find dynamical differences linked to color. According to radiative transfer analysis models, the color change resulted from the diffusion of a colored compound that interacted with the solar photons at the upper levels of the oval. Partial absorptions have been observed in Jupiter’s Great Red Spot interactions with large ovals.
Color changes are relatively common in Jupiter’s atmosphere. Color changes in the red coloration of tropical anticyclones have been described in Sánchez-Lavega et al. Their analysis of tropical vortices concluded that the vertical structure and dynamics of the anticyclones are not the causes of their coloration, and they propose that the red chromophore forms when the background material is stirred and exposed to ultraviolet radiation or mixed with other chemical compounds inside the vortex. In addition, planetary scale disturbances in the Jovian atmosphere can modify the zonal albedo pattern of the planet as for example in the cycle of the South Equatorial Belt (SEB) with convective Disturbances, large-scale color changes and Fades. The North Temperate Belt (NTB) also experiences this kind of event leading to the entire band becoming totally disturbed. The NTB Disturbances (NTBD) start with the outbreak of one or more convective plumes seen as bright spots moving with a velocity slightly faster than the zonal wind that interacts with the surrounding cloud patterns altering them and forming turbulence in their wake. The last NTBD developed in October 2016 with an outburst of four plumes that disturbed the entire latitudinal band and led to the formation of a very different reddish band.
Now, in the present work, Barrado–Izagirre and colleagues have followed the evolution of an anticyclone located in the boundary between the North Tropical Zone (NTrZ) and the North Equatorial Belt (NEB) at 19º N planetographic latitude (Figures 1 and 2). This oval has existed at least since 2008 but it is probably older than that, so it is one of the longest living ovals observed in Jupiter. The oval is interesting because of its large size and longevity (third after the GRS and oval BA), its color changes between white and red, and its interactions with close vortices including a major vortex merger in February 2013, and by the interaction with the NTBD in 2012, 2016 and 2020. They referred this anticyclone as NTrZ-Oval (NTrO).
In order to describe the historic evolution of the properties of NTrZ-Oval, astronomers used JunoCam and Hubble Space Telescope (HST) images to measure its size, obtaining a mean value of (10,500±1,000) x (5,800±600) km² and its internal rotation finding a value of –(2±1)·10¯5 s¯1 for its mean relative vorticity.
Furthermore, they used HST and PlanetCam-UPV/EHU multi-wavelength observations to characterize its color changes and Junocam images to unveil its detailed structure. The color and the altitudeopacity indices showed that the oval is higher and has redder clouds than its environment but has lower cloud tops than other large ovals like the GRS, and it is less red than the GRS and oval BA.
They also showed that in spite of the dramatic environmental changes suffered by the oval during all these years, its main characteristics are stable in time and therefore must be related with the atmospheric dynamics below the observable cloud decks.
Featured image: The White Tropical Oval and its environment from JunoCam observations on June 21, 2019. The image was originally projected in planetocentric coordinates. The inset shows zonal winds as measured during the Cassini flyby in black or in purple with only minor changes in this latitude range. © Barrado–Izagirre et al.
Reference: N. Barrado-Izagirre, J. Legarreta, A. Sánchez-Lavega, S. Pérez-Hoyos, R. Hueso, P. Iñurrigarro, J. F. Rojas, I. Mendikoa, I. Ordoñez-Etxeberria, the IOPW Team, “Jupiter’s third largest and longest-lived oval: Color changes and Dynamics”, Icarus, Volume 361, June 2021, 114394. https://doi.org/10.1016/j.icarus.2021.114394 https://www.sciencedirect.com/science/article/abs/pii/S001910352100083X
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