The zonal jets on the giant planets have been thought to be stable in time. A decline in the velocity of Saturn’s equatorial jet has been identified, on the basis of a comparison of cloud-tracking data across two decades, but the differences in cloud speeds have since been suggested to stem from changes in cloud altitude in combination with vertical wind shear, rather than from temporal changes in wind strength at a given height. Here, we combine observations of cloud tracks and of atmospheric temperatures taken by two instruments on the Cassini spacecraft to reveal a significant temporal variation in the strength of the high-altitude equatorial jet on Saturn. Specifically, we find that wind speeds at atmospheric pressure levels of 60 mbar, corresponding to Saturn’s tropopause, increased by about 20 m s^(−1) between 2004 and 2008, whereas the wind speed has been essentially constant over time in the southern equatorial troposphere. The observations further reveal that the equatorial jet intensified by about 60 m s^(−1) between 2005 and 2008 in the stratosphere, that is, at pressure levels of 1–5 mbar. Because the wind acceleration is weaker near the tropopause than higher up, in the stratosphere, we conclude that the semi-annual equatorial oscillation of Saturn’s middle atmosphere is also damped as it propagates downwards

Achterberg, Richard K.

Baines, Kevin H.

Conrath, Barney J.

Del Genio, Anthony D.

Ewald, Shawn P.

Fletcher, Leigh N.

Gierasch, Peter J.

Ingersoll, Andrew P.

Jiang, Xun

Li, Liming

Nixon, Conor A.

Orton, Glenn S.

Porco, Carolyn C.

Simon-Miller, Amy A.

Vasavada, Ashwin R.

West, Robert A.

The zonal jets on the giant planets are generally thought to be stable with time. Recently, there are still some debates about the general thought. Here, we report a significant temporal variation of the equatorial jet at high-altitude on Saturn. Long-term (2004-2009) observations by Cassini reveal that wind speed at the 60-mbar level increased from 270 m/s in 2004 to 290 m/s in 2008, while the wind speed has been mostly constant over time at the 500-mbar level in the southern equatorial region. The Cassini observations further reveal that the equatorial jet intensified approximately 60 m/s in the stratosphere (1-5 mbar) from 2005 to 2008. The fact that the wind acceleration is weaker at the 60-mbar level (approximately 20 m/s) than at the 1-mbar level (approximately 60 m/s) demonstrates that the equatorial oscillation is damped when it propagates downwards to the tropopause around 60 mbar. The direct measurement of the varying equatorial jet around the tropopause also serves as a key boundary condition when deriving the thermal wind fields in the stratosphere

DelGenio, Anthony D.

Jian, Xun

NASA Technical Reports Server

Equatoiral winds on Saturn and the Stratospheric Oscillation 
Liming Lil', Xun Jiang l, Andrew P. Ingerso1l2, Anthony D. Del Geni03, Carolyn C. Porco4, 
Robert A. West5, Ashwin R. Vasavada5, Shawn P. Ewald2, Barney J. Conrath6, Peter J. Gierasch6, 
Amy A. Simon-Miller7, Conor A. Nixon8, Richard K. Achterberg8, 
Glenn S. Orton5, Leigh N. Fletcher9, Kevin H. Baines5 
I Department 0/ Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA. 
2 Division of Geological and Planetary Sciences, Caltech, Pasadena, CA, USA. 
3 NASA Goddard Institute /01' Space Studies, New York, NY, USA. 
4 CICLOPS/Space Science Institute, Boulder, CO, USA. 
5 Jet Propulsion Laboratory, Caltech, Pasadena, CA, USA. 
6 Department 0/ Astronomy, Cornell University, Ithaca, NY, USA. 
7 NASA Goddard Space Flight Center, Greenbelt, MD, USA. 
8 Department 0/ Astronomy, University 0/ Maryland, College Park, MD, USA. 
9 Atmospheric, Oceanic and Planetary Physics, University ofOx/ord, Oxford, UK 
, To whom all correspondence should be addressed. E-mail: lli7(iiimail.uh.edu 
To be re-submitted to Nature-Geosciences, July 12, 2011 
1 
https://ntrs.nasa.gov/search.jsp?R=20110023014 2019-08-30T18:07:17+00:00Z
The zonal jets on the giant planets are generally thought to be stable with time l -). Recently, 
there are still some debates about the general thought4-5• Here, we report a significant 
temporal variation of the equatorial jet at high-altitude on Saturn. Long-term (2004-2009) 
observations by Cassini reveal that wind speed at the 60-mbar level increased from 270 mls 
in 2004 to 290 mls in 2008, while the wind speed has been mostly constant over time at the 
500-mbar level in the southern equatorial region. The Cassini observations further reveal 
that the equatorial jet intensified - 60 mls in the stratosphere (1-5 mbar) from 2005 to 
2008. The fact that the wind acceleration is weaker at the 60-mbar level (- 20 m/s) than at 
the I-mbar level (- 60 m/s) demonstrates that the equatorial oscillation6-7 is damped when it 
propagates downwards to the tropopause around 60 mbar. The direct measurement of the 
varying equatorial jet around the tropopause also serves as a key boundary condition when 
deriving the thermal wind fields in the stratosphere. 
Does the equatorial jet vary with time on the giant planets? A comparison4 of zonal winds 
between Voyager (l981-1982) and Hubble Space Telescope (l995-2002) observations suggested 
that velocity of Saturn's equatorial jet has decreased with time. Conversely, an analysis5 of 
observations by the Imaging Science Subsystem (ISS) on Cassini argued that the decrease of the 
equatorial jet discussed in the previous study) could instead be explained by changes in altitude 
of the tracked clouds within a zonal wind stlUcture with vertical shear. Studies8,9 based on data 
from the Composite Infrared Spectrometer (CIRS) on Cassini determine that vertical shear is 
present in the equatorial zonal winds. However, other studies I 0,11 suggest that the vertical shear is 
not strong enough to explain the wind decrease. The above debates reveal the need to further 
characterize the equatorial jet on Saturn with altitude and time. Such results also are critical for 
2 
discriminating between different theories of large-scale circulation on the giant planets because 
they provide some important constraints on stability that must be fulfilled. 
The equatorial thermal and dynamical structure of planetary atmospheres is also 
perturbed by quasi-periodic oscillations. A temperature oscillation with a period of 4-5 Earth 
years, known as the Quasi-Quadrennial Oscillation (QQO), was discovered in the equatorial 
stratosphere of Jupiter'2. A recent study'3 suggests that there may be a wind oscillation on Jupiter 
that is related to the temperature oscillation, similar to the Quasi-Biennial Oscillation (QBO) on 
Earth. Cassini observations4,5 recently showed a Semi-Annual Oscillation (SAO) of stratospheric 
temperature in the equatorial region of Saturn. However, we have little information about the 
temporal variation of equatorial stratospheric winds. Such observations would not only enrich 
our knowledge of giant-planet dynamics but also help us explore the full phenomenology of 
Saturn's SAO. 
Here we use long-term (2004-2009) observations from ISS and CIRS on board Cassini to 
examine the temporal variations of the equatorial jet in both the troposphere and stratosphere of 
Saturn. In order to obtain accurate measurements of zonal winds in the equatorial region, we 
select only the ISS image pairs with relatively long time separations and high spatial resolutions 
to decrease the uncertainty of wind measurements. In addition, the selected ISS images contain 
long segments of the planetary limb so that they can be navigated well. The selected images on 
two dates (May 10, 2004 and December 30, 2008) and the image processing steps are described 
in the Supplementary Information. Figure 1 shows examples of the processed maps at the strong 
methane filter (MT3) and the corresponding continuum filter (CB3). The MT3 observations are 
3 
restricted to the hydrocarbon hazes or other photolysis products in the upper atmosphere 11 ,14, 
which are - 60 mbar in the equatorial region of Saturn l5 , The CB3 observations are generally 
thought to represent ammonia clouds around 500 mbar, The map scale of Fig, I is 0, I o/pixel in 
latitude and longitude, which corresponds to a spatial resolution of - 105 km/pixel at the equator, 
The region very close to the equator (0-5°S) has low contrast due to the scattered sunlight from 
Saturn's rings. We identified few cloud features in this region, and exclude it from this study, 
After selecting the ISS images, we searched for simultaneous thermal observations in the 
CIRS data. We found CIRS observations on December 29, 2008, nearly simultaneous to the ISS 
observations on December 30, 2008, Unfortunately, we did not find CIRS observations 
corresponding to the ISS observations on May 10, 2004. The closest CIRS observations of the 
equatorial region were taken on April 8, 2005. Details of the selected CIRS observations and the 
procedure of temperature retrieval are also discussed in the Supplementary Information, 
Based on the ISS and CIRS observations, we apply three methods to measure the zonal 
winds in the equatorial region of Saturn. The first method is a manual feature tracking. By 
tracking cloud features that do not substantially alter their shape over time, we can convert their 
movements into the zonal winds, The derived winds are averaged in I ° latitude bins, The second 
method is similar to tbat of Limaye l6 , in which east-west strips of pixels are extracted from 
image pairs and shifted to find the maximal cross-correlation coefficient The third method 
applies tbe thermal wind equation to the CIRS temperature maps, as described in our previous 
stud/, We use winds measured from the ISS CB3 images to define the boundary level of the 
4 
thermal wind integration, in order to maintain approximate consistence in time between the 
boundary condition and the CIRS temperature. 
Wind measurements in Saturn's upper troposphere (i.e., -50-500 mbar) are shown in Fig. 
2. The uncertainty of wind measurements based on the ISS images comes from three sources: the 
standard deviation of multiple wind measurements, the optical characteristics of ISS cameras, 
and the accuracy of locating cloud/haze features in the ISS images. Uncertainty is discussed in 
the Supplementary Information. Panel A ofFig.2 shows that the two image-based methods give a 
consistent result fi·om the ISS CB3 images. The CB3 zonal wind near 500 mbar increased a few 
m/s from 2004 to 2008, which is within the uncertainty of the measurement. Measurements on 
the ISS MT3 images (panel B), which probe the relatively high hazes around 60-mbar, reveal an 
increase of - 20 m/s from 2004 to 2008 around [2°S, significantly larger than the uncertainty. 
We conclude that the wind speed at the level of the high-altitude hazes (- GO mbar) increased, 
while the wind speed at the level of ammonia clouds (- 500 mbar) remained constant (within 
uncertainty) from 2004 to 2008. 
To examine the temporal variation of zonal winds revealed by the ISS images, we further 
analyze the thermal winds in the equatorial region of Saturn. Panel B of Fig.2 shows the profile 
of thermally derived zonal winds at 60 mbar, which is based on Fig. S 1 in the Supplementary 
Information. The GO-mbar thermal winds also show an increase of - 20 m/s around 12°S between 
2005 and 2008. The increase is mainly due to the warming of equatorial region and the 
corresponding changes of the temperature gradient in the troposphere of Saturn (Fig. S 1). The 
uncertainty of thermal winds is closely related to the uncertainty of the retrieved temperature and 
5 
the corresponding meridional gradient, which is difficult to estimate quantitatively. The 
uncertainty of the CIRS retrieved temperature is - I K in the upper troposphere", smaller than 
the magnitude of temperature variation of - 2 K in the equatorial region shown in Fig. S 1. The 
retrieval process leaves some uncertainty in the magnitude of warming around the equator, but 
the warming trend from 2005 to 2008 is robust l7 The warming trend of atmospheric temperature 
intensifies the zonal winds via the thermal wind relationship, which explains the qualitative 
consistency of the high-altitude winds between the ISS measurements and the CIRS observations. 
On the other hand, the temperature of the relatively deep atmosphere around 500 mbar does not 
change significantly ,CFig. S I), which is probably due to the long time scale of radiative 
adjustment of the deep atmosphere. 
We also apply the thermal wind equation to the stratospheric temperature retrieved from 
the CIRS nadir observations (Fig. S2 in the Supplementary Information). Fig. S2 shows that the 
thermal winds in the stratosphere increased - 60 mls from 2005 to 2008, which confirms the 
previous results from the CIRS limb observations 18 The temporal variation of the equatorial jet 
in the stratosphere is related to the varying temperature, which has been observed previouslyI7-19. 
In these previous studies 18.19, the varying temperature and the related thermal winds are further 
linked to the vertical propagation of Saturn's equatorial oscillation. 
In this study, we report the temporal variation of equatorial jet speed at different altitudes 
in the troposphere and stratosphere of Saturn. We find that the wind speed of the equatorial jet 
around the tropopause (- 60 mbar) increased - 20 mls from 2004 to 2008. This is the first change 
of the equatorial jet around the tropopause to be observed at the time scale of a few Earth years. 
6 
At the time scale of one Saturn year (- 29.4 Earth years), inter-annual variations of cloud 
morphologlO and heat budget21 have been discovered on the giant planets. The season of Saturn 
will be the same in 2010-2011 as it was during the Voyager epoch (1980-1981). A detailed 
comparison of the wind field between the Cassini extended mission and the Voyager mission 
will shed light on the inter-annual variation of equatorial jet structure on Saturn. 
As well as enriching our knowledge of the equatorial jet and oscillation, the temporal 
variation of zonal winds has important implications for atmospheric dynamics on Saturn. The 
high-altitude equatorial jet is at 60-mbar, which is near the tropopause l7 . The direct measurement 
of zonal winds ncar the tropopause will benefit the exploration of thermal winds in stratosphere. 
Fouchet et a!. 4 and Guerlet et al. 18 integrated the thermal wind equation in Saturn's low-latitude 
stratosphere by assuming a null wind as the boundary condition around 20 mbar. Obviously, 
measured winds ncar the tropopause will be a better boundary condition for deriving the thermal 
winds in the stratosphere, even though the different boundaty conditions do not change the 
vertical structure of zonal winds discussed in the previous studies4,18 The weaker time variation 
of the equatorial jet at the 60-mbar (- 20 m/s) than at the 1-mbar (- 60 m/s) further implies a 
commonality in the vertical propagation of the equatorial oscillation between Saturn and Earth22 
Finally, the difference in behavior in the time variation of zonal winds between the troposphere 
(e.g., 500-mbar) and the stratosphere (e.g., I-mbar) suggests that the driving mechanisms of 
zonal winds are probably different between the stratosphere, where the radiative effects are 
important and the troposphere where the convection and turbulence are important. 
7 
References 
I. Irwin, P. G. 1. in Giant planets olour solar .Iystem, 141-142 (Praxis Publishing, 2003). 
2. Cho, 1. Y., Polvani, L. M., The morphogenesis of bands and zonal winds in the atmospheres 
on the giant outer planets. Science 19, 335-337 (1996). 
3. Galperin, B., Sukoriansky, S., Huang, H. Universal n-5 spectrum of zonal flows on giant 
planets. Physics of Fluids 13, 1545-1549 (200 I). 
4. Sanchez-Lavega, A., Perez-Hoyos, S., Rojas, 1. F. & French, R. G. A strong decrease in 
Saturn's equatorial jet at cloud level. Nature 423, 623-625 (2003). 
5. Porco, C. C. et 01. Cassini imaging science: Initial results on Saturn's atmosphere. Science 
307,1243-1247 (2005). 
6. Fouchet, T., Guerlet, S., Strobel, D. F., Simon-Miller, A. A., Bczard, B. & Flasar, F. M. An 
equatorial oscillation in Saturn's middle atmosphere. Nature 453, 200-202 (2008). 
7. Orton, G. S. et 01. Semi-annual oscillations in Saturn's low-latitude stratospheric temperatures. 
Na/ure 453, 196-199 (200S). 
S. Flasar, F. M. et 01. Temperatures, winds, and composition in the Saturnian system. Science 
307,1247-1251 (2005). 
9. Li, L., Gierasch, P. 1., Achterberg, R. K., Conrath, B. 1., Flasar, F. M., Vasavada, A. R., 
Ingersoll, A. P., Banfield, D., Simon-Miller, A. A. & Fletcher, L. N. Strong jet and a new 
thermal wave in Saturn's equatorial stratosphere. Geophys. Res. Lett. 35, L2320S (200S). 
10. Perez-Hoyos, S. & Sanehez-Lavega, A. On the vertical wind shear of Saturn's equatorial jet 
at cloud level. Icarus 180,161-175 (2006). 
8 
II. Sanchez-Lavega, A., Hueso, R. & Perez-Hoyos, S. The three-dimensional structure of 
Saturn's equatorial jet at cloud level. Icarus 187, 510-519 (2007). 
12. Leovy, C. B., Friedson, AJ. & Orton, G. S. The quasi-quadrennial oscillation of Jupiter's 
equatorial atmosphere. Na/ure 354,380-382 (1991). 
[3. Simon-Miller, A. A. & Gierasch, P. l On the Long-Term Variability of Jupiter's Winds and 
Brightness as Observed from Hubble. !cams 210, 258-269 (2010). 
14. GarciaL.Melendo, E., Sanchez'Lavega, A., Rojas, l F., PereziHoyos, S. & Hueso, R. 
Vertical shears in Saturn's eastward jets at cloud level. Icarus 201, 818-820 (2009). 
15. Garcia-Melendo, E., Sanchez-Lavega, A., Leg31Teta, l, Perez-Hoyos, S. & Hueso, R. A 
strong high altitude narrow jet detected at Saturn's equator. Geophys. Res. Lell. 37, L22204 
(2010). 
16. Limaye, S. S. Jupiter: new estimates of the mean zonal flow at the cloud level. Icarus 65, 
335-352 (1986). 
]7. Fletcher, L. N., Achterberg, R. K., Greathouse, T. K., Orton, G. S., Conrath, B. 1., Simon-
Miller, A. A., Guerlet, S., Irwin, P. G. 1. & Flasar, F. M. Seasonal Change on Saturn from 
Cassini/CIRS Observations, 2004-2009. IcG/'us 208, 337-352 (2010). 
18 Guerlet, S., Fouchet, T., Bezard, B., Flasar, F. M., Simon-Miller, A. A. Evolution of the 
equatorial oscillation in Saturn's atmosphere between 2005 and 2010 from Cassini/CIRS limb 
data analysis. Geophys. Res. Lell. 38, LOnOI (2011). 
19 Schinder, P. 1., Flasar, F. M., Marouf, E. A., French, R. G., McGhee, C. A., Kliore, A. 1., 
Rappaport, N. 1., Barbinis, E., Fleischman, D., Anabtawi, A. Saturn's equatorial oscillation: 
evidence of descending thermal structure from Cassini radio occultations. Geophys. Res. Lell. 
38, L08205 (2011). 
9 
20, Baines, K. 1-1, e/ ai, Polar lightning and deeadallscale cloud variability on Jupiter. Science 
318,226-229 (2007), 
21. Li, L., Conrath, B" Gieraseh, p" Achterberg, R" Nixon, C, Simon-Miller, A, A" Flasar, F, 
M" Banfield, 0" Baines, K" West, R" Vasavada, A" Mamoutkine, A" Segura, M" Bjoraker, 
G" Orton, G" Fletcher, L., Irwin, p, & Read, p, Emitted power of Saturn, 1. Geophys, Res, 
115, EI1002 (2010), 
22, Baldwin, M, p" e/ ai, The Quasi-biennial oscillation, Reviews of Geophysics 39, 179-229 
(2001), 
10 
Correspondence 
To whom all correspondence and requests for materials should be addressed to Dr. Li with E-
mail: lli7@mail.uh.edu 
Aclmowledgements 
NASA Cassini Data Analysis Program (CDAP) funded this work. We acknowledge Enrique 
Garcia-Melendo, Ricardo Hueso, Agustin Sanchez-Lavega, and Santigo Perez-Hoyos for 
providing valuable comments and constructive discussions. We are also grateful for valuable 
comments and suggestions on this work from two anonymous reviewers. 
Author contributions 
L. L. measured the ISS winds (correlated), computed the thermal winds, and conceived the 
overall research. X. 1. carried the ISS wind measurements (cloud-tracking). X. 1., A. P. I., A. D. 
D. G., C. C. P., R. A. W., A. R. V., S. P. E., B. 1. c., P. J. G., A. A. S-M, C. A. N., R. K. A., G. S. 
0., L. N. F., and K. H. B. provides assistance in interpreting the observational results. 
1 1 
Figure Captions 
Figure I. Southern equatorial maps based on ISS multi-filter images taken in May 10, 2004. The 
spatial resolution of the cylindrical maps is - 105 km/pixel (please refer to Table S I in the 
Supplementary Information). (A) MT3 map at time I (tl). (B) MT3 map at time 2 (t2), which is 
taken by ISS in 9.5 hours later than tl. (C) and (D) are the same as (A) and (B) except for the ISS 
images taken at filter CB3. 
Figure 2. Saturn wind measurements on two different dates. (A) Low-altitude winds from the 
ISS/CB3 images. The CB3 winds are located at or deeper than the SOO-mbar pressure level, and 
used as the boundary condition of thermal winds. (B) High-altitude winds from the ISS/MT3 
images and the crRS thermal observations. The CIRS thermal winds at 60 mbar are taken from 
the tropospheric thermal winds in the Supplementary Information. The horizontal solid lines and 
dashed lines are the uncertainties for the cloud-tracking winds and the correlation winds, 
respectively. 
12 
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Equatorial Winds on Saturn and the Stratospheric Oscillation

Caltech Authors - Main

Equatorial winds on Saturn and the stratospheric oscillation

https://authors.library.caltech.edu/28358/2/ngeo1292-s1.pdf

Equatorial winds on Saturn and the stratospheric oscillation

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