A stratospheric oscillation detected at the equator of Saturn (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Thierry Fouchet
Observatoire de Paris, LESIA and CNRS
Tel: 33 1 45 07 71 11 Fax: 33 1 45 07 71 44

8 May 2008

A stratospheric oscillation detected at the equator of Saturn

A team led by researchers from Paris Observatory found evidences for
temperature vertical oscillations in the stratosphere of Saturn. Using the
Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft
orbiting the planet since 2004, they retrieved a temperature map in Saturn's
stratosphere. The meridional gradient of temperature is changing sign
vertically. This allowed the team to calculate the wind direction and wind
magnitude, showing the existence of a westward and eastward equatorial jets
stacked vertically.

The stratosphere is the region of a planetary atmosphere where the
temperature increases with altitude. In this respect, the stratosphere
differs from the troposphere -- where we are living -- where the temperature
does decrease with altitude. The rise in temperature is due to the solar
flux absorption by an atmospheric constituent: ozone for the Earth, methane
for the Giant Planets, and Saturn in particular.

The vertical temperature variations of opposite direction in the troposphere
and stratosphere induce sharply different atmospheric circulations in these
two atmospheric regions. In the troposphere, convection dominates and drives
the Hadley circulation where warm air is lifted at the equator and descends
over polar regions. In the stratosphere, convective movements are forbidden.
Indeed, as it is lifted up, an air parcel cools down, while the environment
warms up. The parcel hence becomes colder than its neighbourhood and sinks
back. This reflex movement triggers temperature and pressure waves that
propagate vertically. With radiative transfer, these waves dominate energy
and heat transport in the stratosphere.

A team led by Paris Observatory researchers just found new evidence for
this. Using the Composite InfraRed Spectrometer (CIRS) aboard the Cassini
mission orbiting Saturn since 2004, they obtained a map of Saturn's
stratospheric temperature as a function of altitude (or pressure) and
latitude. At the equator, the temperature does not increase regularly with
altitude, but rather shows vertical oscillations. In addition, at a given
pressure level, a warm temperature at the equator corresponds to a cool
temperature at tropical latitudes (20 deg S and 20 deg N). From this
temperature gradient, it is possible to calculate the direction and the
magnitude of winds. The map shows the presence two equatorial westward and
eastward jets stack one above the other.

Such a structure also exists in the equatorial stratosphere of the Earth, as
well as in Jupiter. It is driven by the interaction of the zonal mean wind
with atmospheric waves. Atmospheric waves transport either westward or
eastward angular momentum depending on their nature, and transfer it to the
equatorial jets when they are damped. In addition, the vertical wind
structure is pulled downward. Hence, at a given altitude, the wind
alternates quasi-periodically between westward and eastward. On Earth, the
period is about 26 months, leading to the name of quasi-biennial
oscillation. On Jupiter, the period is about 4 years (quasi-quadriennial
oscillation). In parallel to the CIRS/Cassini observations, US astronomers,
after a 20-year long ground-based observation campaign, established that the
oscillation period for Saturn is about 15 years.

Hence, Saturn's stratosphere behaves like its terrestrial and jovian
siblings. The comparison between the three planets will help us to
understand how atmospheric waves triggers this kind of oscillation on Earth
and elsewhere in the Solar System.

Reference

Fouchet, T., Guerlet, S., Strobel, D.F., Simon-Miller, A.A., Bezard, B.,
Flasar, F.M., 2008
An equatorial oscillation in Saturn's middle atmosphere.
Nature, 453, 200-202 (8 May 2008), doi:10.1038/nature06912

For the observations, see also: Orton et al. 2008, Semi-Annual Oscillations
in Saturn's Low-Latitude Stratospheric Temperatures, Nature, same number.

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/apr08/saturn-f1.gif (107KB)]
Temperature measured by CIRS/Cassini in Saturn~s stratosphere as a function
of latitude and pressure (negative latitudes correspond to the Southern
Hemisphere, positive to the Northern Hemisphere). At mid-latitudes, the
temperature increases with altitude, hence with decreasing pressure. In
contrast, at the equator, the temperature oscillates vertically. In
addition, at 1 hPa, the warm equator corresponds to cold 20 deg S and 20 deg
N latitudes, and vice-versa at 0.1 hPa and 10 hPa.

[Figure 2:
http://www.obspm.fr/actual/nouvelle/apr08/saturn-f2.gif (85KB)]
From the temperature map above, it is possible to infer the direction and
magnitude of the winds in Saturn's stratosphere. The equatorial temperature
oscillation implies the existence of several strong jets stacked vertically.
Only atmospheric waves can generate such a structure.