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°S and 20°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.
