January 3, 2018

Cassini - Tracking Saturn’s great storm

A giant storm raged in Saturn’s atmosphere from 2010 to 2011. To understand the effects of this planetary-scale disturbance, scientists pored over the 13 years of data from the Cassini spacecraft to analyse temperatures and wind dynamics. We talk to Sandrine Guerlet, research scientist at the LMD dynamic meteorology laboratory and co-author of an article on this subject in the journal Nature Astronomy.

Thanks to the instruments on the Cassini spacecraft orbiting Saturn up to September last year, the scientific community has been able to track the evolution of the “Great White Spot” in Saturn’s northern hemisphere and its effects on the rest of the planet. The storm that raged on the planet Saturn from December 2010 to July 2011, almost as large as Earth and with winds of up to 430 km/h, significantly disrupted the planet’s atmosphere. New bands of cloud appeared in the troposphere at latitudes between 35 and 40°N, encircling the entire planet in a matter of months and changing how it looked in visible wavelengths.

More surprisingly, this storm caused extreme heating in the stratosphere of +80°C at around 200 km above the cloud deck near 40°N, perceptible in the infrared. A new study also reveals a sharp drop of 10 to 12°C in the stratosphere across the equatorial band at the same time as the storm. But in addition to these temperature excursions, the entire alternating east-west wind regime in the stratosphere was disrupted.

“This is a dynamic atmospheric phenomenon we’ve known about on Earth since the 1960s, and a similar pattern was discovered on Saturn about 15 years ago,” explains Sandrine Guerlet. “In Earth’s stratosphere, winds at the equator change direction every 28 months on average (22 to 32 months). This cycle, known as the equatorial oscillation, is caused by the east-west winds—which are sitting on top of one another—propagating slowly downwards as a result of atmospheric waves generated in the troposphere. These waves propagate vertically and can cause the winds to strengthen or slacken. A similar phenomenon occurs on Saturn and we think the same mechanism is at work. However, while the oscillation in Earth’s stratosphere is quasi-biennial, on Saturn it lasts 15 years or half a Saturnian year. We therefore had to sift through all of Cassini’s data to study it over such a long period.”

The 2010-2011 storm in Saturn’s northern hemisphere, viewed by Cassini. Credit: NASA/JPL-Caltech/Space Science Institute

Calm after the storm on saturn

The sharp change in atmospheric temperature as a result of the storm shows that the cycle’s slow and steady evolution was disrupted. The first change was noted in 2011 when the storm erupted, then things returned to normal in 2014 when the anomaly waned. In 2016, the normal cycle returned with a phase of westerly winds as would be expected if no anomaly had occurred. But for Sandrine Guerlet, it’s still too early to tell if the cycle was just paused and resumed late, or if the 2016 cycle has returned as if nothing had happened. And while the equatorial oscillation was slowly returning on Saturn, a similar disturbance was unfolding back here on Earth:

“In 2016, an anomaly affected our atmosphere’s quasi-biennial oscillation,” says Sandrine Guerlet. “And that year precisely, the El Niño warming episode in the Pacific was exceptionally strong. The two phenomena could be linked, because the disturbances propagate via atmospheric waves, transferring angular momentum that can affect the atmosphere even in far-away regions. On Earth, such long-distance links are known as atmospheric teleconnection. And the parallel here with Saturn is interesting: we’ve found indications that suggest the giant storm of 2011 at mid-northern latitudes also strongly impacted Saturn’s equatorial oscillation thousands of kilometres away. These studies are examples of comparative planetology and meteorological dynamics on Earth and Saturn.”

Earth’s equatorial oscillation as a function of altitude up to July 2016. Easterly winds are in blue and white, westerly winds in green and brown: the disturbance can be seen from the start of 2016. Source: Newman, P. A., L. Coy, S. Pawson, and L. R. Lait (2016), The anomalous change in the QBO in 2015–2016, Geophys. Res. Lett., 43, 8791–8797, doi:10.1002/ 2016GL070373. 