Wherever you go in life, everything has a natural shape: tides, girls, history, oak leaves, and even planetary atmospheres. Sometimes we’re surprised and elated to see them clearly. Observers working on NASA’s Cassini mission to Saturn and Titan had exactly that reaction when the found a polygon over Saturn’s north pole.
Contrary to NASA’s article, however, patterns like this aren’t unheard of on Earth. Indeed, a similar, somewhat stable, dynamic pattern in atmospheric flow can be observed around the 500 millibar pressure-height for at least a few weeks most years.
Let’s take northern-hemisphere winter as an example. The overall motion of the atmosphere north of the tropics is west-to-east, that is, the Earth rotates counter-clockwise from the point of view of a satellite looking down over the north pole, and the atmosphere in this region rotates even faster than the ground. So, weather systems approach from the west and depart to the east. The paths taken by these weather systems are usually dictated by changes in the longwave pattern of the atmosphere, observable around the 500-millibar pressure-height. Shortwaves that generate weather move along the boundaries of a longwave pattern that spans the globe. These longwaves are called Rossby waves. You can get a feeling for how they work by looking at a plot of 500-mb heights like this.
If you look closely, you can see something not unlike a distorted pentagon forming in the image. The polygon of pressure heights in Earth’s atmosphere is much less stable and more distorted than that on Saturn, because Earth’s atmosphere is relatively thin, and the surface has wildly non-uniform thermal properties. So differences in cloud cover and how well the surface absorbs sunlight in neighboring regions keeps us from seeing nice, pretty shapes that last a long time.
On Earth, the number of nodes (i.e., number of sides to the “polygon”) in a pattern of atmospheric waves like this (called Rossby waves) changes regularly, and the rotation of the polygon about the pole depends on the number of nodes, k. In the above example, k=5. If k is large, waves propagate upstream slower than the atmosphere moves downstream, so they move west to east over the surface. If k is small, the individual waves are longer, and propagate upstream faster relative to the atmospheric motion. In that case, they can seem not to move relative to the surface, or even move east-to-west. There’s usually some possible k for which the nodal pattern in the atmosphere, as observed from the surface or from a geostationary satellite, seems stable.
Why go through all this? Well, first of all, according to current understanding, Rossby waves form in the atmosphere because of the changes in the Coriolis effect with latitude. As particles go north, the surface of the planet gets closer to the spin axis, so they have to speed up toward the east in order to conserve angular momentum, and that process eventually generates these wave patterns. Observations like this from Saturn, then, can tell us something about the horizontal motion of particles lower in its atmosphere. Secondly, these phenomena aren’t unique to planetary atmospheres. Fluid patterns eerily similar to those imaged on Saturn have been observed in the laboratory (thanks to user “hendric” over at Unmanned Spaceflight for pointing this out). Intriguingly, these laboratory observations cannot be explained by the Coriolis effect, because they are on far too small a scale for changes in latitude to matter. Whatever is happening on Saturn’s north pole is clearly related to what happens on Earth’s north pole, but it may also yield insight into forces and phenomena of fluid dynamics, not yet well understood, that can affect atmospheric behavior. It’s all very complicated and speculative and wicked awesome.
