The subtropical jet is a band of fast eastward winds concentrated more than 10 kilometers above the ground. It affects the propagation of upper-level atmospheric waves, thus affecting surface weather and climate. Climate models show that it will strengthen in response to anthropogenic climate change.

The subtropical jet is driven by angular momentum advection from the deep tropics in the Hadley circulation. Because Earth rotates faster at the equator, due to the larger rotation radius, air moving poleward in the upper troposphere brings higher angular momentum, creating a jet at the subtropical edge of the Hadley cell. While this mechanism is well-understood, an outstanding question remains: What enables the maintenance of the subtropical jet at the Hadley cell edge in the presence of eddies? We know that atmospheric eddies (weather systems) tend to drive another jet in midlatitudes, by mixing the air and transporting momentum poleward. Idealized models show that a single jet in midlatitudes is a much more dynamically stable state than a subtropical jet at the Hadley cell edge.