Rossby Waves
The polar jet stream is a good indicator of the location of the polar front that separates the cooler polar air mass from warmer subtropical air masses. Jet streams are found in places with a strong horizontal temperature gradient. The meandering band of the jet around the globe follows the pattern of Rossby waves closely.
Rossby waves and thus the jet stream show varying wave amplitudes in space and time. They can have a strong zonal component as indicated in Figure 1a or show a strong meridional component as seen in Figure 1c.
Figure 1: Planetary Rossby waves with increasing amplitude (from a to c). Blue represents the polar air mass, and orange the warmer air masses. The pink ribbon marks the position of the jet stream. © Fred the Oyster, CC-BY-SA.
The shape of Rossby waves has a direct impact on the propagation speed of low-pressure systems that form along the polar front. Low pressure systems move faster with westerly winds and tend to remain stationary in the case of high amplitude Rossby waves.