Why Do Rossby Waves Form?
The Coriolis force is a pseudo force that acts on any air parcel that does not move parallel to the axis of rotation of the earth. This force increases with wind speed and is null when the air parcel is immobile. Hence, the Coriolis force acts in particular on the ribbon of the jet stream that circles the Earth. Additionally, pressure gradient forces also affect the path of the jet stream. Differential heating of the Earth's surface influences the repartition of high and low-pressure areas. Finally, orographic features such as mountain ranges can influence the flow of the jet around the globe.
All these factors act together and possibly lead to an amplification of Rossby waves as indicated in Figure 1.