The ‘answer’ is that there is no SINGLE ANSWER.
The Jet Stream is caused by different factors than ground wind. Ground wind is caused by various forces, including the respiration of the trees and plants. One of the larger factors is the movement from high to low pressure zones. The movement of the pressure zones is largely controlled by the Jet Stream. (at least in zones surrounding the jet stream).
Moisture evaporation (Heat from Sun) causes updrafts, electrical discharges cause certain flows of wind around them, traffic flow along a highway, a train speeding down the tracks, and butterflies flapping their wings.
The Jet Streams control the movement of the pressure zones, but what they are looking into with this paper is what creates the pressure zones. Obviously it is not temperature because we would have nothing but low pressure systems in winter and high pressure systems in summer. My 2 cents anyway.