Embedded within the jet stream are jet streaks. The high momentum air associated with a jet streak helps build troughs and ridges. When a stream of air on a straight-line path moves into a region where it must follow a curved path, a centrifugal force results. The centrifugal force is an outward fleeing force. It is synonymous to a car turning a sharp corner. The car remains on a curved path due to friction, however, your body and separate items in the car want to remain on a straight-line path. The driver will feel their body wanting to go through the side of the car. Air in the atmosphere works in a similar fashion, except air does not turn corners as easily as a car on the road due to a low value of friction in the upper atmosphere. A jet streak of air on the left side of the trough will tend to amplify the trough. The diagram below shows a time series of a jet streak amplifying a trough.
Another perspective in looking at how a jet streak on the left side of a trough amplifies a trough is by studying the left front divergence quadrant of a jet streak. Mass is evacuated from the left front quadrant of a jet streak. As the left front quadrant moves south, the trough will build to the south. The diagram below shows the left front quadrant of a jet streak that begins to enter the base of a trough.
As a jet streak turns into the base of a trough, the left front quadrant remains just to the south of the axis of rotation associated with the trough. Because divergence is greater to the south of the axis of rotation as compared to the north, the trough builds to the south. Strong upper level divergence lowers heights and surface pressure. The diagram below shows the position of the left front quadrant with respect to the position of the axis of rotation.
It is the left front quadrant and the production of PVA that are responsible for divergence to the north of a jet streak axis in the base of a trough. The diagram below shows the positions of a vort max and the left front quadrant within the base of a trough. The divergence from PVA and the left front Quadrant being south of the axis of rotation allows the trough to build south and amplify.
Haby sez: The momentum of a jet streak and divergence created from the left front quadrant and PVA allow a jet streak on the left side of the trough, which is entering into the base of the trough, to deepen and amplify toward the south.