Wind is in geostrophic balance when pressure gradient force and Coriolis force are in balance. This is often the case in the upper troposphere at jet level. However, when the air is accelerated or decelerated (for example in jet streaks), this balance is disturbed. The disturbance of the geostrophic balance is caused by the ageostrophic component of the jet and its effect becomes manifest in the jet cross circulation.

Acceleration and deceleration of air in the region of a jet streak causes the airflow to deviate slightly to the left in the entrance region and slightly to the right in the exit region when we look along the air trajectory. This "strange" behavior of the air flow is caused by the inertial-forces acting on an air parcel moving through a jet streak and being accelerated or decelerated. In the jet entrance region, the pressure gradient force is slightly stronger than the Coriolis force, as the latter is depending on the speed of the air parcel and the speed is a bit lagging behind due to inertia. In other words, the Coriolis force does not compensate the pressure gradient force anymore and the air parcel moves towards lower pressure. Vice versa, at the exit region, the air is decelerated, but again, due to inertia, the speed of the air parcel is a bit higher than pressure gradient forces would require for a geostrophic balance and Coriolis force prevails; this results a deviation of the flow to the right.

Figure 1: Schematic depicting the relationship between pressure gradient force (PGF) and Coriolis force (CF) in the area of a jet streak.

A deviation of the jet stream to the left in the entrance region causes a divergence in the right entrance region of the jet and a convergence in the left entrance region. A deviation to the right in the exit region has the opposite effect (see figure 2).

Figure 2: Location of convergence (CON) and divergence (DIV) areas in the entrance and exit region of a jet streak, © NOAA

Convergence aloft creates divergence in lower levels and divergence aloft convergence below. The combination of convergence and divergence at different levels will create the jet cross circulation as depicted in figure 3.

Figure 3: Schematic of the cross circulation at the jet entrance and exit region. © COMET.

There is one more effect to consider when speaking about the ageostrophic wind caused by inertia. Wind speeds at lower levels are usually not as strong as in upper levels, what means that air parcels lifted from lower levels are continuously accelerated when rising to higher levels (vertical wind shear). When rising, their actual speed is slightly slower than ambient geostrophic wind speed, which results in a deviation to the region of lower pressure as Coriolis force cannot fully compensate pressure gradient force. Descending air masses show the opposite effect, i.e. a deviation to high pressure regions because Coriolis forces dominate. This is why rising or descending air masses are tilted and divergence/convergence are not exactly vertically superimposed.

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