The Jet Cross Circulation

The Jet Cross Circulation and the Effect of Shear

Wind is in geostrophic balance when the 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, producing a cross circulation.

Acceleration and deceleration of the 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, looking along the air trajectory. This anomalous behavior of the air flow is caused by the inertial forces acting on an air parcel that moves through a jet streak and is accelerated or decelerated. In the jet entrance region, the pressure gradient force is slightly stronger than the Coriolis force, as the latter depends on the speed of the air parcel, which is somewhat reduced due to inertia. In other words, the Coriolis force no longer balances the pressure gradient force and the air parcel moves towards lower pressures. Conversely, in the exit region, the air is decelerated but, again due to inertia, the speed of the air parcel is somewhat higher than pressure gradient forces would require for geostrophic balance and the 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 (wind from west to east). In the entrance region, PGF prevails (brown arrows) and in the exit region CF prevails (green arrows)

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 causes divergence at lower levels and divergence aloft causes convergence below. The combination of convergence and divergence at different levels creates the jet cross circulation, as depicted in Figure 3.

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

There is one more effect to consider when describing the ageostrophic wind caused by inertia. Wind speeds at lower levels are usually not as high as at upper levels, which 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 the ambient geostrophic wind speed, which results in a deviation to the region of lower pressure, as the Coriolis force cannot fully balance the pressure gradient force. Descending air masses show the opposite effect, i.e., a deviation to high pressure regions because the Coriolis force is greater. This is why ascending or descending air masses are tilted, and divergence and convergence are not precisely vertically superimposed.

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