7.4: Jet streams

Polar and Subtropical Jet Streams

There is one final important piece of general circulation that deserves discussion: jet streams. In Figure \(\PageIndex{1}\) you can see two jet streams: the subtropical jet and the polar jet. The figure shows the average positions of the jet streams in the Northern Hemisphere during winter, along with their relationship to the tropopause. The figure shows jet streams flowing from west to east. We can see that there are two jets located right under the tropopause. The subtropical jet stream is located near 30° latitude, about 13 km up, above the tropical high. The polar jet stream is located near the polar front, about 10 km above the surface, at latitudes of 50° to 60 °. The difference in height of these jets is due to their location at the tropopause, and to the fact that the tropopause is higher in tropical regions than in polar regions, reflecting differences in the average temperature of the troposphere below. The troposphere is thinner in polar regions than in tropical regions because air is colder and denser at the poles.

Figure \(\PageIndex{1}\): Cross-section of the northern hemisphere circulation, and the positions of the polar and subtropical jet streams. Details in the above paragraph (Public Domain; National Weather Service via Wikimedia Commons). Alternative Description of Image.

If the general circulation of the atmosphere is like a giant meandering river of air around the globe, then jet streams are swiftly flowing currents within that river. Jet streams are thousands of kilometers in length and hundreds of kilometers in width. In the core of a jet stream (called a jet streak), wind speeds are often higher than 100 knots and are occasionally higher than 200 knots. The polar jet can sometimes merge with the subtropical jet if it sweeps southward enough, and it occasionally splits into two jet streams.

Figure \(\PageIndex{2}\): A global schematic highlighting the main jet streams in each hemisphere. Two subtropical jet streams encircle the planet in the midlatitudes, forming fast-moving bands linked to strong temperature contrasts between tropical and subtropical air. Closer to the poles, narrower polar jet streams mark the boundaries between cold polar air and warmer midlatitude air. The pattern appears in both hemispheres, illustrating how steep temperature gradients at different latitudes produce distinct high-altitude wind bands that strongly influence weather systems (CC BY-NC-SA 4.0; Roland Stull via Practical Meteorology).

The existence of jet streams ultimately results from the energy imbalance between the tropical and polar regions. How exactly do they form? As mentioned before, the polar front is a boundary between colder polar air and warmer subtropical air. As a result, the strongest temperature gradient occurs along the polar front. This rapid change in temperature with distance also causes a rapid change in pressure, due to the thermal wind effect (a vertical shear in the geostrophic wind caused by a horizontal temperature gradient). This strong pressure gradient across the polar front generates intense winds that form the jet stream. The temperature contrast between north and south along the polar front is more intense during the winter than during the summer, so the polar jet is also stronger during the winter. During winter, the leading edge of the cold polar air extends farther south into subtropical regions. During the summer, the polar front retreats into higher latitudes and is weakened.

The subtropical jet stream tends to form just above the descending branch of the Hadley cell, at about 12 km altitude. Here, a boundary exists between warmer equatorial air and cooler air that has been cycled up and around the Ferrel cell from the polar front. This is sometimes called the subtropical front, but it does not extend to the surface. Here, the temperature gradient is strongest aloft near the tropopause, which induces a sharp pressure gradient and strong winds.