Jet streams and extratropical storms
Jet streams are fast-flowing air currents located in the upper atmosphere, primarily in the troposphere and lower stratosphere, encircling the Earth and influencing weather patterns. There are two main types of jet streams: the subtropical jet, found outside tropical latitudes, and the polar jet, situated where midlatitudes meet polar regions. These winds predominantly blow from west to east and shift position with the seasons.
Extratropical storms, which often develop at the boundaries of differing air masses, are significantly affected by the presence of jet streams. These storms, including extratropical cyclones, rely on the energy generated by the jet streams, particularly from their areas of strongest winds known as jet streaks. The dynamics of the jet streams can create turbulence, impacting air travel and weather forecasting.
The relationship between jet streams and climate change is increasingly relevant, as global warming is believed to be causing these currents to migrate poleward. This shift may result in changes to storm tracks and the regions vulnerable to tropical storms. Additionally, while some research suggests a potential weakening of jet stream intensity due to reduced temperature gradients, others argue that increased humidity and energy release may still fuel storm activity. Understanding the behavior of jet streams is crucial for predicting weather patterns and assessing the implications of climate change on storm dynamics.
Subject Terms
Jet streams and extratropical storms
Definition
A jet stream is a band of high-velocity atmospheric current that encircles the Earth. This band of strong winds is typically found in the upper troposphere and lower stratosphere. In both the Northern and the Southern Hemisphere, there are two distinctive jet streams: one is located just outside the tropical latitudes, in the subtropics; the other is located at the boundary of the midlatitudes and the polar region. In these latitudes, the jet-stream winds are westerlies, blowing from west to east. The two jet streams are named, respectively, “the subtropical jet” and “the polar jet.” These jets shift locations seasonally.
The rotation of the Earth around its own axis causes the air that surrounds the Earth to move as a result of the drag exerted by the Earth’s solid surface. This movement of air is called “wind.” Wind can blow in any direction, even though the Earth rotates from west to east. Two of the major forces in the atmosphere that determine wind direction are the pressure force and the Coriolis effect. Because of differential solar heating between the tropics and a polar region, there is a strong tendency of the atmosphere to move outward from the warmer equator to the colder poles, distributing heat. This tendency would cause winds to blow in the north-south directions in the Northern and Southern Hemispheres at different altitudes.
Owing to the Earth’s rotation, these two major north-south circulations break up, forming several smaller cells of circulation. These smaller cells include the Hadley circulation, the Ferrel circulation, and the polar circulation, listing them from the equator to the poles in each hemisphere. At upper atmospheric levels, when air travels to the north in the Northern Hemisphere, the Coriolis effect turns this airstream to its right, turning a southerly wind into a westerly wind. Furthermore, because the becomes stronger as air travels farther north and the effect of Earth’s surface drag is smaller at upper atmospheric levels, these westerly winds become very strong. As a result, jet streams form outside the tropics, toward high latitudes, at upper atmospheric levels.
Significance for Climate Change
Because it is a high-velocity wind current, a jet stream can be an important energy source for extratropical cyclones and storms. The position of a jet stream serves as an important indicator for the location of surface front and storm development. Both the subtropical and the polar jet are located at the boundaries of two different air masses. The boundaries of different air masses are the locations of surface fronts, at which extratropical cyclones develop. Therefore, a jet stream is an important component of a synoptic weather system.
Inside a jet stream, there is often a core region where winds become even stronger than they are in other parts of the jet. This core region is called the “jet streak.” Because winds increase toward the jet streak, large horizontal and vertical wind shears exist at the boundaries of a jet stream. The sheared winds can cause a flow current to become curved and unstable. Therefore, in the vicinity of a jet stream, atmospheric waves and turbulence are often generated. Upper air turbulence may threaten aircraft.
Several studies have indicated that global warming tends to cause jet streams to move poleward. The implication of this migration is that extratropical storms or “storm tracks” will extend poleward as well. Furthermore, the combined effects of global warming and the poleward movement of jet streams leave a larger spatial area in which tropical storms may develop. Therefore, the area of coastal land that is vulnerable to hurricanes may increase.
Some scientists have pointed out that, as a result of a continuous increase in Earth’s average surface temperature and of the temperature throughout the troposphere, the temperature gradient from the equator to the poles may be decreased. This possibility seems to suggest that the jet stream’s intensity may decrease as the climate warms. The weakening of jet streams may in turn decrease the activity of extratropical cyclones. However, other scientists have argued that global warming may not lead to a warming of the entire and a corresponding decrease in jet-stream intensity. Moreover, because global warming would also tend to increase atmospheric humidity, release may supply additional energy for extratropical cyclones despite a decreased global temperature gradient.
Bibliography
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