When jet stream winds were first discovered, they were thought to be part of a solitary stream extending around the globe over the mid-latitudes. Research proved this not to be the case, and as other jet streams were discovered, they were named. Their names are derived from the regions over which the jet streams occur and, in some cases, their association with other features. The jet streams covered in this unit are as follows:

Most studies center around the middle-latitude jet streams of the Northern Hemisphere, because the availability of upper-level data in this region far exceeds that of other areas of the world, and because these are the jet streams that impact the majority of the worlds population. With this in mind, most of our discussion will focus on the polar-front and subtropical jet streams.

POLAR-FRONT JET STREAMS. These jet streams are found in both hemispheres and are associated with the principal frontal zones and low-pressure systems of the middle and subpolar latitudes. They appear as single or multiple systems on upper-level charts. A single polar-front jet occurs characteristically during high index cycles, when there are few long waves and the westerlies are predominantly zonal. Multiple jets occur during low index cycles, when there are numerous long waves and the westerlies are predominantly meridional.

The Single Polar-front Jet. When the upper-level westerlies are well organized, a single polar-front jet exists. The stream lies west to east, with cold air north of the axis and warm air south. The wind along the axis shows areas of alternating maximum and minimum wind speeds, known as jet maxima and jet minima. Each maximum is associated with a short-wave trough and moves with a speed proportional to the short-waves speed at the 700-mb level. The wind speed in these maxima may easily exceed 250 knots, and the distance between each maximum varies from about 10 to 25 longitude. The difference in wind speed between the maxima and minima can be as much as 100 knots. As the westerlies begin to shift from zonal to meridional, the single polar-front jet often splits into two or more branches. We are then faced with multiple polar-front jet streams.

Multiple Polar-front Jets. When the upper-level westerlies become predominantly meridional, outbreaks of polar and/or arctic air become common. A jet accompanies each outbreak and is directly associated with the frontal zone. On the average, each jet drifts equatorward at a rate of 30 nautical miles per day. They intensify over the mid-latitudes, but weaken as their associated polar or arctic air moderates in the lower latitudes. This equatorward migration is not regular, because some jets become stationary or even drift back to the north. The wind along their axes is more or less uniform, and the jet maxima and minima disappear. Split jet streams are common, and the most important are those associated with the formation of blocks in the upper-level circulation. Numerous small offshoots of jet flow, known as jet fingers, are also common. They are weaker than the primary jet, and at times the fingers may be separated by as little as 5 latitude. Figure 8-3-6 illustrates these features. Multiple jets usually retain their identity around the hemi-sphere, but occasionally, they merge. When jets merge, the wind speed in the stream intensifies. As the circulation pattern changes, the westerlies once again organize into strong zonal flow, and a single polar-front jet is reestablished.

From the above information, you can see that on any given day, the number of polar-front jets, their strength, and location may vary. However, climatologists portray them as a single feature on climatic charts and provide mean statistics about them. In the absence of real-time data, climatic information becomes very important.

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