The Discovery Of Jet Streams
Late nineteenth-century observers of high-level cloud motion noted the occasional existence of strong upper winds, but their regularity and persistence were not suspected at the time. The recognition that there are coherent bands of very strong winds in the upper troposphere was an operational discovery by Allied bomber pilots flying over Europe and the North Pacific during the Second World War. Flying westward, headwinds were sometimes encountered that approached the air speed of the planes. The term jet stream, used earlier for certain ocean current systems, was introduced in 1944 and soon became widely adopted. The corresponding German word Strahlstrome had in fact first been used in the 1930s.
Bands of strong upper winds are associated with intense horizontal temperature gradients. Locally enhanced equator to pole temperature gradients are associated with westerly jets and pole to equator gradients with easterly jets. The principal westerly jet streams are the subtropical westerly jet stream at about 150 to 200 mb, and one associated with the main polar front at around 250 to 300 mb. The former is located between latitudes 30 to 35° and the latter between 40 to 50° in both hemispheres. The strongest jet cores tend to occur over East Asia and eastern North America in winter. There may be additional jet-stream bands associated with a strong arctic frontal zone. In the tropics there are strong easterly jet streams in summer at 100 mb over southern India and the Indian Ocean and over West Africa (see Figure 7.8). These are linked to the monsoon systems.
arcticfrontjet streams (Chapter 9E), are associated with the steep temperature gradient where polar and tropical air and polar and arctic air, respectively, interact, but the subtropical jet stream is related to a temperature gradient confined to the upper troposphere. The polar front jet stream is very irregular in its longitudinal location and is commonly discontinuous (Plate 15), whereas the subtropical jet stream is much more persistent. For these reasons, the location of the mean jet stream in each hemisphere and season (Plate D) reflects primarily the position of the subtropical jet stream. The austral summer (DJF) map shows a strong zonal feature around 50°S, while the boreal summer jet is weaker and more discontinuous over Europe and North America. The winter maps (Plate D, [A] and [D]) show a pronounced double structure in the southern hemisphere from 60°E eastward to 120°W, a more limited analogue over the eastern and central North Atlantic Ocean (0 to 40°W). This double structure represents the subtropical and polar jets.
The synoptic pattern of jet stream occurrence may be complicated further in some sectors by the presence of additional frontal zones (see Chapter 9E), each associated with a jet stream. This situation is common in winter over North America. Comparison of Figures 7.3, 7.4 and Plate D indicates that the main jet-stream cores are associated with the principal troughs of the
Rossby long waves. In summer, an easterly tropicaljet stream forms in the upper troposphere over India and Africa due to regional reversal of the S-N temperature gradient (p. 284). The relationships between upper tropospheric wind systems and surface weather and climate will be considered below.
In the southern hemisphere, the mean jet stream in winter is similar in strength to its northern hemisphere winter counterpart and it weakens less in summer, because the meridional temperature gradient between 30° and 50°S is reinforced by heating over the southern continents (Plate D).
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