Geoscience Reference
In-Depth Information
7.2 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
World War II. 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 been first used in the 1930s.
Bands of strong upper winds are associated with intense horizontal temperature gradients.
Locally enhanced Equator-Pole temperature gradients are associated with westerly jets and Pole-
Equator gradients with easterly jets. The principal westerly jet streams are the subtropical westerly
jet stream at about 150-200mb, and one associated with the main polar front at around 250-300mb.
The former is located between latitudes 30-35
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 an Arctic frontal zone. In summer there is a
persistent, albeit generally weak Arctic jet stream feature, most apparent over Eurasia, which owes
its existence to differential surface heating between the cold Arctic Ocean and surrounding snow-
free land. In the tropics there are strong easterly jet streams in summer over southern India and
the Indian Ocean and over West Africa. These are linked to the monsoon systems.
°
and the latter between 40-50
°
and longitudinal locations and is commonly
discontinuous, whereas the Subtropical Jet Stream
is much more persistent and varies far less in
latitude. For these reasons, the location of the
mean jet stream in each hemisphere and season
(
Plate 7.2
) primarily reflects the position of the
Subtropical Jet Stream. The austral summer (DJF)
map shows a strong zonal feature around 50
Chapter 9E), each associated with a jet stream.
This situation is common in winter over North
America. Comparison of
Figure 7.4
and
Plate 7.2
indicates that the main jet stream cores are
associated with the principal troughs of the Rossby
longwaves. In summer, an
Easterly Tropical Jet
Stream
forms in the upper troposphere over India
and Africa due to regional reversal of the S-N
temperature gradient (p. 355). The relationships
between upper tropospheric wind systems and
surface weather and climate will be considered
later.
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
S,
while the boreal summer jet is weaker and more
discontinuous over Europe and North America.
The winter maps (
Plate 7.2
[A] and [D]) show a
pronounced double structure in the Southern
Hemisphere from 60
°
W, a more
limited analogue over the eastern and central
North Atlantic Ocean (0-40
°
E eastward to 120
°
W). This double
structure represents the subtropical and polar jets.
The synoptic pattern of jet stream occurrence
may be further complicated in some sectors by
the presence of additional frontal zones (see
°
S is
reinforced by heating over the southern continents
(
Plate 7.2
).
°
and 50
°
