Geoscience Reference
In-Depth Information
THE DISCOVERY OF JET STREAMS
box 7.2
significant
20th-c. advance
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 intro-
duced 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.
arctic front jet 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 loca-
tion 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 tropical jet
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).
4 Surface pressure conditions
The most permanent features of the mean sea-level
pressure maps are the oceanic subtropical high-pressure
cells (Figures 7.9 and 7.10). These anticyclones are
located at about 30° latitude, suggestively situated
below the mean subtropical jet stream. They move a
few degrees equatorward in winter and poleward in
summer in response to the seasonal expansion and con-
traction of the two circumpolar vortices. In the northern
hemisphere, the subtropical ridges of high pressure
weaken over the heated continents in summer but are
thermally intensified over them in winter. The principal
subtropical high-pressure cells are located: (1) over the
