Geoscience Reference
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associated with the height of air parcels above the
surface. Gradients in potential energy on a
pressure surface provide conditions to convert
potential to kinetic energy. This was set forth in
the work of Margules (1901) who showed that the
potential energy of a typical depression is less than
10 percent of the kinetic energy of its constituent
winds. In Stockholm V. Bjerknes' group concen-
trated on frontal development (Bjerknes, 1897
and 1902) but its researches were particularly
important during the period 1917-1929 after J.
Bjerknes moved to Bergen and worked with
Bergeron. In 1918 the warm front was identified,
the occlusion process was described in 1919,
and the full Polar Front Theory of cyclone
development was presented in 1922 (J. Bjerknes
and Solberg). After about 1930, meteorological
research concentrated increasingly on the impor-
tance of mid- and upper-tropospheric influences
for global weather phenomena. This was led by Sir
Napier Shaw in Britain and by Rossby, with
Namias and others, in the USA. The airflow in
the 3-10km high layer of the polar vortex of the
Northern Hemisphere westerlies was shown to
form large-scale horizontal (Rossby) waves due to
latitudunal gradients in the Coriolis parameter,
the influence of which was simulated by rotating
'dish pan' experiments in the 1940s and 1950s.
The number and amplitude of these waves appear
to depend on the hemispheric energy gradient, or
'index'. At times of high index, especially in
winter, there may be as few as three Rossby waves
of small amplitude giving a strong zonal (i.e., west
to east) flow. A weaker hemispheric energy
gradient (i.e., low index) is characterized by four
to six Rossby waves of larger amplitude. As with
most broad, fluid-like flows in nature, the upper
westerlies were shown by observations in the
1920s and 1930s, and particularly by aircraft
observations in World War II, to possess narrow
high-velocity threads, termed 'jet streams' by
Seilkopf in 1939. The higher and more important
jet streams approximately lie along the Rossby
waves. The most important jet stream, located at
10km, clearly affects surface weather by guiding
the low pressure systems which tend to form
beneath it. In addition, air subsiding beneath the
jet streams strengthens the subtropical high
pressure cells.
F THE POLAR REGIONS
The earliest view of the Arctic's atmospheric
circulation can be traced to the late nineteenth-
century work of von Helmholtz, who argued that
the region was dominated by a more or less
permanent surface high pressure cell, a view
developed in the earlier part of the twentieth
century by Hobbs in his 'glacial anticyclone'
theory. In 1945, Hobbs elaborated further on this
basic idea, advocating the existence of a persistent
anticyclone over the Greenland ice sheet, having
strong impacts on middle latitudes. Given the
general lack of data until the 1940s and 1950s,
such a misconception is not surprising. Sea level
pressure analyses in the US Historical Weather
Map Series produced during World War II
contained strong positive biases prior to the 1930s
away from the North Atlantic sector. Part of the
problem, as noted by Jones, was that these maps
were prepared by relatively untrained analysts,
who tended to extrapolate into the data-poor
Arctic with the prevailing mindset of an Arctic
high pressure cell. Even by the early 1950s, some
studies erroneously depicted traveling cyclones as
largely restricted to the periphery of the Arctic
Ocean. The emergence in North America of more
modern views of the Arctic circulation in the late
1950s and 1960s, fostered by the growing database
of upper-air data and surface observations,
appeared in the work of research groups at McGill
University led by F. K. Hare and the University
of Washington led by R. J. Reed. R. G. Barry
participated in the work at McGill, and made
many contributions. Interestingly, in the Soviet
Union, a relatively modern view of the summer
circulation had already been formulated in 1945
by B. L. Dzerdzeevskii.
Knowledge of Antarctica lagged behind that of
the Arctic. The remoteness and extremely harsh
