Geoscience Reference
In-Depth Information
• Geopotential height surfaces at 200 hPa are more than 12 km from the
surface near the equator and slope down below 11 km at higher latitudes in
both hemispheres.
• At 900 hPa, the highest geopotential heights are preferentially located over
the oceans.
• The
equatorial trough
is the region of relatively low geopotential heights
deep in the tropics.
• Poleward of the
subtropical highs
in both hemispheres, geopotential heights
decrease through the midlatitudes all the way to the poles. (Geopotential
height values at 900 hPa over East Antarctica are not realistic because the
topography rises above this level.)
• The presence of the continents disturbs the zonal (east/west) uniformity of
the geopotential height lines.
The distribution of geopotential heights has strong seasonal dependence, par-
ticularly at lower levels. To represent seasonal changes in this field and others,
climatologies for the December, January, and February mean, designated DJF, are
used to display Northern Hemisphere winter and Southern Hemisphere summer.
For the opposite season, June, July, and August averages are displayed, and de-
noted as JJA. Seasonal ranges are quantified by plotting the difference DJF−JJA.
DJF and JJA geopotential heights at 900 hPa are shown in
Figures 2.4a
and
b,
respectively, and their difference is in
Figure 2.4c.
• Geopotential heights are higher over the oceans than over the land in the
hemisphere experiencing summer; the opposite is the case in the winter
hemisphere.
• Seasonal differences in geopotential heights are greater over land surfaces
than over the oceans.
• The subtropical highs centered over the oceans that were noted in the annual
mean (
Fig. 2.3)
are primarily a summer feature in the Northern Hemisphere.
• In the Southern Hemisphere, geopotential heights are more zonally uniform
in the winter (JJA), and located slightly closer to the equator.
• The monsoon regions of the world—southeastern Asia, northern Africa,
tropical South America, the southwestern United States, and Australia—are
characterized by low geopotential heights in the summer.
Geopotential height distributions at 200 hPa are displayed in
Figure 2.5.
The lines of constant geopotential height are much more zonally uniform than
at the 900 hPa level, but some effects of continentality are still discernible.
• The low-level monsoon lows at 900 hPa are overlain by regions of high
geopotential heights. This means that the distance between the 900 hPa and
200 hPa levels, known as the
thickness,
is greater in these regions.
• North of about 45°N in DJF there is a wavy pattern in the geopotential
height lines, with two wave cycles encircling the globe. This pattern brings
low heights to lower latitudes on the east coasts of Asia and North America,
along with high geopotential height gradients. These are the
storm tracks
of
the Northern Hemisphere, running southwest to northeast over the North
Pacific and Atlantic Oceans.
• The most pronounced seasonal differences at this level occur over
northeastern Asia (Siberia) and, to a lesser degree, northern North America.
