Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 244

strong subpolar westerlies, which reduce the severity of
winter weather over much of mid- to high-latitude conti-
nental regions. The recent decadal trend in this direction is
suspected as having a partly human cause (though as we
write this, in winter 2005, the huge blocking high from
Azores to Iceland has been around for over a month and it
hardly feels like it).

this vortex (formed as southward- and downward-flowing
air along the Tibetan anticyclonic north to south pressure
gradient and turned westward by Coriolis force) enhances
the downward flow to add to the strength of lower-level
summer monsoonal winds. Thus there is a correlation
between strength of the easterly subtropical jet stream and
the amount of monsoonal precipitation. Such a correlation
is further evidence of the enhancement role of the Tibetan
Plateau in monsoon development. Thus the monsoonal
winds from the Indian Ocean (which ought to be flowing,
like all trade winds in the northern hemisphere, to the
southwest) turn to penetrate high into the foothills (par-
ticularly the eastern foothills) and ranges of the Himalayas,
contributing additional latent heat to the rising dry air on
the Tibetan Plateau to the north, which is itself markedly
arid during the summer.

6.1.7 Monsoonal circulations

These can be most simply regarded as continent-scale
sea/land breezes. They result from seasonal variations in
the Hadley circulation because of changing thermal gradi-
ents, notably the trans-equatorial migration of the ITCZ
and substantial enhancement of subtropical highs in winter
over the cool continents and their diminution and change
to deep lows in summer as the land warms up. Generally
(Fig. 6.11), as land warms up the overlying air is heated,
expands, and ascends, and hence creates higher pressures
above. The high level land-to-seaward pressure gradient
causes divergence or diffluence and net outflow of air in
the upper levels, causing a low pressure at the land surface.
Air now flows in from the ocean along the resulting sea
level pressure gradient. The overall effect is the formation
of a top to bottom convective circulation of air. The effect
in the subtropical maritime zones of Asia and Africa,
together with smaller regional zones like the southwestern
USA, is noteworthy because it leads to seasonal reversals
(remember that the Coriolis force is low at low latitudes)
of the trade winds that lead to the inflow of moist warm
maritime air onto the continents. It is the ascent and cool-
ing of these monsoonal air masses onto the Himalayas that
causes some of Earth's highest and most intense rainfall.
In recent years the role of large continental plateaux,
notably the Tibetan Plateau and, to a lesser extent, the
western USA, has come center-stage in efforts to explain
not only regional but also world-wide climate. Tibet in
particular seems to play a major role in enhancing the
Indian Ocean monsoon. It is a site of high sensible heat
transfer from the atmosphere in late spring and summer
and a ground-level low-pressure system results, with
upward-flowing air defining an upper-atmosphere Tibetan
anticyclone. The area thus contrasts greatly with the high-
pressure trade wind deserts of Africa at similar latitudes,
which lie in the continental interior at much lower ground
elevations. A strong (

6.1.8 High-latitude climates

The polar and subpolar ( latitude) regions of Earth

are influenced by highly seasonal shortwave insolation that
varies from nonexistent to low. Longwave diffuse reradia-
tion predominates in the totally dark winters. Polar skies
are often cloudy and it might be thought that these clouds
should trap more reradiated energy than they actually do.
The clouds are in fact very “thin,” with a sparse content of
water droplets present in the very cold and undersaturated
air. Albedo is high all year round over permanent ice- and
snowfields and high in winter everywhere because of sea-
sonal snowfall. The net radiation deficit is partly filled by
the poleward atmospheric transport of heat described
above and partly by oceanic transport. In northern lati-
tudes there is a major radiation deficit over the Norwegian
Sea associated with the formation and descent of deep
water, one of Earth's major heat sinks. Over the poles
themselves, radiative sinking dominates, there existing
more-or-less permanent but weak high-pressure systems of
descending air that diverge surfaceward to form the polar
easterly air masses (Fig. 6.10) that help define the Arctic
and Antarctic fronts.
In the southern hemisphere the climate over the high lat-
itudes is dominated by the very strong circumpolar wester-
lies acting about the polar vortex. Like the northern
hemisphere NAM (see Section 6.1.6), strengthening of the
Southern Hemisphere Annular Mode ( SAM , also known as
the Antarctic Oscillation ) seems to have occurred in recent
decades, interpreted as due to the effects of Antarctic
ozone depletion. This is because in the stratosphere,
observed ozone depletion has induced cooling over the
polar icecap as a result of reduced absorption of springtime

70

110 km h 1 ) easterly subtropical jet
stream forms south of the Tibetan Plateau during the
monsoon in response to strong temperature and pressure
gradients between the warm rising Tibetan Plateau air and
the cooler Indian Ocean air (Fig. 6.11). The magnitude of