Geoscience Reference
In-Depth Information
Main outbursts
MPH pathways
MPH boundaries mid-latitudes
Trade Winds
MPH boundaries in tropics
Relief affecting MPH pathways
Pathways for mobile polar highs (MPH) and resulting trade wind circulation in the tropics (based on Leroux, 1993).
Fig. 2.5
approaches the continent from the southwest, and then
loops across the continent towards the equator, and
drifts in the Hadley belt out into the Tasman Sea.
Highs rarely move directly northward across the conti-
nent, or sweep up from the Tasman Sea. In North
America, mobile polar highs regularly surge southward
across the Great Plains for the same reason.
Because mobile polar highs consist of dense air, they
deflect less dense, warm air upward and to the side.
The deflection is greatest in the direction they are
moving. Hence, the polar high develops an extensive
bulbous, high-pressure vortex, surrounded downdrift
by a cyclonic branch or low-pressure cell (Figure 2.6).
Typically, the high-pressure cell is bounded by an
arching, polar cold front with a low pressure cell
attached to the leading edge. However, in the northern
hemisphere, individual outbreaks tend to overlap so
that the low-pressure vortex becomes contained
between two highs. This forms the classic V-shaped,
wedged frontal system associated with
extratropical
lows or depressions
(Figure 2.6). Thus lows, and upper
westerly jet streams, are a product of the displacement
and divergence of a mobile polar high. The intensity of
the low-pressure cell becomes dependent upon the
strength of the polar high, and upon its ability to
displace the surrounding air. Strong polar highs
produce deep lows; weak highs generate weak lows. In
a conceptual sense, a deeper Icelandic or Aleutian Low
must be associated with a stronger mobile polar high.
If a mobile polar high is particularly cold, it can cause
the air above to cool and settle. This creates low
pressure above the center of the high-pressure cell,
which can develop into a trough and then a cell as
upper air flows inwards. If a mobile polar high has lost
its momentum and stalled forming a 'blocking high',
the low-pressure cell can intensify, propagate to the
ground, and generate a surface storm. This process
occurs most often on the eastern sides of continents
adjacent to mountain ranges such as the Appalachians
in the United States and the Great Dividing Range in
Australia. These storms will be discussed further in the
next chapter.
Mobile polar highs tend to lose their momentum
and stack up or
agglutinate
at particular locations over
the oceans. A blocking high is simply a stagnant mobile
polar high. Air pressure averaged over time thus
produces the illusion of two stable, tropical high-
pressure belts - known as Hadley cells - on each side
of the equator. Mobile polar highs can propagate into
the tropics, especially in winter. Here, their arrival
tends to intensify the easterly trade winds. More
important, yet little realized, is the fact that strong
