The monsoon regions of the planet encompass wide swaths of the tropics and sub-
tropics and are, classically, experienced in Asia and parts of central Africa. From
December through February the primary wind direction is northeast to southwest,
while June through September brings the reversal of this flow. The former period is
quite dry while the latter is prodigiously wet.
Monsoons have their roots in the tropical and subtropical parts of the global
wind and pressure belts. The shifting declination of the vertical sun causes parts
of the global circulation to expand, contract, and change latitudes. Monsoons are
features in the tropical and subtropical portions of the planetary circulation.
Differential heating of land versus water is the simplest explanation of the wind
reversals. Continents heat more dramatically in the summer than do oceans and
thesetemperaturegradients set up wind gradients transporting air from the oceans
to the continents. In the winter, the thermal gradients reverse and cause the surface
air to reverse flow. Continental air is dry while oceanic air is moist and associated
with great amounts of precipitation. The monsoon is most prominent around Asia
because of the immense size of the Asian land mass and the complexities of its
coasts.
The air flow high in the troposphere is known as a partial cause of monsoons.
The subtropical jet stream is most active in the winter season because of the great
temperature difference between equator and poles. With its mean winter position
to the south of the Himalaya Mountains and Tibetan Plateau this strong upper flow
complements the cooling of the Asian continent and keeps the oceanic air out of
the continent. In the summer season, the subtropical jet weakens and jumps to
the north of these southern Asia highland areas and the oceanic air is allowed to
incur into the continent.
In Asian monsoon terminology, the Intertropical Convergence Zone is known as
the monsoon trough. This broad zone ofshallow low pressure is able to make
exceptionally large forays about the equator (especially to the north) because of
the great size of the Asian continent. The surface mechanics of the wet (summer)
monsoon are that the monsoon trough arrives and behind it is a deep, moist mari-
timetropicalor equatorial air. These air masses are unstable and host many slow
moving low pressure disturbances containing many convectional storms with high
rainfall rates. Monsoon lows and monsoon depressions are common with the for-
mer moving over an individuallocationabout once every 5 days while the latter
can be expected every 15 days. Both have mean diameters that can approach
3,000 km and can produce copious precipitation. The monsoon depression is the
more organized and has winds that can reach as high as 18 m per second. The
summer monsoon “bursts” as the monsoonal trough passes a location. There is
another burst of precipitation along the retreating monsoon trough, and this is
closely followed by the low sun season of dryness.
Monsoon 231