Geoscience Reference
In-Depth Information
winds caused by temperature differences between the land and sea and lasting for
several months. Monsoons are seasonal shifts in wind direction, where moisture-
laden air moves from ocean to land during the summer, and drier air moves from
land to sea in the winter. Moist maritime tropical air moves from ocean to land,
where it encounters hills and mountains that create uplift, cooling, and cloud
development with intense convective precipitation. In the winter, the subtropical
high-pressure system moves into more tropical latitudes and dominates with dry
continental air and a prolonged dry season.
Although monsoons occur in a number of countries, the most well known of
these is the Asian monsoon. The large Asian landmass, including Pakistan, India
(and nearby Sri Lanka), Bangladesh, and Myanmar, and the large ocean sur-
rounding the region (the Arabian Sea and the Indian Ocean) provide the perfect
conditions for a monsoonal climate and a dramatic transfer of heat between land
and sea.
Around April, premonsoon heat builds over the land, resulting in rising air and
forming areas of low pressure over north India and the Himalayas. The ocean heats
up more slowly, creating a temperature difference of as much as 36
C)
between the land and sea. Over the oceans, the air is cooler and denser and linked
to areas of high pressure. To maintain a balance of energy in the atmosphere, air
begins to flow from the oceans to the land (high pressure to low pressure) bringing
moisture-laden southwest winds across southern Asia (see Figure 4.4). With this
shift, the rains come. Rains usually start in late May, hitting Sri Lanka and moving
up from the Bay of Bengal into parts of northeast India and Bangladesh. As the
land and ocean begin to cool in late summer and into autumn, the land loses heat
more quickly than the ocean. The winds reverse and dry continental winds prevail,
beginning the long dry season.
Monsoons also control much of the rainfall in the tropical seasonal forests of
Africa. The high-intensity, high-water-content monsoonal airmasses can bring tor-
rential rains over short periods of time. As much as 20-30 in (500-700 mm) of daily
rainfall have been recorded in sites in Africa and India.
While the winter in the Northern Hemisphere experiences clear skies and a dry
season, farther south, strong north and northeast winds originating from cold,
northern Asia mix with moist tropical winds and bring severe weather including
heavy rainfall and typhoons to Australasia. This climate pattern has created sea-
sonal forests on several islands in the Pacific as well as Australia.
F (20
Soils
Soils of the Tropical Seasonal Forest Biome are varied, but tend to be similar to
those of the rainforest (see Chapter 2). In South America and Africa, soils are an-
cient, with a high rate of leaching that washes away most of the nutrients. These
older soils are derived from the Precambrian continental shield and are deeply
