designated at Pp are distinguished by anomalously high surface temperatures, which, together
with the high evaporation, contribute to instability of the atmosphere and precipitation. If orographic rain also falls on the coast, the amounts
are sufficient to develop lush vegetation on oceanic islands, such as the Polynesian Islands and
the continental coast of Brazil, south of Bahia, as
well as northeastern Australia.
3.3.4 740 Trade Winds Division
In these regions, the North and South Equatorial
Currents, which Dietrich classified as Pw, move
westward uniformly and persistently. The uniformity of current corresponds to uniformity of
wind and weather. These regions have little precipitation and since evaporation is high, the sea
surface is highly saline. Because the region is
free of divergences and the annual variation of
surface temperature is small, there is no vertical
mixing to renew nutrients from deeper layers.
Because the plankton population is, therefore,
low, only a few higher marine organisms exist
here.
High sea-surface temperatures—above 27
C
in these latitudes—are important in the development of tropical storms, which originate in this
area. Warming of air at low levels creates instability and predisposes the area toward the formation of storms. Once formed, the storms move
westward through the trade wind belt. They are
by far the most violent storms on the Earth and
are known as typhoons in East Asia, as Mauritius
hurricanes in the South Indian Ocean, and as
hurricanes in the West Indies.
3.3.5 750 Equatorward Trades
Division
This type consists of currents known as the
Canaries and Benguela Currents in the North
and South Atlantic Oceans, as California and
Peru (Humbolt) Current in the North and
South Pacific Oceans, and as the West
Australian Current in the South Indian Ocean.
In the Dietrich system, areas of this type are
designated as Pa¨. Since they flow from higher
to lower latitudes, they carry water that is colder
than average for the corresponding latitudes.
These temperature differences become larger
because the winds which blow parallel to the
coast, deflect surface water seaward, thereby
causing upwelling. This brings cold water,
abundant in nutrients, to surface layers. It
contributes to the extraordinary development
of plankton. The abundance of plankton shows
in the green coloring of the sea water, in contrast to the cobalt blue of the neighboring
regions of the horse latitudes. The great abundance of plankton produces large amounts of
fish in these areas, attracting endless numbers
of sea birds. They are responsible for the formation of guano deposits at these coasts. When this
upwelling is disturbed or replaced by a motion
directed toward the pole (the so-called El Nino
at the north Peruvian coast, which is not
associated with upwelling but carries warm,
nutrient depleted, equatorial water), then masses
of fish, and subsequently birds, die.
In general, surface temperatures are lower
than air temperatures, especially in the vicinity
of coasts. Hence, the formation of lasting and
frequent fogs, known under the name Garua at
the Peruvian coast, is common. Low rainfall is
the result of the stabilization of the air mass over
the cold water surface. These areas have the
lowest precipitation on the globe, including
inland deserts. Oceanic islands in these regions,
such as the Cape Verde Islands in the North
Atlantic Ocean and the Galapagos Islands in the
South Pacific Ocean, have desert-type climates.
The coastal deserts of Namib in Southwest
Africa and of Atacama in north Chile are located
at approximately the same latitude adjacent to
these cold currents.
Another consequence of the low surface temperature of these regions is the fact that no coral
reefs are found in these areas. Their development
requires not only clear water but also
temperatures of at least 20
C in the coldest
month of the year (Fig. 3.5).
3.3 700 Tropical Domain
23
with the high evaporation, contribute to instability of the atmosphere and precipitation. If orographic rain also falls on the coast, the amounts
are sufficient to develop lush vegetation on oceanic islands, such as the Polynesian Islands and
the continental coast of Brazil, south of Bahia, as
well as northeastern Australia.
3.3.4 740 Trade Winds Division
In these regions, the North and South Equatorial
Currents, which Dietrich classified as Pw, move
westward uniformly and persistently. The uniformity of current corresponds to uniformity of
wind and weather. These regions have little precipitation and since evaporation is high, the sea
surface is highly saline. Because the region is
free of divergences and the annual variation of
surface temperature is small, there is no vertical
mixing to renew nutrients from deeper layers.
Because the plankton population is, therefore,
low, only a few higher marine organisms exist
here.
High sea-surface temperatures—above 27
C
in these latitudes—are important in the development of tropical storms, which originate in this
area. Warming of air at low levels creates instability and predisposes the area toward the formation of storms. Once formed, the storms move
westward through the trade wind belt. They are
by far the most violent storms on the Earth and
are known as typhoons in East Asia, as Mauritius
hurricanes in the South Indian Ocean, and as
hurricanes in the West Indies.
3.3.5 750 Equatorward Trades
Division
This type consists of currents known as the
Canaries and Benguela Currents in the North
and South Atlantic Oceans, as California and
Peru (Humbolt) Current in the North and
South Pacific Oceans, and as the West
Australian Current in the South Indian Ocean.
In the Dietrich system, areas of this type are
designated as Pa¨. Since they flow from higher
to lower latitudes, they carry water that is colder
than average for the corresponding latitudes.
These temperature differences become larger
because the winds which blow parallel to the
coast, deflect surface water seaward, thereby
causing upwelling. This brings cold water,
abundant in nutrients, to surface layers. It
contributes to the extraordinary development
of plankton. The abundance of plankton shows
in the green coloring of the sea water, in contrast to the cobalt blue of the neighboring
regions of the horse latitudes. The great abundance of plankton produces large amounts of
fish in these areas, attracting endless numbers
of sea birds. They are responsible for the formation of guano deposits at these coasts. When this
upwelling is disturbed or replaced by a motion
directed toward the pole (the so-called El Nino
at the north Peruvian coast, which is not
associated with upwelling but carries warm,
nutrient depleted, equatorial water), then masses
of fish, and subsequently birds, die.
In general, surface temperatures are lower
than air temperatures, especially in the vicinity
of coasts. Hence, the formation of lasting and
frequent fogs, known under the name Garua at
the Peruvian coast, is common. Low rainfall is
the result of the stabilization of the air mass over
the cold water surface. These areas have the
lowest precipitation on the globe, including
inland deserts. Oceanic islands in these regions,
such as the Cape Verde Islands in the North
Atlantic Ocean and the Galapagos Islands in the
South Pacific Ocean, have desert-type climates.
The coastal deserts of Namib in Southwest
Africa and of Atacama in north Chile are located
at approximately the same latitude adjacent to
these cold currents.
Another consequence of the low surface temperature of these regions is the fact that no coral
reefs are found in these areas. Their development
requires not only clear water but also
temperatures of at least 20
C in the coldest
month of the year (Fig. 3.5).
3.3 700 Tropical Domain
23