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Chapter 9:
Control of
Chang Yu Pei
• Monsoon circulations exist on Earth today
because the land responds to seasonal
changes in solar radiation much more
quickly than does the ocean.
• In this chapter will examine evidence that
changes in insolation over orbital time
scales have driven major change in the
strength of the summer monsoons.
Monsoons Circulations
• Sea & Land
• Orbital-Scale Control
• Tectonic
• Monsoon Circulations
• Orbital-Scale Control of Summer
Evidence of Orbital-Scale Changes in Summer Monsoons
• Joint Tectonic and Orbital Control of
Monsoons Circulations ( summer )
In summer, strong solar
radiation causes rapid warming
of the land, but slower and much
less intense warming of the
ocean. Rapid heating over the
continents causes air to warm,
expand, and rise, and the
upward movement of air creates
an area of low pressure at the
Flow of air toward this lowpressure region causes in-andup motion of warm, moist air
from the oceans.
Monsoons Circulations ( winter )
In winter, when solar
radiation is weaker, air
over the land cools off
rapidly, increases in
density in relation to air
over the still-warm ocean,
and sinks from higher
levels in the atmosphere.
The overall atmospheric
flow in winter is a downand-out movement of
cold, dry air from the
land to the sea.
• Here focus on past
variation of the North
African monsoons.
• North African lies far from
the high-latitude that
might complicate the direct
response of land surface to
solar heating.
• The ocean around North
Africa yield a rich variety of
climate records showing
monsoon-related signals.
Monsoon circulations over North Africa
• Strong solar heating in the
northern hemisphere’s
summer, a low-pressure
region develops over westcentral North Africa, drawing
moisture-bearing winds in
from the tropical Atlantic to
the south.
• In winter, solar radiation over
North African land surface by
back radiation causes sinking
of air from above, and a highpressure cell develops at the
surface over the northwestern
Sahara Desert.
Orbital-Scale Control of Summer Monsoons
• The idea that changing insolation could control
the strength of monsoons over orbital time scales
was proposed by the meteorologist John
Kutzbach in 1980s.
• This hypothesis has been widely accepted.
• Monsoon circulation are linked to changes in the
strength of solar radiation during summer and
winter, long-term, orbital-scale changes in the
strength of summer and winter insolation should
have affected the strength of the monsoons in
the same manner in past.
The orbital monsoon hypothesis
• Stronger in-and-up
monsoon flows in summer
should occur at the same
time in the past as
stronger down-and-out
monsoon flows in winter.
• It seem that the climatic
effects of these opposed
insolation trends in the two
seasons might cancel each
• Precipitation.
Three assumptions
• First, assume a threshold insolation level.
• Second, stronger insolation should drive
stronger monsoons and fill lakes to higher
• Third, the strength of the monsoon at any
time in the past as recorded in the record
of lake levels is composite of the average
monsoon strength over many individual
Evidence of Orbital-Scale Changes in
Summer Monsoons
• Between 85,000 and
130,000 years ago, when
the summer insolation
curve reached strong
maxima because of
modulation of prcession by
• The weaker insolation
maxima near 35,000 and
60,000 years ago are
predicted to create much
weaker peak monsoons.
Monsoons and Nile floods
• The critical link is the Nile
River which gathers most
of its water from the
highlands of eastern North
Africa at tropical latitudes.
• Receive summer rains from
the relatively weak tropical
Freshwater Diatoms in the Tropical Atlantic
• These Atlantic sediments
have distinct layers
containing concentration of
the opaline (SiO2.H2O)
shells of a species of
freshwater diatoms.
• Only way for lake diatoms
to get there is by being
blown in by winds.
Diatom deposition in the Atlantic
When the monsoon is weak and
the thermocline is shallow,
cooler nutrient-rich waters rise
to shallow depths that receive
plenty of sunlight for
When the monsoon is strong
and the thermocline is deep,
sunlight cannot penetrate down
to the nutrient-rich subsurface
waters, and the warmer nearsurface water are low in nutrient.
Combined tectonic and orbital forcing of monsoons
The End