Insolation Control of Monsoons

Monsoonal circulation
results from seasonal
changes in solar
radiation
 Logical to assume
that orbital scale
seasonal changes in
insolation
 Can cause changes
in the strength of
monsoonal
circulation
Modern Monsoons
Strong summer monsoons exist in N.
hemisphere
 Large landmasses in tropical regions
 Weaker in S. hemisphere
 Land masses in tropical and subtropical
regions are generally smaller
 N. Africa good example
 Strong summer monsoon
 Sediments deposited off-shore document
a record of monsoons in region

Monsoon Circulation over N. Africa


Strong summer heating
creates low-pressure over
west-central N. Africa
drawing moisture from
tropical Atlantic
 Wet summer monsoon
Winter cooling creates
high pressure in northwest
Sahara Desert enhancing
flow of the northern trade
winds
 Dry trade winds inhibit
precipitation
Summer Monsoon Controls Vegetation
Most rainfall in N.
Africa from
summer monsoon
 Vegetation
patterns driven by
summer monsoon
rainfall patterns
 Rainforest near
equator
 Desert scrub in
the Sahara

Orbital Monsoon Hypothesis

Strength of monsoons are linked with the
strength of insolation on orbital time scales
Greater summer
insolation intensified
wet summer monsoon
John Kurtzbach
Decreased winter
insolation
intensified
dry winter
monsoon
Nonlinear Response of Climate



More intense summer insolation maxima and deeper
winter minima always occur together at same
location
So why don’t the effects simply cancel?
 One season dominates response
 Significant rainfall only during summer
 Orbital-scale changes in winter insolation have
no affect on annual rainfall
An example of nonlinear response
 A strong net response to insolation
 Even though rainfall sensitive to only one
season
Evidence for Orbital-Scale Changes




Evidence should be in the
23,000 year cycle
Calculated June insolation at
30°N
 Today insolation low
 10,000 years ago high
Assumed that a critical
threshold must be reached
 Needed to drive strong
summer monsoons
Lake levels in N. Africa provide
a test of hypothesis
Three Assumptions: First

Assume a critical threshold level
 Below level summer monsoon weak
 No geologic record produced
 Context of N. Africa monsoon
 Rainfall must have been high enough to
fill lakes
• Above a level that prevented
evaporation during dry winter
 No lakes in Sahara Desert today
• Threshold insolation level well above
modern day level
Second Assumption
N. African lake level directly proportional to
strength of the summer monsoon
 i.e., the extent to which summer insolation
exceeds the critical threshold
 Reasonable assumption
 Greater summer insolation
 Should drive stronger monsoon
circulation
• Increase rainfall
• Increase lake levels

Third Assumption

Lake level records an average of several
individual monsoon summers
 Lake level is an average of several seasonal
signals
 Represents rainfall in summer
 Since winters are dry
• Blends the strength of several
summer monsoons
– Can be said of many geologic climate
records
Predicted Monsoon Response

Response mimics shape
of insolation curve
 Truncated at a
threshold level
 Below which lakes
will not record
rainfall
 Evaporate in dry
winter
 Note strong signals
at 85,000 and
130,000
Lake Deposits

No good geological or stratigraphic evidence
for deposition of N. African lakes
 However, fresh water diatoms found in
tropical Atlantic Ocean sediments
 Diatoms could only have grown in fresh
water lakes
 Blown by strong winds off shore
(sometimes 1000’s of kilometers)
 Concentrated in discrete stratigraphic
horizons
Fresh Water Diatoms


Diatoms must have
grown in N. African
Lakes
 During strong
summer monsoon
During strong winter
monsoon
 Lakes dry
 Winds strong
 Deflation occurs
 Diatoms blown off
shore as aeolian
sediments
Diatom Deposition Lags Insolation Maximum
If the sequence of events is correct
 Deposition of diatoms off shore
 Must lag insolation maximum
 Time needed for lakes to dry out
 In addition, pulses of diatoms off shore
 Should coincide with high amplitude
 June insolation
• Since larger lakes would be expected
– More diatom-rich sediments
available to blow off shore

Marine Deposition of Freshwater Diatoms
Lakes dry out when monsoon
weakens therefore diatoms
pulses at insolation minimum
Evidence for Monsoon Record


Sapropel deposition in Mediterranean may provide
evidence for a 23,000 monsoonal cycle
Today, well oxygenated water give rise to
deposition of beige colored mud with benthic fauna
 Circulation due to
 High evaporation
 Dense water
formation along
the north margin
Sapropel Deposition


May record strong summer monsoon
 Sapropel units rich in organic carbon suggesting
high surface productivity
 No benthic fauna suggesting anoxic bottom
waters
Deep water formation cut off by low salinity cap
 High runoff
 Nile River
 Stopped bottom
water formation
 Supplied nutrients
Sapropel Deposition during High Runoff
Sapropel deposition due to fresh
water inputs to Mediterranean
Fresh Water Mediterranean?


Nile River drains
eastern N. Africa
 Strong monsoon
should bring rainfall
to Nile River
headlands
 Ancient river beds
found in Sahara
Desert in Sudan and
Chad
Strong summer monsoon
should have driven high
fresh water discharge
into Mediterranean
Sapropel Deposition on 23,000 Cycle?
Beige clay
deposition
Sapropel
deposition
Sapropel Deposition on 23,000 Cycle!
Summer Monsoon and Atlantic Upwelling



Strong N. African summer monsoon winds modify
equatorial Atlantic Ocean circulation
Counter normal SE trade winds that drive strong
upwelling
Results in weak upwelling and deep thermocline
Normal Equatorial Atlantic Upwelling



During weak summer monsoon, strong SE trade
winds push warm waters offshore
Enhance upwelling of cold, nutrient-rich waters
Cause the thermocline to shallow
Strong Summer Monsoon
Plankton preferring
warm nutrientpoor water
favored
When strong summer
monsoon winds
weaken the SE
trade winds
Weak Summer Monsoon
Plankton preferring
cool nutrientrich water
favored
When weak summer
monsoon allows strong
SE trade winds to
blow warm surface
water away from
equator
Faunal Changes Preserved

Record of faunal changes preserved in
tropical Atlantic sediments
 Ecosystem shifts change with upwelling
 Upwelling changes with strength of
summer monsoon
 Ecosystems preserved in sediments
• Record the strength of N. African
summer monsoon
 Changes in the relative abundance of
environmentally-sensitive species
 Record 23,000 year precessional cycle
Fauna Preserve Record of Monsoons
Complications with Orbital Monsoon Hypothesis

Peak monsoon development lags summer peak
insolation maximum
 Interactions with other parts of climate system?
 Perhaps development of monsoon influenced by
N. hemisphere ice sheets
 Or by cooler ocean surface temperatures
during glacial intervals
• Cold ocean poor source of latent heat
 Peak development of summer monsoon may be
in phase with July 21 insolation
• July 21 insolation forcing N. African
summer monsoon
 Any of these explanations would only modify
hypothesis
More complications

Response of monsoon to
insolation changes is not linear
 There is a threshold
dependence
 As a result of this clipping
• Only a portion of the
23,000 y cycles
recorded
• Can distort the way
monsoons are recorded
in climate record
– Cause artifacts
Clipping Artifacts

If climate record
sensitive only to one side
of cycle
 Dominate signal may
show up as eccentricity
cycle
 Eccentricity
modulates amplitude
of precession
 Changes in eccentricity
not forcing the
response
 Precession forcing
 Yet without full record
 Eccentricity appears
strong
Harmonics

Shorter cycles generated by clipping
 For the 23,000 year cycle
 Harmonics have periods of
• N/2 = 11,500 years
• N/3 = 7,600 years
• N/4 = 5,750 years, etc.
 Harmonic cycles not present in original orbital
signal
 Or in change in the strength of monsoon
 Artifacts of biased way climate system
recorded response to orbital changes in
insolation