Insolation%20Control%20of%20Monsoons

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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

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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

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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

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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

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Orbital Monsoon Hypothesis

• Strength of monsoons are linked with the strength
  of insolation on orbital time scales

Greater summer insolation intensified wet summer
monsoon
Decreased winter insolation intensified dry
winter monsoon
John Kurtzbach
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Nonlinear Response of Climate

• More intense summer insolation maxima and deeper
  winter minima always occur together at same
  location
• So why dont 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

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Evidence for Orbital-Scale Changes

• Evidence should be in the 23,000 year cycle
• Calculated June insolation at 30N
• 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

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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

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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

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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

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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

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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 1000s
  of kilometers)
• Concentrated in discrete stratigraphic horizons

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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

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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

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Marine Deposition of Freshwater Diatoms
Lakes dry out when monsoon weakens therefore
diatoms pulses at insolation minimum
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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

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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

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Sapropel Deposition during High Runoff
Sapropel deposition due to fresh water inputs to
Mediterranean
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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

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Sapropel Deposition on 23,000 Cycle?
Beige clay deposition
Sapropel deposition
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Sapropel Deposition on 23,000 Cycle!
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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

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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

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Strong Summer Monsoon
Plankton preferring warm nutrient- poor
water favored
When strong summer monsoon winds weaken the
SE trade winds
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Weak Summer Monsoon
Plankton preferring cool nutrient- rich
water favored
When weak summer monsoon allows strong SE trade
winds to blow warm surface water away from equator
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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

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Fauna Preserve Record of Monsoons
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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

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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

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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

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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