1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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

17. 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

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

19. 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

21. Sapropel Deposition on 23,000 Cycle?
Beige clay
deposition
Sapropel
deposition

22. Sapropel Deposition on 23,000 Cycle!

23. 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

24. 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

25. Strong Summer Monsoon Plankton preferring warm nutrient- poor water
favored
When strong summer
monsoon winds
weaken the SE
trade winds

26. 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

27. 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

28. Fauna Preserve Record of Monsoons

29. 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

30. 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

31. 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

32. 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