Insolation Control of Monsoons Monsoonal circulation Monsoonal circulation

results from seasonal results from seasonal changes in solar changes in solar radiationradiation Logical to assume Logical to assume

that orbital scale that orbital scale seasonal changes in seasonal changes in insolationinsolationCan cause Can cause changes in the changes in the strength of strength of monsoonal monsoonal circulation circulation

Modern Monsoons Strong summer monsoons exist in N. Strong summer monsoons exist in N.

hemispherehemisphere Large landmasses in tropical regionsLarge landmasses in tropical regions

Weaker in S. hemisphereWeaker in S. hemisphere Land masses in tropical and subtropical Land masses in tropical and subtropical

regions are generally smallerregions are generally smaller N. Africa good exampleN. Africa good example

Strong summer monsoonStrong summer monsoon Sediments deposited off-shore document Sediments deposited off-shore document

a record of monsoons in regiona record of monsoons in region

Monsoon Circulation over N. Africa

Strong summer heating Strong summer heating creates low-pressure creates low-pressure over west-central N. over west-central N. Africa drawing moisture Africa drawing moisture from tropical Atlanticfrom tropical Atlantic Wet summer monsoonWet summer monsoon

Winter cooling creates Winter cooling creates high pressure in high pressure in northwest Sahara Desert northwest Sahara Desert enhancing flow of the enhancing flow of the northern trade windsnorthern trade winds Dry trade winds inhibit Dry trade winds inhibit

precipitationprecipitation

Summer Monsoon Controls Vegetation

Most rainfall in N. Most rainfall in N. Africa from Africa from summer monsoonsummer monsoon

Vegetation Vegetation patterns driven by patterns driven by summer monsoon summer monsoon rainfall patternsrainfall patterns Rainforest near Rainforest near

equatorequator Desert scrub in Desert scrub in

the Saharathe Sahara

Orbital Monsoon Hypothesis Strength of monsoons are linked with the Strength of monsoons are linked with the

strength of insolation on orbital time scalesstrength of insolation on orbital time scales

John KurtzbachJohn Kurtzbach

Greater summerGreater summerinsolation intensifiedinsolation intensifiedwet summer monsoonwet summer monsoon

Decreased Decreased winterwinterinsolation insolation intensifiedintensifieddry winter dry winter monsoonmonsoon

Nonlinear Response of Climate More intense summer insolation maxima and More intense summer insolation maxima and

deeper winter minima always occur together at deeper winter minima always occur together at same locationsame location

So why don’t the effects simply cancel?So why don’t the effects simply cancel? One season dominates responseOne season dominates response

Significant rainfall only during summerSignificant rainfall only during summerOrbital-scale changes in winter insolation Orbital-scale changes in winter insolation

have no affect on annual rainfallhave no affect on annual rainfall An example of nonlinear responseAn example of nonlinear response

A strong net response to insolationA strong net response to insolationEven though rainfall sensitive to only one Even though rainfall sensitive to only one

season season

Evidence for Orbital-Scale Changes

Evidence should be in the Evidence should be in the 23,000 year cycle23,000 year cycle

Calculated June insolation at Calculated June insolation at 30°N30°N Today insolation lowToday insolation low 10,000 years ago high10,000 years ago high

Assumed that a critical Assumed that a critical threshold must be reachedthreshold must be reached Needed to drive strong Needed to drive strong

summer monsoonssummer monsoons Lake levels in N. Africa Lake levels in N. Africa

provide a test of hypothesisprovide a test of hypothesis

Three Assumptions: First Assume a critical threshold levelAssume a critical threshold level

Below level summer monsoon weakBelow level summer monsoon weakNo geologic record producedNo geologic record produced

Context of N. Africa monsoonContext of N. Africa monsoonRainfall must have been high enough Rainfall must have been high enough to fill lakesto fill lakes•Above a level that prevented Above a level that prevented evaporation during dry winterevaporation during dry winter

No lakes in Sahara Desert todayNo lakes in Sahara Desert today•Threshold insolation level well Threshold insolation level well above modern day levelabove modern day level

Second Assumption N. African lake level directly proportional N. African lake level directly proportional

to strength of the summer monsoonto strength of the summer monsoon i.e.i.e., the extent to which summer , the extent to which summer

insolation exceeds the critical thresholdinsolation exceeds the critical threshold Reasonable assumptionReasonable assumption

Greater summer insolationGreater summer insolationShould drive stronger monsoon Should drive stronger monsoon circulationcirculation•Increase rainfallIncrease rainfall•Increase lake levelsIncrease lake levels

Third Assumption Lake level records an average of several Lake level records an average of several

individual monsoon summersindividual monsoon summers Lake level is an average of several Lake level is an average of several

seasonal signalsseasonal signalsRepresents rainfall in summerRepresents rainfall in summerSince winters are drySince winters are dry

•Blends the strength of several Blends the strength of several summer monsoonssummer monsoons–Can be said of many geologic Can be said of many geologic climate recordsclimate records

Predicted Monsoon Response Response mimics Response mimics

shape of insolation shape of insolation curvecurve Truncated at a Truncated at a

threshold levelthreshold level Below which lakes Below which lakes

will not record will not record rainfallrainfallEvaporate in dry Evaporate in dry winterwinter

Note strong signals Note strong signals at 85,000 and at 85,000 and 130,000130,000

Lake Deposits No good geological or stratigraphic No good geological or stratigraphic

evidence for deposition of N. African lakesevidence for deposition of N. African lakes However, fresh water diatoms found in However, fresh water diatoms found in

tropical Atlantic Ocean sedimentstropical Atlantic Ocean sedimentsDiatoms could only have grown in Diatoms could only have grown in fresh water lakesfresh water lakes

Blown by strong winds off shore Blown by strong winds off shore (sometimes 1000’s of kilometers)(sometimes 1000’s of kilometers)Concentrated in discrete stratigraphic Concentrated in discrete stratigraphic horizonshorizons

Fresh Water Diatoms Diatoms must have Diatoms must have

grown in N. African grown in N. African LakesLakes During strong During strong

summer monsoonsummer monsoon During strong winter During strong winter

monsoonmonsoon Lakes dryLakes dry Winds strongWinds strong

Deflation occursDeflation occursDiatoms blown off Diatoms blown off

shore as aeolian shore as aeolian sedimentssediments

Diatom Deposition Lags Insolation Maximum

If the sequence of events is correctIf the sequence of events is correct Deposition of diatoms off shoreDeposition of diatoms off shore

Must lag insolation maximumMust lag insolation maximumTime needed for lakes to dry outTime needed for lakes to dry out

In addition, pulses of diatoms off shoreIn addition, pulses of diatoms off shore Should coincide with high amplitudeShould coincide with high amplitude

June insolationJune insolation•Since larger lakes would be expectedSince larger lakes would be expected

–More diatom-rich sediments More diatom-rich sediments available to blow off shoreavailable to blow off shore

Marine Deposition of Freshwater Diatoms

Lakes dry out when monsoonLakes dry out when monsoonweakens therefore diatomsweakens therefore diatomspulses at insolation minimumpulses at insolation minimum

Evidence for Monsoon Record Sapropel deposition in Mediterranean may Sapropel deposition in Mediterranean may

provide evidence for a 23,000 monsoonal cycleprovide evidence for a 23,000 monsoonal cycle Today, well oxygenated water give rise to Today, well oxygenated water give rise to

deposition of beige colored mud with benthic deposition of beige colored mud with benthic faunafauna Circulation due toCirculation due to

High evaporationHigh evaporationDense water Dense water

formation alongformation along

the north marginthe north margin

Sapropel Deposition May record strong summer monsoonMay record strong summer monsoon

Sapropel units rich in organic carbon Sapropel units rich in organic carbon suggesting high surface productivitysuggesting high surface productivity

No benthic fauna suggesting anoxic bottom No benthic fauna suggesting anoxic bottom waterswaters

Deep water formation cut off by low salinity capDeep water formation cut off by low salinity cap High runoffHigh runoff

Nile RiverNile River Stopped bottomStopped bottom water formationwater formation Supplied nutrientsSupplied nutrients

Sapropel Deposition during High Runoff

Sapropel deposition due to freshSapropel deposition due to freshwater inputs to Mediterraneanwater inputs to Mediterranean

Fresh Water Mediterranean? Nile River drains Nile River drains

eastern N. Africaeastern N. Africa Strong monsoon Strong monsoon

should bring rainfall should bring rainfall to Nile River to Nile River headlandsheadlands

Ancient river beds Ancient river beds found in Sahara found in Sahara Desert in Sudan and Desert in Sudan and ChadChad

Strong summer Strong summer monsoon should have monsoon should have driven high fresh water driven high fresh water discharge into discharge into MediterraneanMediterranean

Sapropel Deposition on 23,000 Cycle?

SapropelSapropel depositiondeposition

Beige clayBeige clay depositiondeposition

Sapropel Deposition on 23,000 Cycle!

Summer Monsoon and Atlantic Upwelling Strong N. African summer monsoon winds modify Strong N. African summer monsoon winds modify

equatorial Atlantic Ocean circulationequatorial Atlantic Ocean circulation Counter normal SE trade winds that drive strong Counter normal SE trade winds that drive strong

upwellingupwelling Results in weak upwelling and deep thermoclineResults in weak upwelling and deep thermocline

Normal Equatorial Atlantic Upwelling

During weak summer monsoon, strong SE trade During weak summer monsoon, strong SE trade winds push warm waters offshorewinds push warm waters offshore

Enhance upwelling of cold, nutrient-rich watersEnhance upwelling of cold, nutrient-rich waters Cause the thermocline to shallowCause the thermocline to shallow

Strong Summer Monsoon

Plankton preferringPlankton preferringwarm nutrient-warm nutrient-poor waterpoor waterfavoredfavored

When strong summerWhen strong summermonsoon winds monsoon winds weaken the SEweaken the SEtrade windstrade winds

Weak Summer Monsoon

Plankton preferringPlankton preferringcool nutrient-cool nutrient-rich waterrich waterfavoredfavored

When weak summerWhen weak summermonsoon allows strongmonsoon allows strongSE trade winds toSE trade winds toblow warm surfaceblow warm surfacewater away fromwater away fromequatorequator

Faunal Changes Preserved Record of faunal changes preserved in Record of faunal changes preserved in

tropical Atlantic sedimentstropical Atlantic sediments Ecosystem shifts change with upwellingEcosystem shifts change with upwelling Upwelling changes with strength of Upwelling changes with strength of

summer monsoonsummer monsoonEcosystems preserved in sedimentsEcosystems preserved in sediments

•Record the strength of N. African Record the strength of N. African summer monsoonsummer monsoon

Changes in the relative abundance of Changes in the relative abundance of environmentally-sensitive speciesenvironmentally-sensitive speciesRecord 23,000 year precessional Record 23,000 year precessional cyclecycle

Fauna Preserve Record of Monsoons

Complications with Orbital Monsoon Hypothesis

Peak monsoon development lags summer peak Peak monsoon development lags summer peak insolation maximuminsolation maximum Interactions with other parts of climate Interactions with other parts of climate

system?system?Perhaps development of monsoon Perhaps development of monsoon

influenced by N. hemisphere ice sheetsinfluenced by N. hemisphere ice sheetsOr by cooler ocean surface temperatures Or by cooler ocean surface temperatures

during glacial intervalsduring glacial intervals•Cold ocean poor source of latent heatCold ocean poor source of latent heat

Peak development of summer monsoon may Peak development of summer monsoon may be in phase with July 21 insolationbe in phase with July 21 insolation• July 21 insolation forcing N. African July 21 insolation forcing N. African

summer monsoonsummer monsoon Any of these explanations would only modify Any of these explanations would only modify

hypothesishypothesis

More complications Response of monsoon to Response of monsoon to

insolation changes is not linearinsolation changes is not linear There is a threshold There is a threshold

dependencedependenceAs a result of this clippingAs a result of this clipping

•Only a portion of the Only a portion of the 23,000 y cycles recorded23,000 y cycles recorded

•Can distort the way Can distort the way monsoons are recorded monsoons are recorded in climate recordin climate record– Cause artifactsCause artifacts

Clipping Artifacts If climate record If climate record

sensitive only to one sensitive only to one side of cycleside of cycle Dominate signal may Dominate signal may

show up as show up as eccentricity cycleeccentricity cycle

Eccentricity Eccentricity modulates modulates amplitude of amplitude of precessionprecession

Changes in Changes in eccentricity not eccentricity not forcing the responseforcing the response

Precession forcingPrecession forcing Yet without full recordYet without full record

Eccentricity Eccentricity appears strongappears strong

Harmonics Shorter cycles generated by clippingShorter cycles generated by clipping

For the 23,000 year cycleFor the 23,000 year cycleHarmonics have periods ofHarmonics have periods of

•N/2 = 11,500 yearsN/2 = 11,500 years•N/3 = 7,600 yearsN/3 = 7,600 years•N/4 = 5,750 years, etc.N/4 = 5,750 years, etc.

Harmonic cycles not present in original Harmonic cycles not present in original orbital signalorbital signalOr in change in the strength of monsoonOr in change in the strength of monsoonArtifacts of biased way climate system Artifacts of biased way climate system

recorded response to orbital changes in recorded response to orbital changes in insolationinsolation