Insolation Control of Monsoons n Monsoonal circulation results

Insolation Control of Monsoons n Monsoonal circulation results from seasonal changes in solar radiation u Logical to assume that orbital scale seasonal changes in insolation t Can cause changes in the strength of monsoonal circulation

Modern Monsoons Strong summer monsoons exist in N. hemisphere u Large landmasses in tropical regions n Weaker in S. hemisphere u Land masses in tropical and subtropical regions are generally smaller n N. Africa good example u Strong summer monsoon u Sediments deposited off-shore document a record of monsoons in region n

Monsoon Circulation over N. Africa n n Strong summer heating creates low-pressure over west-central N. Africa drawing moisture from tropical Atlantic u Wet summer monsoon Winter cooling creates high pressure in northwest Sahara Desert enhancing flow of the northern trade winds u Dry trade winds inhibit precipitation

Summer Monsoon Controls Vegetation Most rainfall in N. Africa from summer monsoon n Vegetation patterns driven by summer monsoon rainfall patterns u Rainforest near equator u Desert scrub in the Sahara n

Orbital Monsoon Hypothesis n 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 n n n More intense summer insolation maxima and deeper winter minima always occur together at same location So why don’t the effects simply cancel? u One season dominates response t Significant rainfall only during summer t Orbital-scale changes in winter insolation have no affect on annual rainfall An example of nonlinear response u A strong net response to insolation t Even though rainfall sensitive to only one season

Evidence for Orbital-Scale Changes n n Evidence should be in the 23, 000 year cycle Calculated June insolation at 30°N u Today insolation low u 10, 000 years ago high Assumed that a critical threshold must be reached u Needed to drive strong summer monsoons Lake levels in N. Africa provide a test of hypothesis

Three Assumptions: First n Assume a critical threshold level u Below level summer monsoon weak t No geologic record produced u Context of N. Africa monsoon t Rainfall must have been high enough to fill lakes • Above a level that prevented evaporation during dry winter t 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 u i. e. , the extent to which summer insolation exceeds the critical threshold n Reasonable assumption u Greater summer insolation t Should drive stronger monsoon circulation • Increase rainfall • Increase lake levels n

Third Assumption n Lake level records an average of several individual monsoon summers u Lake level is an average of several seasonal signals t Represents rainfall in summer t Since winters are dry • Blends the strength of several summer monsoons – Can be said of many geologic climate records

Predicted Monsoon Response mimics shape of insolation curve u Truncated at a threshold level u Below which lakes will not record rainfall t Evaporate in dry winter u Note strong signals at 85, 000 and 130, 000

Lake Deposits n No good geological or stratigraphic evidence for deposition of N. African lakes u However, fresh water diatoms found in tropical Atlantic Ocean sediments t Diatoms could only have grown in fresh water lakes u Blown by strong winds off shore (sometimes 1000’s of kilometers) t Concentrated in discrete stratigraphic horizons

Fresh Water Diatoms n n Diatoms must have grown in N. African Lakes u During strong summer monsoon During strong winter monsoon u Lakes dry u Winds strong t Deflation occurs t Diatoms blown off shore as aeolian sediments

Diatom Deposition Lags Insolation Maximum If the sequence of events is correct u Deposition of diatoms off shore t Must lag insolation maximum t Time needed for lakes to dry out n In addition, pulses of diatoms off shore u Should coincide with high amplitude t June insolation • Since larger lakes would be expected – More diatom-rich sediments available to blow off shore n

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

Evidence for Monsoon Record n n 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 u Circulation due to t High evaporation t Dense water formation along the north margin

Sapropel Deposition n n May record strong summer monsoon u Sapropel units rich in organic carbon suggesting high surface productivity u No benthic fauna suggesting anoxic bottom waters Deep water formation cut off by low salinity cap u High runoff t Nile River u Stopped bottom water formation u Supplied nutrients

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

Fresh Water Mediterranean? n n Nile River drains eastern N. Africa u Strong monsoon should bring rainfall to Nile River headlands u 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 n n n 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 n n n During weak summer monsoon, strong SE trade winds push warm waters offshore Enhance upwelling of cold, nutrient-rich waters Cause 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 n Record of faunal changes preserved in tropical Atlantic sediments u Ecosystem shifts change with upwelling u Upwelling changes with strength of summer monsoon t Ecosystems preserved in sediments • Record the strength of N. African summer monsoon u Changes in the relative abundance of environmentally-sensitive species t Record 23, 000 year precessional cycle

Fauna Preserve Record of Monsoons

Complications with Orbital Monsoon Hypothesis n Peak monsoon development lags summer peak insolation maximum u Interactions with other parts of climate system? t Perhaps development of monsoon influenced by N. hemisphere ice sheets t Or by cooler ocean surface temperatures during glacial intervals • Cold ocean poor source of latent heat t Peak development of summer monsoon may be in phase with July 21 insolation • July 21 insolation forcing N. African summer monsoon u Any of these explanations would only modify hypothesis

More complications n Response of monsoon to insolation changes is not linear u There is a threshold dependence t 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 n If climate record sensitive only to one side of cycle u Dominate signal may show up as eccentricity cycle t Eccentricity modulates amplitude of precession u Changes in eccentricity not forcing the response t Precession forcing u Yet without full record t Eccentricity appears strong

Harmonics n Shorter cycles generated by clipping u For the 23, 000 year cycle t Harmonics have periods of • N/2 = 11, 500 years • N/3 = 7, 600 years • N/4 = 5, 750 years, etc. u Harmonic cycles not present in original orbital signal t Or in change in the strength of monsoon t Artifacts of biased way climate system recorded response to orbital changes in insolation