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Quaternary Science Reviews 26 (2007) 3043–3057

Sensitivity of the Australian summer monsoon to tiltand precession forcing

Karl-Heinz Wyrwolla,�, Zhengyu Liub, Guangshan Chenc, John E. Kutzbachb, Xiaodong Liuc

aSchool of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA 6009, AustraliabCenter for Climatic Research, University of Wisconsin-Madison, WI, USA

cState Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China

Received 24 November 2006; received in revised form 31 May 2007; accepted 12 June 2007

Abstract

The response of the Australian summer monsoon to orbital forcing is studied using a coupled General Circulation Model (GCM) with

the focus on the relative roles of tilt and precession on the forcing of the northern Australian summer monsoon. It was found that unlike

the Northern Hemisphere monsoons, which are dominated by precession forcing, the Australian monsoon can be enhanced significantly

not only by precession forcing, but also by tilt forcing coupled to oceanic feedback. The new insights obtained from a series of

experiments with differing tilt-precession configurations allow an interpretation of the Australian Late Quaternary monsoon record in

which insolation forcing plays a significant role.

r 2007 Elsevier Ltd. All rights reserved.

1. Introduction

1.1. Aims and context

The recognition of Milankovitch insolation variation asa first-order forcing mechanism of low-latitude monsoonregimes has a long history and has met with considerablesuccess (e.g. Kutzbach, 1981; Kutzbach and Street-Perrott,1985; Liu et al., 2003b; Ruddiman, 2006). In the context ofinsolation forcing of the Australian summer monsoon,differing emphases prevail: (i) the suggestion that low-latitude summer insolation maxima/minima are likely to beof considerable importance for explaining monsoon states(Chappell and Syktus, 1996; Bowler et al., 2001; Wyrwolland Valdes, 2003; Holbourn et al., 2005); (ii) the rejectionof the importance of Southern Hemisphere insolation andan appeal to inter-hemispheric forcing through the strengthof the East Asian winter monsoon, related to insolationlows in that region (Magee et al., 2004; Miller et al., 2005);(iii) the view that land-cover changes resultant onAboriginal vegetation burning practices may have playedan important role over late Quaternary time-scales

e front matter r 2007 Elsevier Ltd. All rights reserved.

ascirev.2007.06.026

ing author. Tel.: +618 6488 2714; fax: +61 8 6488 1037.

ess: wyrwoll@segs.uwa.edu.au (K.-H. Wyrwoll).

(Johnson et al., 1999; Miller et al., 2005) and (iv) that seasurface temperatures can over-ride insolation controls (Liuet al., 2003b, 2004).A full evaluation of these claims is hampered by the

absence of a broadly based conceptual understanding ofthe likely sensitivity of the Australian monsoon system toinsolation forcing. To overcome this we have undertaken aseries of model experiments in which the precession and tiltorbital parameters differ and through this to prescribevariations in insolation. We evaluate the model response ofthe Australian summer monsoon to the imposed insolationchanges and assess the sensitivity of the ‘model’ monsoonregime to such changes. The model findings are thenrelated to the Late Quaternary record of Australianpalaeomonsoon events, and their skill in explaining thestratigraphic record evaluated.

1.2. Model experiments

The model used is the fast ocean–atmosphere model(FOAM), a fully coupled ocean–atmosphere model with-out flux adjustment. The atmosphere component ofFOAM is a fully parallel version of the NCAR CCM2 inwhich the atmospheric physics are replaced by those of

ARTICLE IN PRESSK.-H. Wyrwoll et al. / Quaternary Science Reviews 26 (2007) 3043–30573044

CCM3 (Jacob, 1997). The atmosphere component has R15resolution (equivalent to grid spacing of about 7.51longitude and 41 latitude). The ocean component wasdeveloped following the Geophysical Fluid Dynamics

Fig. 1. Model climatology January–February–March (JFM): (a) precipi-

tation (mm/day); (b) sea level pressure (hPa); (c) sea surface temperature

(1C).

Laboratory Modular Ocean Model (GFDL MOM) witha resolution of 1.41 latitude and 2.81 longitude, 32 levels.The model climatology shows a reasonable approxima-

tion of the present conditions prevailing over the monsoonregion of northern Australia (Fig. 1), with the qualificationthat in the Control experiment the centre of the heat lowover western Australia sits too far south by �101, a featurethat may be associated partly with the coarse resolution ofthe atmosphere model.In order to evaluate the possible influence of orbital

changes on the Australian monsoon, combinations of tiltand precession were specified in four sensitivity experi-ments (Table 1), with the resultant insolation changesshown in Fig. 2. Each sensitivity experiment is integratedfor 400 years starting from the end state of a long control.The orbital parameters for these sensitivity experimentsfollow the atmospheric model experiments of Prell andKutzbach (1987), with a high tilt/low tilt (TH/TL), and ahigh (Northern Hemisphere) precession/ low (SouthernHemisphere) precession (PH/PL). Over the SouthernHemisphere, a high (Northern Hemisphere) precession(Fig. 3(b)) or a low tilt (Fig. 3(a), with the minus sign)corresponds to a low summer insolation relative to theNorthern Hemisphere, or an increased insolation gradienttowards the Southern Hemisphere, which should favour areduced summer monsoon over Australia.To better understand the combined as well as the

individual roles of tilt and precession, the four experimentswill be discussed in two sets of anomaly differences. In thefirst set, each experiment will be discussed in terms of itsdifference from the Control (CTRL) that is, THPH-CTRL,TLPH-CTRL, THPL-CTRL and TLPL-CTRL. Thisreflects the combined tilt and precession effects. Note inTable 1 that low precession (PL ¼ Southern Hemisphereprecession) is very similar to the precession of the Controlconfiguration. The difference of THPL-CTRL and TLPL-CTRL mainly reflects the tilt effect. In the second set, theexperiments will be discussed as the differences of THPH-TLPH, THPL-TLPL and THPH-THPL and TLPH-TLPL:the first pair of experiments mainly reflect the increased tilteffect alone, with a slight modulation due to a differentprecession; the last pair of experiments reflect the increasedprecession effect alone, with a slight difference modulatedby different tilt.

Table 1

Experimental design—the orbital configurations defined for the model

experiments

Experiment Perihelion Tilt (1)

Control January 4 23.45

Tilt high precession high (THPH) June 15 24.45

Tilt high precession low (THPL) December 15 24.45

Tilt low precession high (TLPH) June 15 22.45

Tilt low precession low (TLPL) December 15 22.40

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Fig. 2. Insolation changes (W/m2) for different tilt and precession configurations and mean annual insolation: (a) THPH-CTRL; (b) TLPH-CTRL; (c)

THPL-CTRL; (d) TLPL-CTRL.

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2. General climatology of the model experiments

The model climatology—precipitation, wind and sealevel pressure and sea surface temperatures (SSTs) for thefour experiments are shown in the respective panels of Figs.4, 5 and 7.

2.1. General trends in insolation–precipitation response of

the monsoon region

As a first approximation, the model results demonstratea general relationship in the trends between insolation andprecipitation. With a reduction in summer insolation overthe low latitudes of the Southern Hemisphere (Figs. 2(a)and (b)) associated with a high precession (i.e. NorthernHemisphere precession), precipitation shows a generalpattern of lower daily precipitation rates for much of themonsoon region of northern Australia (Fig. 4(a) and (b)).In addition, the model results indicate an important rolefor tilt forcing on precipitation. This is evident by the factthat the precipitation reduction is weaker in THPH (Fig.4(a)) than in TLPH (Fig. 4(b)), which results fromincreased Southern Hemisphere insolation associated withincreased tilt in the former configuration. In both experi-ments the reduction of precipitation over northernAustralia is accompanied by an increase in daily precipita-tion over the Maritime Continent. This south–north

contrast is strongest when the reduction in summerinsolation forms a focused band extending to the highlatitudes of the Southern Hemisphere, reaching values of�80W/m2.For the two experiments dominated by tilt effects

relative to the Control (THPL, TLPL), the combined tiltand precession changes result in insolation increases overnorthern Australia that amount to slightly in excess of20W/m2. The two experiments differ significantly intheir respective insolation distribution patterns, due toopposing tilt effects (TH/TL), with these having differingclimate expressions. With high tilt, a strong southwardinsolation gradient for the Southern Hemispheresummer (Fig. 2(c)) leads to a significant increase in dailyprecipitation of up to 3mm/day for the northern half ofthe continent (Fig. 4(c)). For this experiment, the modelresults show that the region north of the Australianlandmass, and the Cape York region of Australia, bothexperience a reduction in the daily rainfall rate amountingto �3mm/day. Conversely, with a low tilt, the insolationchanges extend into the winter Northern Hemisphere,reducing the southward insolation gradient (Fig. 2(d)).This results in a very weak increase in the dailyprecipitation rate of up to 1mm/day over a very restrictedregion of northern Australia with an equally minorreduction in precipitation over the Maritime Continent(Fig. 4(d)).

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Fig. 3. Insolation changes (W/m2) for separate effects of tilt and

precession and mean annual insolation: (a) THPH-TLPH ¼ tilt; (b)

THPH-THPL ¼ precession.

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2.2. Wind and pressure changes over the monsoon region of

Australia

Over the monsoon region of northern Australia surfacewind changes reveal expected trends that correspond to theprecipitation patterns outlined above and match inflow(outflow) patterns into the northern parts of the Australiancontinent, as well as corresponding sea level pressurechanges (Fig. 5(a) and (b)). The most marked pressureresponse over northern Australia is related to the strongerSouthern Hemisphere focused insolation decrease (Fig.2(b)) resulting in an increase in pressure of up to 2 hPa.Under this orbital configuration (TLPH-CTRL) outflowfrom East Asia is significantly reduced, as is inflow intonorthern Australia.

The results of the reduced insolation experiment (THPH-CTRL) show no significant sea level pressure change overnorthern Australia but with reduced sea surface pressureover the adjacent eastern Indian Ocean. A significantpressure increase occurs over East Asia, resulting in anEast Asia–northern Australia pressure gradient change of4 hPa (Fig. 5(a)). The increased pressure gradient results instrong outflow from East Asia, extending over theMaritime Continent but fails to reach northern Australia.Instead, a significant decrease in the inflow into northern-most Australia is evident, but with stronger inflow, sourcedfrom the tropical Indian Ocean, affecting southerncontinental margins of the northwest Australia monsoon

region and extending further south. The inflow isassociated with a region of increased precipitation fringingthe continental margin (Fig. 4(a)).For the two increased insolation experiments only

THPL-CTRL shows an increase of inflow into thecontinent. This is sourced from the tropical Indian Oceanand the Maritime Continent. With the prescribed orbitalconfiguration and resultant increased insolation, nopressure change is registered over northern Australia. ForTLPL-CTRL, a minor reduction of inflow into northernAustralia is evident but with no accompanying pressurechange (Fig. 5(d))—these insignificant changes are mir-rored by very minor changes in precipitation (Fig. 4(d)).

2.3. Precipitation changes and land–ocean contrast

Pronounced differences between the northern monsoonregion of the Australian continent and the adjacentnorthwest Indian Ocean are evident in the results. As anexample, the results of the experiment THPH-CTRL showa very striking difference between the land area of northernAustralia which experiences a reduction in precipitation,while the adjacent northwest Indian Ocean sees asignificant increase (Fig. 4(a)). Differences between landand ocean response are also evident in TLPH-CTRL andto a lesser extent TLPL-CTRL (Fig. 4(b) and (d)). In orderto highlight the differences in land-ocean response, withdifferent orbital configurations, zonal averages wereobtained for the longitude bands 801E–1101E and1101E–1401E on the oceanic and land points, respectively(Fig. 6). The results of two experiments (THPH-CTRL andTLPH-CTRL) are of special significance because theyhighlight the importance of variations in tilt in provokingdifferent land–ocean precipitation responses, in combina-tion with a high precession:

(i)

with tilt high and precession high (THPH-CTRL) thezonal sector 801E–1101E (ocean) shows a precipitationpeak (�3mm/day) extending from 101S to 201S, withprecipitation increasing by �3mm/day. A decrease inthe daily rate of precipitation amounting to 2.5mm/dayoccurs on the equatorward side of the peak. The regionof increase in precipitation corresponds to a stronggradient in SSTs from �0.5 1C in the east to �1.5 1C inthe west (Fig. 7(a)), strong convergence characterisesthis region with resulting instability accounting forincreased precipitation. Over the land sector(1101E–1401E), the monsoon region of Australia showsa strong decrease in precipitation, with the precipita-tion peak displaced north by �51,

(ii)

with tilt low and precession high (TLPH-CTRL) theocean sector (801E–1101E) shows a very slight pre-cipitation increase of up to 0.5mm/day centred on 201S,with a general decrease in precipitation between 201Sand 201N. Under this orbital configuration the changesin the land sector (1101E–1401E) are both different andstronger, with a 51 northward displacement of peak

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Fig. 4. January–February–March precipitation (mm/day) and surface wind: (a) THPH-CTRL; (b) TLPH-CTRL; (c) THPL-CTRL; (d) TLPL-CTRL.

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precipitation to approximately 51S. A significantdecrease in precipitation amounting to more than2mm/day centred on 181S is evident, with a higherdaily precipitation increase at 51S.

In both cases there is clearly a strong difference in theresponse of the two zonal sectors, with the land sectorshowing a strong response, while the ocean sector hasregistered relatively small changes. The marked differencein the response of the two experiments is related to changesin tilt. Similarly, a land–ocean contrast, albeit weak, isevident for TLPL-CTRL, with the land sector showinglittle change in precipitation while the ocean sector registersa decrease of �1mm/day centered on 201S.

2.4. Separating the relative importance of tilt and precession

changes in controlling monsoon activity

The seemingly complex response to insolation forcing, asoutlined above, can be clarified through separating therespective roles of tilt and precession. Fig. 3 shows theinsolation differences that result from separating tilt andprecession changes. It is evident that with tilt high (THPH-

TLPH) the low latitudes of the Southern Hemisphereexperience an increase in summer insolation, whiledecreases occur with precession high (THPH-THPL).Separating precession and tilt provoke quite a differentclimate response. A change to a Southern Hemispheresummer perihelion (PL), with tilt kept at high, results in astrong increase in the precipitation rate over northernAustralia, to as much as 5mm/day (Fig. 8(a)). Thisincreases contrasts with a decrease in the precipitation rateover the adjacent Indian Ocean, coinciding with increasesin sea surface pressure and higher SSTs (Fig. 8(b) and (c)).The response is almost identical for TLPL-TLPH (notshown), reflecting little modulation on the precession effectby tilt. Similarly, increasing tilt, but with the precessionkept at high (THPH-TLPH) results in a higher precipita-tion rate over a broad area of northern Australia and theadjacent Indian Ocean and Pacific Ocean domains(Fig. 9(a)). The region of increased precipitation corre-sponds in part to sea surface pressure decreases andincreases in SSTs that extend into the more southern partsof the Indian Ocean (Fig. 9(b) and (d)). Similar results areobtained for THPL-TLPL, reflecting the small modulationrole by precession on the tilt effect.

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Fig. 5. January–February–March sea level pressure (hPa): (a) THPH-CTRL; (b) TLPH-CTRL; (c) THPL-CTRL; (d) TLPL-CTRL.

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It is interesting that in spite of the similar responses totilt (high tilt) and precession (low precession) forcing overland, the precipitation responses over ocean differ drama-tically. With low precession (Fig. 8(a)), rainfall is reducedover the adjacent eastern Indian Ocean, opposite to therainfall increase over northern Australia. In contrast, withhigh tilt (Fig. 9(a)), the precipitation is increased over theadjacent eastern Indian Ocean, the same as over northernAustralia. These different responses reflect the differentseasonality of the two forcings. Precession forcing iscompletely seasonal, and therefore has a dominantseasonal monsoon response. As pointed out by Liu et al.(2003b, 2004), with an increased summer insolation in theSouthern Hemisphere at a low precession, the subtropicalhigh strengthens over the Indian Ocean (Fig. 8(a) and (b)),the intensified coastal wind increases the coastal upwellingand leads to an anomalous coastal cooling along thewestern coast of Australia, indicated by a smaller SSTincrease along the coast than in the interior ocean(Fig. 8(c)). The wind then diverges from the Australiancontinent towards the Indian Ocean, reducing the rainfallover the eastern Indian Ocean, in combination with themonsoon rainfall along the western coast of Australia, the

latter countering the increased rainfall further inlandinduced by the increased insolation forcing.Unlike precession forcing, tilt forcing has a significant

mean annual component (see Fig. 3(a), right panel). Hightilt cools the equatorial region, and therefore imposes anannual mean poleward insolation gradient. This forcingtends to provoke a similar temperature change over landand ocean on the same latitude, with less land–seatemperature contrast (Liu et al., 2003c). With an increasedtilt, the equatorial belt tends to be colder, forcing aconverging surface flow towards the extratropics over boththe land and ocean. Therefore, opposite to the precessionforcing, the same land–sea rainfall response now enhancesthe Australian monsoon response. As a result, in spite of aweak summer insolation value over the Australiancontinent for tilt (20W/m�2, Fig. 3(a)) than for precession(80W/m�2, Fig. 3(b)), the enhanced Australian monsoonrainfall becomes almost comparable.Having separated the roles of tilt and precession, we can

now better understand the sensitivity experiments relativeto the Control (Fig. 4). Over the land sector the climateresponse is straightforward with precession high (i.e.Northern Hemisphere precession) inducing a monsoon

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Fig. 6. Zonal averaged precipitation response for Experiments-CTRL in:

(a) the ‘ocean’ sector as the area average in the longitude belt of

801E–1101E; (b) the ‘land’ sector as the area average of longitude belt of

1101E–1401E.

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precipitation decrease response over the northern Austra-lian region. In the two experiments in which precessionhigh is specified (THPH and TLPH) the monsoon regionshows changes in daily precipitation rates of more than�5mm/day. The precipitation decrease with respect to theControl experiment is greatest with TLPH, while withTHPH a less significant decrease occurs.

Over the ocean sector the concomitant changes are morecomplex. Here SST changes correspond to changes inprecipitation (Fig. 7). There is a minor increase inprecipitation (with respect to CTRL) of 1–2mm/day withTLPH, with the precipitation increase significantly greaterwith THPH amounting to as much as 5mm/day (withrespect to CTRL). In the THPH-CTRL experiment thearea of precipitation increase is centred on 181S–901E andcoincides with convergence, SST decreases of 0.5–1.5 1Cand sealevel pressure changes of between �1 and �2 hPa.

Over the land sector, the strongest increase in precipita-tion over the Australian monsoon region occurs withthe orbital configuration tilt high and precession low(i.e. Southern Hemisphere precession; THPL-CTRL, see

Fig. 4). Changing tilt (TLPL-CTRL) decreases precipita-tion both over the continent and ocean, with ocean changesamounting to as much as 3mm/day.In summarising the respective roles of tilt and precession

in forcing monsoon changes in the Australian summermonsoon regime, we emphasise the different responses ofthe ocean sector in the longitudes east of Australia,801E–1101E (Fig. 10(a)) and the land sector in thelongitudes of the Australia continent, 1101E–1401E(Fig. 10(b)):

(i)

a clear contrast exists between the ocean and landsectors in their response to both tilt and precessionforcing. The summer rainfall over Australia in the landsector is enhanced by high tilt (THPH-TLPH, THPL-TLPL), and is reduced by high precession (THPH-THPL, TLPH-TLPL). This is in contrast to theresponse over the eastern Indian Ocean adjacent toAustralia where rainfall is increased for both high tiltand high precession. The increase of monsoon rainfallover northern Australia (centred at 201S) due to tiltchanges is significant, and in magnitude is comparableto the increase due to reduced precession forcing. Theresponse of the Australian monsoon to both tilt andprecession forcing is in contrast to the NorthernHemisphere Asian monsoon, which shows a muchstronger response to precession than to tilt (Prell andKutzbach, 1987; Liu et al., 2003a);

(ii)

under all the forcings, the rainfall responses overnorthern Australia and the adjacent eastern IndianOcean are opposite to those to the north in theequatorial region, reflecting a migration of the rainfallbelt between the Equator and to the south;

(iii)

the monsoon response over Australia seems to bedominated by the response to local Southern Hemi-sphere insolation. Indeed, there is little simultaneousresponse in the Northern Hemisphere to the tilt forcingand only a minor response to the precession forcing, asshown in Fig. 10.

3. Significance of modelled insolation changes for the

interpretation of the Australian Quaternary palaeomonsoon

record

In the context of the FOAM model domain, theAustralian summer monsoon is responsive to both tiltand precession, with the added qualifier that tilt plays arole of greater importance than expected from its minorcontribution to overall insolation changes. Stratigraphicsupport for the importance of precession in regulating theAustralian summer monsoon is provided by Holbourn etal. (2005). These authors obtained a high-resolution recordfor the last 460 ka of palaeoproductivity events in theTimor Sea (easternmost Indian Ocean). The record acts asa proxy for northwest Australian monsoon activity and

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Fig. 7. January–February–March sea surface temperature (1C): (a) THPH-CTRL; (b) TLPH-CTRL; (c) THPL-CTRL; (d) TLPL-CTRL.

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spectral analysis exhibits a strong peak in the precessionalband, with high coherence with the 21 December insolationat 251S. Statistically significant spectral peaks at 41 ka(obliquity) are present in two of their proxies—benthicforaminiferal (90% significance level) and chlorins con-centration (80% significance level) records. Additionalsupport for the importance of both precession andobliquity in driving northern Australian monsoon climatesis provided by the results of Kershaw et al. (2003), whoshow a strong obliquity signal in pollen records recoveredfrom deep sea cores located in the wider Australianmonsoon region.

Other model experiments (Chappell and Syktus, 1996;Wyrwoll and Valdes, 2003) support the present results thatSouthern Hemisphere insolation forcing is a direct controlof the Australian summer monsoon. In contrast to this isthe view that the Australian summer monsoon is not forcedby Southern Hemisphere orbital driven insolation varia-tions, but by the strength of the East Asian Winter High(Magee et al., 2004; Miller et al., 2005). Outflow from theEast Asian winter monsoon and its influence on the activityof the Australian summer monsoon regime has been widely

discussed (Suppiah, 1992; McBride, 1998; Wang et al.,2003; Hung et al., 2004) and generally recognised to be anelement in triggering active monsoon phases associatedwith ‘cold surges’ which provoke deep convection over theMaritime Continent. Hence, there is good reason to expectthe strength of outflow from the East Asian wintermonsoon to be a control of the Australian summermonsoon over Quaternary time scales (e.g. Liu and Ding,1998; An, 2000). This argument requires the realisationthat a heat low ‘attractor’ has to exist over the monsoonregion of Australia to direct low-level westerly flow into thecontinent.Fig. 11 shows Late Quaternary ‘lake high events’ at Lake

Eyre (an ephemeral lake located to the southwest of anextensive catchment (1.2� 106 km2) in central Australia)and corresponding insolation for January 201S andJanuary 501N for the last �140 ka (see Magee et al.,2004). The figure makes it clear that any claim that theselake events are directly driven by either the strength of theEast Asian High—the surrogate for this being insolationreductions over East Asia—or the strength of insolationover the low latitudes of Australia encounters problems,

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Fig. 8. The effect of precession changes with high tilt (THPL-THPH) for

January–February–March: (a) precipitation (mm/day) and surface wind;

(b) sea level pressure (hPa); (c) sea surface temperature (1C).

Fig. 9. The effect of tilt changes with precession high (THPH-TLPH) for

January–February–March: (a) precipitation (mm/day) and surface wind;

(b) sea level pressure (hPa); (c) sea surface temperature (1C).

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with points of correspondence and disagreement possiblefor both claims.

A more convincing interpretation is obtained from theresults of the FOAM experiments presented here. Thesemake it clear that it is not simply total insolation receivedthat should be considered, but indicate the additional need

to partition the roles of precession and tilt in forcing theAustralian summer monsoon (Fig. 12). Interpreting thestratigraphic record within this context indicates thatmonsoon intensity over northern Australia is strongestwith THPL, so high lake stages are to be expected, centredon �130, �90, �45 ka, taking into consideration the

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Fig. 10. Tilt and precession effects on precipitation for: (a) the ‘ocean’

sector as the area average in the longitude belt of 801E–1101E; (b) the

‘land’ sector as the area average of longitude belt of 1101E–1401E.

Lake Eyre

January insolation at 50°N

January insolation at 20°S

400

200

200

220

240

250

1050

1000

950

900

850

-140 -120 -100 -80

Time (ka)

Insola

tion W

. m-2. d

-1In

sola

tion W

. m-2. d

-1Lake v

olu

me (

km

3)

-60 -40 -20 0

0

Fig. 11. Location of Lake Eyre catchment (grey shaded area), lake

volumes of Lake Eyre and January insolation at 201S and 501N (after

Magee et al., 2004).

K.-H. Wyrwoll et al. / Quaternary Science Reviews 26 (2007) 3043–30573052

decreasing insolation trend. Such claimed events showagreement with the stratigraphic record. In addition,monsoon events can be expected to occur at �115 and�70 ka. For these two events, while insolation is high,corresponding with a Southern Hemisphere summerprecession, tilt is low. These events correspond to theFOAM TLPL-CTRL experiment, indicating a slightincrease in monsoon activity (above CTRL). In general,there is a reasonable correspondence between lake eventsand insolation–tilt–precession relationships. A point ofcontention is the lake high stage centred on 120 ka. Thislake phase (V) is claimed to have lasted from �130 to106 ka, being consistently high during this time. If it isassumed that insolation is a direct driver of paleomonsoonevents, it would seem likely that a decrease in monsoonactivity can be expected centred on 125 ka, for which nostratigraphic evidence has been indicated (Magee et al.,2004). In addition, our model results would suggest that alake full stage prevailed at a time centred on 140 ka, priorto which the monsoon would have been considerablyweaker and Lake Eyre should have been dry. The claimdoes not appear to meet with the stratigraphic findings, but

given the poor resolution of the dates during this timeperiod (see Magee et al., 2004) it is not really possible todraw a conclusion either way. A further issue is theposition of the lake event centred at 63.571.3 ka, which isdisplaced from the insolation event that occurs approxi-mately 5 ka earlier.The presence of an active Australian monsoon during

the latest Pleistocene–Holocene boundary coinciding witha time of low insolation has attracted some attention(Wyrwoll et al., 1992; Wyrwoll and Miller, 2001; Liu et al.,2003b, 2004; Magee et al., 2004; Miller et al., 2005). Theearly work claimed an absence of or reduced monsoonactivity until the early Holocene (Wyrwoll et al., 1992).When additional stratigraphic evidence became availableindicating an active monsoon by ca 14 ka this view had tobe revised, but doubt still remained as to the strength of themonsoon during the millennia centred on the Pleistoce-ne–Holocene boundary, coinciding with the insolation low

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at that time (Wyrwoll and Miller, 2001). From firstprinciples, it would appear that an apparent activemonsoon at a time of insolation low challenges thepossibility of direct low latitude Southern Hemisphereinsolation forcing of monsoon activity (Miller et al., 2005).In fact, this may not be as much of an issue as previouslythought, as the more recent stratigraphic evidence clearlyshows the prevalence of a monsoon at that time which wasconsiderably weaker than other active monsoon periods ofthe last 130 ka (Magee et al., 2004). An inference that couldbe drawn is that this is due to the summer insolation low,which prevailed over the low latitudes of the SouthernHemisphere during the Pleistocene–Holocene transition.A response of the Australian monsoon to the lowinsolation at that time is also suggested by Holbourn etal. (2005) and Holbourn (pers. comm.), who note thatdecreasing insolation is paralleled by the decrease inproductivity proxy values.

The model results make it evident that an active, albeitweak, monsoon can be expected over northern Australiadespite an insolation low over the region. This results fromthe fact that the pressure gradient between 601N (winter)and the summer monsoon region in the Australian sector isgreatest with THPH amounting to 6 hPa (with respect toCTRL) over the longitude band 801E–1101E, the moreappropriate longitude band for ‘capturing’ the East AsianWinter High (Fig. 13(a)). Ultimately, this leads tomonsoon inflow into the margins of the continent, but itonly does so through the controlling role of SSTs. TheTHPH-CTRL simulation shows a region of increased(decreased) SSTs (pressure) in the Indian Ocean to the

Tilt(degees)

24.5

24

23.5

23

22.5

22

0.06

Precession(e sinω)

0.04

0.02

0

-0.02

-0.04

-0.06

-160 -120

T

Fig. 12. January insolation and orbital variations at 151S for the last 150 ka for

and relative Lake Eyre lake levels (meters relative to Australian Height Datum

northwest of Australia (Fig. 7(a)), which corresponds tostrong convergence of the surface winds and higherprecipitation (Fig. 4(a)). The overall pattern of change inthe climate variables shows a strong resemblance to thatdescribed by Liu et al. (2003b) in their 11 ka simulation.Their model results indicate that with the development ofwarm SSTs off northwestern Australia inflow is enhancedand with it monsoon precipitation, despite the thenprevailing insolation low. Their experiments showed thatwithout the SST changes, the monsoon over northwesternAustralia would have been weaker in response to theinsolation low. From the two sets of experiments it is clearthat despite the Southern Hemisphere summer insolationlow at ca 9 ka of �440W/m2 coupled to a high tilt value, areasonably active monsoon can be expected, but onlybecause it occurred in association with a high tilt and thecontrolling influence of SSTs.The importance of the SST anomaly and the resultant

atmospheric response is further evidenced in the zonalaverage (801E–1101E) U (latitudinal) wind. This attains itshighest value at �171S, under a THPH orbital configura-tion, potentially giving rise to strong inflow into north-western Australia (Fig. 14(a)). However, for the sector1101E–1401E, the strongest U wind anomalies correspondto both THPH and TLPH, and are centred on the Equatorto about 101S, corresponding to a strong precipitationincrease over the Indonesian–New Guinea region. In thissector there is a decrease in the U wind component centredat 201S, with THPH (Fig. 14(b)). Clearly, the strong Uwind anomaly at 171S in the sector 801E–1101E, has alimited ‘inland’ extent, accounting for the rim of increased

Insolation(Wm-2)

-80

ime (ka)

-40 0

500

480

460

440

420

400

the monsoon region of northern Australia (after Berger and Loutre, 1991)

)—indicated by ‘rectangles’ (after Magee et al., 2004).

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precipitation over the margins of the Australian continent(Fig. 4(a)). A continental ‘rim’ of monsoon precipitation isalso evident in the earlier results of Liu et al. (2003b).Similarly, the 11 ka NCAR CSM summer precipitationpattern shows a slight increase (1mm/day) above present,confined to the margins of northwestern Australia, with theremainder of the northwest Australian monsoon regionexhibiting a decrease of �1mm/day in the summer seasonprecipitation. The inference that can be drawn from themodel results is that an active but spatially restrictedmonsoon can be expected over the Australian monsoonregion during the latest Pleistocene–early Holocene, whichsits comfortably with the stratigraphic record, thatprovides evidence of a weak monsoon over the morecentral parts of the Australian continent (Magee et al.,2004).

4. Discussion

The experiments have demonstrated that, within thecontext of the limitations imposed by the model, theAustralian monsoon is sensitive to variations in Milanko-vitch-scale insolation changes. At the most general level,

Fig. 13. Zonal average sea level pressure changes: (a), (b) combined tilt an

(1101E–1401E); (c),(d) Individual tilt effects (black) and precession effects (wh

the results obtained follow expected trends with increases(decreases) in insolation provoking an increase (decrease)in precipitation which, through the corresponding pressureand wind field changes, are taken as indicators of summermonsoon activity over the Australian continent. Animportant precursor to the onset of the monsoon overnorthern Australia is the shallow sensible heating that isalready pronounced during the spring when the Inter-Tropical Convergence Zone (ITCZ) is still located in theNorthern Hemisphere (Hung and Yanai, 2004) This sets upa thermally induced shallow ocean–land circulation thatpromotes the advection of low level moist air into northernAustralia. With the onset of the active monsoon phase, theshallow sensible heat source merges with the southwarddisplaced ITCZ to trigger the full monsoon (Hung andYanai, 2004). Given the shallow summer heating pattern ofthe Australian continent, it is therefore not surprising that,as our model results indicate, increased (decreased)insolation should provoke a direct monsoon response.It is clear from the model results that the limited size of

the Australian landmass and the absence of strongtopographic control are not obstacles in preventing amonsoon response to insolation variations, albeit through

d precession effects for ocean sector (801E–1101E) and for land sector

ite) for corresponding ocean sector and land sector.

ARTICLE IN PRESS

Fig. 14. Zonal average of surface wind latitudinal (U) component: (a), (b) combined tilt and precession effects for ocean sector (801E–1101E) and for land

sector (1101E–1401E); (c),(d) Individual tilt effects (black) and precession effects (white) for corresponding ocean sector and land sector.

K.-H. Wyrwoll et al. / Quaternary Science Reviews 26 (2007) 3043–3057 3055

a combination of land/ocean controls. There may in fact besome value in seeing the Australian monsoon regime asbeing more sensitive to a range of likely forcing mechan-isms, including insolation changes, because of the absenceof a strong controlling relief influence as provided by theTibetan Plateau for the Asian system with its attendantheat and moisture fluxes (see Hung et al., 2004). It isprecisely because of this sensitivity brought about by theabsence of strong land area/topographic control, thatchanges in tilt, which make a minor contribution to theoverall insolation changes, but are important through theirlatitudinal influence, can become a determinant of mon-soon activity for the Australian regime.

Tilt acts through the fact that TH enhances the meanannual insolation at high latitudes and gives rise to meanannual cooling at the Equator, resulting in a symmetricforcing around the Equator (Fig. 3(a)). The effect of this isevident in changes in the precipitation rate both over landand ocean but is especially pronounced over the oceansector (Fig. 10). The SST response to the insolationchanges shows a concomitant decrease in the equatorial

regions with an accompanying increase in the higherlatitudes of the Indian Ocean (Fig. 9(c)).The model results presented here reinforce the recogni-

tion of the importance of precession in controllingAustralian monsoon activity. In this we follow theprevailing view of the importance of precession incontrolling monsoon activity over Quaternary time scalesand which support earlier models as applied to theAustralian summer monsoon (Chappell and Sytkus, 1996;Wyrwoll and Valdes, 2003). The importance of recognisingthe influence of the strength of outflow from the East AsianHigh in controlling monsoon activity, as advocated byother authors (Magee et al., 2004; Miller et al., 2005) is alsomade evident by our results. But this influence is not solelydriven by the strength of the East Asian High per se. Theprevailing summer sea level pressures over the Australianmonsoon region, both land and ocean, also need to beconsidered. Our findings reinforce the view of Wehausenand Brumsack (2002) that it is the pressure gradientbetween the winter and summer hemisphere that is ofparamount importance. In setting up this pressure gradient

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our results make it clear that both tilt and precession driveninsolation changes play an important role for theAustralian monsoon regime.

The results of this study confirm the more generalfindings of Liu et al. (2003b, 2004) as to the importance ofplacing SST changes into the context of orbital variationsand from this, recognising the monsoon response over theAustralian region. In this we draw attention to theimportance of changes in tilt in accounting for ocean–landcontrasts in the immediate monsoon region of Australia.But related to this is the need to recognise the importantrole played by SST changes in more distant parts of thetropical Indian Ocean (Fig. 7). These wider ocean–basinlinks make it possible to account for the existence of therelatively minor differences in land–ocean response thatwere made evident by the set of experiments whichhighlight the precession effect (TLPL-CTRL and TLPH-CTRL).

In the case of TLPH-CTRL (Fig. 4(b)) a restrictedregion of cyclonic circulation and accompanying higherprecipitation occurs in the northeast Southern IndianOcean, while the monsoon region of the Australiancontinent exhibits a significant reduction in precipitationrates. The region of cyclonic circulation is related towesterly sourced outflow from the eastern Indian Ocean,linked to the outflow from the Australian continent to theeast. The SST field shows that the westerly flow isassociated with a reduction of SSTs of up to �1.51C(Fig. 7(b)).

In the TLPL-CTRL experiment (Fig. 4(d)) there is only aweak increase in precipitation over a very limited region ofthe Australian continent, with a marked decrease in theadjacent Indian Ocean extending to �701E, while thetropical western and northwestern Indian Ocean shows asignificant precipitation increase. The wind fields show anincreased outflow (inflow) from the region of reduced(increased) precipitation. The overall expression of bothprecipitation and circulation show correspondence to anincrease in SSTs of up to 1 1C in the western andnorthwestern Indian Ocean (Fig. 7(d)). In these resultsthere is some resemblance to the idealised ‘aqua-planet’monsoon model results of Yano and McBride (1998), whorecognised forcing due to increased SSTs south of theEquator during summer.

In controlling the present day seasonal onset of activephases in the Australian summer monsoon, the Madden–-Julian Oscillation, possible links to mid-latitude systemsand trans-equatorial flow (cold surges) and other con-siderations all play important roles (McBride, 1998;McBride and Wheeler, 2004; Hung and Yanai, 2004; Hunget al., 2004; Li et al., 2005). Similarly, over geological time-scales sea-level fluctuations and the resultant regionalland–ocean geography changes, land–surface changes,SSTs differences and the possible influence of moreglobal-scale climate states need also to be included (e.g.Webster and Streten, 1978; Davidson et al., 1983; Wang etal., 2003; Barrows and Juggins, 2005; Bird et al., 2005), and

at times may well over-ride insolation forcing. Of these, theeffects offered by SSTs at glacial maxima would seem to bea potentially primary control. The likely operation of theseadditional forcing mechanisms make the apparent fitbetween the FOAM model experiments and the palaeohy-drological record of Australian monsoon events, intri-guing. Even more so, as orbital changes imply progressiveweakening of monsoon activity through the Late Quatern-ary, a requirement consistent with the history of lakedepth/extent for Lake Eyre over the last ca 130 ka (Fig. 12).It follows that a weaker Holocene than Last Interglacialmonsoon is to be expected, and this need not be seen asbeing a response to vegetation changes resulting fromAboriginal land use practices (Magee et al. 2004; Miller etal., 2005)—an inference that also sits uncomfortably withthe findings of Holbourn et al. (2005), who show that theprogressive decrease in monsoon activity with decreasinginsolation evident during the closing stages of thePleistocene mirrors similar relationships at TerminationsII, III and IV.It would seem likely that the possible explanations and

relationships that govern the activity of the Australianmonsoon over Quaternary time-scales will involve thresh-old considerations and, given the monsoon regime’sapparent sensitivity, possibly even a degree of ‘complex’behaviour (sensu Webster et al., 1998), which would allowdifferent combinations of forcing mechanisms to come tothe fore over a variety of time scales and differentboundary conditions (Marshall and Lynch, 2006). Butwherever the ultimate explanation lies, at this stage in ourunderstanding of events, it seems plausible to draw theinference that Milankovitch insolation variations make animportant contribution, and at times possibly ‘trigger’the forcing of Australian monsoon activity over LateQuaternary time-scales.

5. Conclusions

Our findings lead us to reject the claim that over LateQuaternary time scales the northern Australian summermonsoon is not related to Milankovitch insolation varia-tions. Rather than demonstrating that the Australiansummer monsoon does not behave like the classic monsoonregime, as claimed by others, our model results lead us toconclude that orbital changes exert a significant control onmonsoon activity. The sensitivity to such changes is aresponse to the limited landmass of the Australiancontinent, the direct near-field ocean–land links promotinginflow into the monsoon region and the absence of strongtopographic forcing. Lacking the overwhelming topo-graphic control exerted by the Tibetan Plateau on theIndian–East Asian summer monsoon, the northern Aus-tralian summer monsoon is less robust than its Asiancounterpart. And it is precisely because of this that it maybe more sensitive to variations in likely forcing controls, ofwhich insolation variations would seem to be of consider-able significance.

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Acknowledgements

We wish to thank Karen Wyrwoll, Pat Behling andMark Marohl for their help. K-H.W. would like toacknowledge with gratitude the important roles that JimBowler and Lu Yanchou played in the early fieldcampaigns, which prompted the questions that ultimatelyled to the modelling work. We thank Martin Williams andan anonymous reviewer for their helpful comments.

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