Australian-Indonesian Monsoon Research Articles

Insights into the Australian mid-Holocene climate using downscaled climate models

Abstract. The mid-Holocene climate of Australia and the equatorial tropics of the Indonesian–Australian monsoon region is investigated using the Community Earth System Model (CESM) and the Weather Research and Forecasting (WRF) model. Each model is used to simulate the pre-industrial (1850) and the mid-Holocene (6000 years before 1950) climate. The results of these four simulations are compared to existing bioclimatic modelling of temperature and precipitation. The finer-resolution WRF simulations reduce the bias between the model and bioclimatic data results for three of the four variables available in the proxy data set. The model results show that temperatures over southern Australia at the mid-Holocene and in the pre-industrial period were similar, and temperatures were slightly warmer during the mid-Holocene over northern Australia and into the tropics compared to the pre-industrial. During the mid-Holocene, precipitation was generally reduced over northern Australia and in the Indonesian–Australian monsoon region, particularly during summertime. The results highlight the improved value of using finer-resolution models such as WRF to simulate the palaeoclimate.

Seasonal response of coccolithophores and its potential to reconstruct paleomonsoon in the eastern Indonesian seas: an overview

Coccolithophore is a group of marine calcifying phytoplankton and its living distribution is sensitive to ecological change within the photic zone. Their coccoliths contribute majorly to the marine sediments and have become a reliable proxy in paleoceanography and paleoclimatology. The upper hydrology of the eastern Indonesian seas has been strongly influenced by seasonal climate change, the Australian-Indonesian monsoon. We review and discuss the result of previous studies on the response of living coccolithophore to seasonal change and the significance of their coccoliths as a proxy for reconstructing past dynamic of monsoon in the region. Coccolithophores abundance, species assemblages and coccolith malformation change in respond to the shift of photic environment condition modulated by reversing monsoon with particular responds across the region. Coccoliths variations in the marine sediment core archives from off southwest Sumba, eastern Banda Sea and Halmahera Sea show an insignificant control of glacial-interglacial climate variability to paleoproducitivity. Primary productivity based on the relative abundance of Florisphaera profunda has contributed to reconstruction of the past dynamic of Australian-Indonesian monsoon. However, more extensive and continuous observation of coccolithophores and their relationship with the photic environment in the eastern Indonesian seas is necessary for a better understanding of their response to monsoon dynamics in the present, past and future.

Climatology of the global summer monsoon rainy seasons: Revisited from a high-resolution satellite climate data record

Reliable information on the monsoon rainfall at finer spatiotemporal scales has always been crucial for applications in hydrometeorology, water resource management, disaster prevention, and assessment of numerical model outputs. Here, the spatial-temporal characteristics of the summer monsoon rainy seasons worldwide are revisited using a global, high-resolution, bias-corrected satellite rainfall climate data record. The Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Climate Data Record (PERSIANN-CDR) provides 0.25° rainfall estimates for the latitude band 60°S–60°N from 1983 to the near present. The global monsoon regimes are defined based on the annual rainfall range and the summer rainfall ratio. The onset, peak, and withdrawal of the rainy season are then determined using a concise yet pertinent rainfall parameter with universal objective criteria.Overall, six major summer monsoon regimes of the globe, the Asian Monsoon, Indonesian–Australian monsoon, North African monsoon, South African monsoon, North American and South American monsoon, are all reasonably represented by PERSIANN-CDR. The progression patterns of climatological onset, peak, and withdrawal phases of these monsoons are consistent with those reported by previous global and regional monsoon studies. Furthermore, compared with other coarser resolution satellite rainfall estimates such as the Climate Prediction Center Merged Analysis of Precipitation (CMAP), the present result is clearly in better agreement with that obtained from ground-based observations since it allows the determination of more detailed information associated with local-scale factors such as the sharp land-sea contrast, local surface heating, and topography effects. However, some “spots”, particularly over complex topography lacking ground truth validations, are revealed to have rainy season characteristics that differ substantially from those over the surrounding areas and have not been fully described by published literature. These distinct features need to be confirmed by further regional monsoon studies.

EA SURFACE PALEOPRODUCTIVITY RECONSTRUCTION BASED ON FORAMINIFERAL ACCUMULATION RATE IN THE WESTERN SAVU STRAIT SINCE THE LAST GLACIAL MAXIMUM (~23 KA BP)

Despite its importance, sea surface paleoproductivity of the western Savu Strait is not well studied. Results from previous studies at the nearby Southwest Sumba and Sumba Strait might not be applicable due to the oceanographic difference. Foraminiferal proxies from gravity core ST10 were applied to generate sea surface paleoproductivity and thermocline depth reconstruction. Foraminiferal Accumulation Rate and Benthic Foraminiferal Accumulation Rate were used as paleoproductivity proxies while the thermocline dwellers’ relative abundance was applied as the thermocline depth proxy. This study suggested paleoproductivity increase during the Last Glacial Maximum (LGM)–~16 ka BP and Holocene (after ~11.65 ka BP) in the western Savu Strait. Thermocline depth was relatively shallower during the LGM–Last Deglaciation and became deeper afterwards. Paleoproductivity increase at LGM–~16 ka BP was caused by the Australian-Indonesian winter monsoon (AIWM)-like condition, characterized by intense coastal upwelling while the Holocene paleoproductivity increase was related to the abrupt rainfall increase which enhanced terrestrial/riverine input. Thermocline depth variability in the western Savu Strait is in-phase with thermocline depth variability in the Java upwelling region, characterized by a shallower thermocline during the LGM–Last Deglaciation (before ~11.65 ka BP) and a deeper thermocline during the Holocene (after ~11.65 ka BP). This thermocline depth shifting indicates a strong Australian-Indonesian Monsoon (AIM) influence on the paleoceanography of the western Savu Strait since LGM.

Antiphase response of the Indonesian–Australian monsoon to millennial-scale events of the last glacial period

Antiphase behaviour of monsoon systems in alternate hemispheres is well established at yearly and orbital scales in response to alternating sensible heating of continental landmasses. At intermediate timescales without a sensible heating mechanism both in-phase and antiphase behaviours of northern and southern hemisphere monsoon systems are recorded at different places and timescales. At present, there is no continuous, high resolution, precisely dated record of millennial-scale variability of the Indonesian–Australian monsoon during the last glacial period with which to test theories of paleomonsoon behaviour. Here, we present an extension of the Liang Luar, Flores, speleothem δ18O record of past changes in southern hemisphere summer monsoon intensity back to 55.7 kyr BP. Negative δ18O excursions (stronger monsoon) occur during Heinrich events whereas positive excursions (weaker monsoon) occur during Dansgaard-Oeschger interstadials—a first order antiphase relationship with northern hemisphere summer monsoon records. An association of negative δ18O excursions with speleothem growth phases in Liang Luar suggests that these stronger monsoons are related to higher rainfall amounts. However, the response to millennial-scale variability is inconsistent, including a particularly weak response to Heinrich event 3. We suggest that additional drivers such as underlying orbital-scale variability and drip hydrology influence the δ18O response.

Australian–Indonesian monsoon rainfall responses to the northern hemisphere climatic changes prior to the Last Glacial Maximum: an early indication

The evidences of Australian-Indonesian monsoon (AIM) rainfall response to the northern hemisphere climatic changes from both marine and terrestrial proxies are well established for the Last Glacial Maximum (LGM) – Deglaciation time interval but not in the previous time interval. Sediment cores from off south Sumba (ST10) and off north Sumba (Sumba strait) (ST14) were analyzed using X-Ray Fluorescence method to obtain elemental proxies. Elemental ratios which reflect terrigenous input (Ti/Ca and K/Ca) are used to infer the AIM rainfall changes since ~42 ka BP. AIM rainfall changes indicate a clear response to the Heinrich Events prior to the LGM (H2, H3, and H4). H2 and H3 are corroborated with the AIM rainfall increases in off south Sumba (~24 – 26 ka BP and ~30 – 31 ka BP) while the AIM rainfall increase (~39 – 41 ka BP) in the Sumba strait indicates a response to H4. The cooler temperature during the Heinrich Events could enhance the Northern Hemisphere (NH) cold surges which eventually pushed the Austral summer Intertropical Convergence Zone (ITCZ) southward in a similar fashion to the Last Deglaciation period (~18 – 11 ka BP). The southward movement of Austral summer ITCZ should increase the research area’s exposure time to the Tropical Rain Belt (TRB) during the Australian – Indonesian summer monsoon (AISM) which eventually triggers the AIM rainfall increase. The Sumba strait AIM rainfall unresponsiveness to H2 and H3 could be related to the constant exposure time to the TRB which indicates that the southern limit of Austral summer ITCZ during its northernmost shift didn’t reach the latitude of Sumba Strait. Comparison with other proxies from the same site and other rainfall proxies from southern Indonesia and the northern Australia regions is needed to confirm the spatial extend of those responses.

Spatio-temporal distribution of chlorophyll-a concentration, sea surface temperature and wind speed using aqua-modis satellite imagery over the Savu Sea, Indonesia

The climatological means and annual variation of chlorophyll-a (Chl-a) concentration, sea surface temperature (SST) and sea surface wind (SSW) from 2002 to 2016 in the Savu Sea (SS) within Indonesia were analysed. Chl-a variability and SST information was used as a reference in determining potential fishing grounds. The research aimed to investigate the variability of Chl-a and SST along with the influencing factor of the Australian-Indonesian Monsoon (AIM) winds. The study was conducted in SS waters, in East Nusa Tenggara (NTT), Indonesia, using 15 years of monthly Aqua MODIS Level 3 data. The retrieved data were corrected geometrically and radiometrically and then analysed to produce mean climatology data. The results indicated that the spatial and temporal AIM winds had a significant influence on the Chl-a concentration and the SST in SS waters. There was a two-month delay in the peak concentration of Chl-a/SST; moreover, the AIM wind speed, which peaks in June, was not followed by either the SST minima or the Chl-a maxima, which occur in August. The level of Chl-a in the SS waters showed seasonal patterns, with the highest concentration occurring during the east season (June–August), the peak concentration being 0.52 mg m−3, alongside a wind speed of 5.42 m s−1 and an SST of 22.89 °C in August. Meanwhile, the lowest concentration of Chl-a 0.17 mg m−3 occurred in February during the west season (December–February), with a wind speed of 2.10 m s−1 and an SST of 29.57 °C. Three specific areas showed characteristics that differed from the general pattern of the monsoon winds’ effects on Chl-a and SST in the SS. This study provides the latest description of Chl-a, SST and wind speed distribution at seasonal and spatial scales and their variability across the Savu Sea. The results of this study are essential for fishing catching activities and fisheries management in the region.

CHANGES OF THERMOCLINE DEPTH AT THE SUMBA ISLAND OFFSHORE BASED ON PLANKTONIC FORAMINIFERAL ASSEMBLAGES AND ITS IMPLICATION TO EUTROPHICATION SINCE THE LAST DEGLACIATION (~18 ka BP): A PRELIMINARY STUDY

Changes of the thermocline depth (DOT) at the Sumba Island offshore are not well-known compared to the DOT changes in the Timor Sea, the main exit passage of the Indonesian Through-flow (ITF). Planktonic foraminiferal assemblages in cores collected from the southwest Sumba offshore (ST08) and Sumba Strait (ST12, ST13, and ST14) were used as a tool to infer the DOT and paleoproductivity changes at the Sumba Island offshore. The DOT changes were indicated from the thermocline and mixed layer dwellers’ relative abundance while the paleoproductivity changes were indicated from the relative abundance of Neogloboquadrina dutertrei. This study suggests a contrast between the DOT pattern at the Sumba Island offshore and the DOT pattern in the Timor Sea during the Last Deglaciation–Holocene. The contrast DOT pattern indicated that the multi-millennial changes of the Australian-Indonesian monsoon (AIM) during the Last Deglaciation– Holocene were the main factors behind the DOT changes in this region while the effects of El Niño Southern Oscillation (ENSO) –like, Indian Ocean Dipole (IOD) –like, and ITF to DOT changes were minimal. Paleoproductivity enhancement at the Sumba Island offshore was not solely related to the monsoon-driven coastal upwelling intensification, which resulted in the DOT shoaling and eutrophic condition. The increase of nutrient availability in surface water due to river runoff increase and changes in the lifted water mass nature were also able to enhance productivity in this region.

Last Deglaciation—Holocene Australian-Indonesian Monsoon Rainfall Changes Off Southwest Sumba, Indonesia

Previous studies suggested the multi-millennial scale changes of Australian-Indonesian monsoon (AIM) rainfall, but little is known about their mechanism. Here, AIM rainfall changes since the Last Deglaciation (~18 ka BP) are inferred from geochemical elemental ratios (terrigenous input) and palynological proxies (pollen and spores). Pollen and spores indicate drier Last Deglaciation (before ~11 ka BP) and wetter Holocene climates (after ~11 ka BP). Terrigenous input proxies infer three drier periods (i.e., before ~17, ~15–13.5, and 7–3 ka BP) and three wetter periods (i.e., ~17–15, ~13.5–7, and after ~3 ka BP) which represent the Australian-Indonesian summer monsoon (AISM) rainfall changes. Pollen and spores were highly responsive to temperature changes and showed less sensitivity to rainfall changes due to their wider source area, indicating their incompatibility as rainfall proxy. During the Last Deglaciation, AISM rainfall responded to high latitude climatic events related to the latitudinal shifts of the austral summer ITCZ. Sea level rise, solar activity, and orbitally-induced insolation were most likely the primary driver of AISM rainfall changes during the Holocene, but the driving mechanisms behind the latitudinal shifts of the austral summer ITCZ during this period are not yet understood.

The origin of Asian monsoons: a modelling perspective

Abstract. The Cenozoic inception and development of the Asian monsoons remain unclear and have generated much debate, as several hypotheses regarding circulation patterns at work in Asia during the Eocene have been proposed in the few last decades. These include (a) the existence of modern-like monsoons since the early Eocene; (b) that of a weak South Asian monsoon (SAM) and little to no East Asian monsoon (EAM); or (c) a prevalence of the Intertropical Convergence Zone (ITCZ) migrations, also referred to as Indonesian–Australian monsoon (I-AM). As SAM and EAM are supposed to have been triggered or enhanced primarily by Asian palaeogeographic changes, their possible inception in the very dynamic Eocene palaeogeographic context remains an open question, both in the modelling and field-based communities. We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 (Sepulchre et al., 2019) earth system model and revised palaeogeographies. Our Eocene climate simulation yields atmospheric circulation patterns in Asia substantially different from modern conditions. A large high-pressure area is simulated over the Tethys ocean, which generates intense low tropospheric winds blowing southward along the western flank of the proto-Himalayan–Tibetan plateau (HTP) system. This low-level wind system blocks, to latitudes lower than 10∘ N, the migration of humid and warm air masses coming from the Indian Ocean. This strongly contrasts with the modern SAM, during which equatorial air masses reach a latitude of 20–25∘ N over India and southeastern China. Another specific feature of our Eocene simulation is the widespread subsidence taking place over northern India in the midtroposphere (around 5000 m), preventing deep convective updraught that would transport water vapour up to the condensation level. Both processes lead to the onset of a broad arid region located over northern India and over the HTP. More humid regions of high seasonality in precipitation encircle this arid area, due to the prevalence of the Intertropical Convergence Zone (ITCZ) migrations (or Indonesian–Australian monsoon, I-AM) rather than monsoons. Although the existence of this central arid region may partly result from the specifics of our simulation (model dependence and palaeogeographic uncertainties) and has yet to be confirmed by proxy records, most of the observational evidence for Eocene monsoons are located in the highly seasonal transition zone between the arid area and the more humid surroundings. We thus suggest that a zonal arid climate prevailed over Asia before the initiation of monsoons that most likely occurred following Eocene palaeogeographic changes. Our results also show that precipitation seasonality should be used with caution to infer the presence of a monsoonal circulation and that the collection of new data in this arid area is of paramount importance to allow the debate to move forward.

The Influence of Seasonal and Interannual Variability on Surface Chlorophyll-a Off the Western Lesser Sunda Islands

Novel and long-term satellite data (2003–2017) are analyzed to investigate the variability of ocean surface chlorophyll-a (Chl-a) concentration off the Western Lesser Sunda Islands (WLSI) under the influence of the Indonesian Australian monsoon, the El Nino-Southern Oscillation, and the Indian Ocean Dipole (IOD). In this article, we first analyzed the seasonal variability of Chl-a, and then describe the relationship among the sea surface Chl-a, sea surface temperature (SST), and sea surface wind stress in the region. Our results demonstrate that prevailing southeasterly wind stress plays a pivotal role in generating the Chl-a maxima off the WLSI. Particularly on seasonal time scale, the strengthening of southeasterly wind stress (up to ∼0.01 N·m−2) during the southeast monsoon season produces enhanced Chl-a concentrations (0.59 mg·m−3) associated with sea surface cooling (∼28.8 °C) in the area of study. In contrast, the Chl-a maxima completely vanished during the northwest monsoon season. On interannual time scale, the largest positive Chl-a and wind stress anomalies and the coolest SST anomaly are observed in 2006 when El Nino and positive IOD events occur at the same time. Meanwhile, the greatest negative Chl-a anomaly is prevailed during the 2016 negative IOD event. This article demonstrates that wind variability is the essential factor in determining the magnitude of the Chl-a maxima off the WLSI.

Attribution of Global Monsoon Response to the Last Glacial Maximum Forcings

Abstract Investigation of global monsoon (GM) responses to external forcings is instrumental for understanding its formation mechanism and projected future changes. Coupled climate model experiments are performed to assess how the individual and full Last Glacial Maximum (LGM) forcings change GM precipitation. Under the full LGM forcing, the annual and local summer-mean GM precipitation are reduced by 8.5% and 10.8%, respectively, compared to the results in the preindustrial control run; and the reduction of Northern Hemisphere (NH) summer monsoon (NHSM) precipitation is twice as large as its Southern Hemisphere (SH) counterpart (SHSM). The NH–SH asymmetric response is mainly caused by the monsoon circulation change–induced moisture convergence rather than the reduction of moisture content, but the root cause is the continental ice sheet forcing. The NHSM precipitation changes dramatically differ among various single-forcing experiments, while this is not the case for their SH counterparts. The moisture budget analysis indicates the NHSM is dynamically oriented, but SHSM is thermodynamically oriented. The markedly different NHSM circulation changes are caused by different forcing-induced sea surface temperature (SST) patterns, including the North Atlantic cooling pattern forced by the continental ice sheet, the mega–La Niña–like pattern resulting from the greenhouse gas forcing, and the Indian Ocean dipole–like SST pattern caused by the land–sea configuration forcing. Moreover, the distinctive change of “monsoonality” in the Australian–Indonesian monsoon is predominantly forced by the exposure of the land shelf, which enhances precipitation during early summer (November–December) but weakens it in the rest of the year.

Spatio-temporal variability of surface chlorophyll-a in the Halmahera Sea and its relation to ENSO and the Indian Ocean Dipole

ABSTRACT Long-term satellite data are used to investigate the variability of ocean surface chlorophyll-a (chl-a) concentration in the Halmahera Sea (HS) under influence of the Australian-Indonesian Monsoon (AIM), the El Niño-Southern Oscillation (ENSO), and the Indian Ocean Dipole (IOD). In this study, we first analysed the seasonal variability of chl-a, and then examine the relationship between surface chl-a, sea surface temperature (SST), and sea surface wind stress in the area. Our results suggest that prevailing southeasterly winds play a fundamental role in generating chl-a blooms in the HS. Particularly on a seasonal timescale, through the mechanism of Ekman mass transport, strengthening of southeasterly wind stress during the Southeast Monsoon season (June – August) produces enhanced chl-a concentrations associated with ocean surface cooling in the area of study. On the other hand, the chl-a bloom completely diminishes during the Northwest Monsoon season (December – February) due to weakening of wind stress and Ekman transport. On an interannual timescale, sea level pressure and wind stress are coherent with ENSO and IOD phases. During El Niño and positive IOD events (La Niña and negative IOD events), both sea level pressure and wind stress greatly increase (decrease) over the HS. These conditions cause an anomaly in southerly (northerly) wind stress, which is favourable to an enhancement (reduction) of the chl-a concentration in the region. This study demonstrates that sea level pressure and wind stress are the critical factors in determining the magnitude of chl-a bloom in the HS.

Glacial aridity in central Indonesia coeval with intensified monsoon circulation

The Last Glacial Maximum was cool and dry over the Indo-Pacific Warm Pool (IPWP), a key region driving global oceanic–atmospheric circulation. Both low- and high-latitude teleconnections with insolation, ice sheets, and sea level have been suggested to explain the pervasive aridity observed in paleoecological and geomorphic data. However, proxies tracking the H- and O-isotopic composition of rainfall (e.g., speleothems, sedimentary biomarkers) suggest muted aridity or even wetter conditions than the present, complicating interpretations of glacial IPWP climate. Here we use multiproxy reconstructions from lake sediments and modern rainfall isotopic measurements from central Indonesia to show that, contrary to the classical “amount effect,” intensified Australian–Indonesian monsoon circulation drove lighter H- and O-isotopic composition of IPWP rainfall during the LGM, while at the same time, dry conditions prevailed. Precipitation isotopes are particularly sensitive to the apparent increase in monsoon circulation and perhaps also decreased moisture residence time implied by our data, explaining contrasts among proxy records while illuminating glacial IPWP atmospheric circulation, a key target for climate models.

Transient coupling relationships of the Holocene Australian monsoon

The northwest Australian summer monsoon owes a notable degree of its interannual variability to interactions with other regional monsoon systems. Therefore, changes in the nature of these relationships may contribute to variability in monsoon strength over longer time scales. Previous attempts to evaluate how proxy records from the Indonesian–Australian monsoon region correspond to other records from the Indian and East Asian monsoon regions, as well as to El Niño-related proxy records, have been qualitative, relying on ‘curve-fitting’ methods. Here, we seek a quantitative approach for identifying coupling relationships between paleoclimate proxy records, employing statistical techniques to compute the interdependence of two paleoclimate time series. We verify the use of complex networks to identify coupling relationships between modern climate indices. This method is then extended to a set of paleoclimate proxy records from the Asian, Australasian and South American regions spanning the past 9000 years. The resulting networks demonstrate the existence of coupling relationships between regional monsoon systems on millennial time scales, but also highlight the transient nature of teleconnections during this period. In the context of the northwest Australian summer monsoon, we recognise a shift in coupling relationships from strong interhemispheric links with East Asian and ITCZ-related proxy records in the mid-Holocene to significantly weaker coupling in the later Holocene. Although the identified links cannot explain the underlying physical processes leading to coupling between regional monsoon systems, this method provides a step towards understanding the role that changes in teleconnections play in millennial-to orbital-scale climate variability.

Dynamics of the Australian–Indonesian monsoon across Termination II: Implications of molecular-biomarker reconstructions from the Timor Sea

Abstract Molecular biomarker-derived proxies in a marine sediment core from the eastern Timor Sea have been employed to track changes in the Australian–Indonesian monsoon system over the time period from the penultimate glacial to the last interglacial. The average chain length of n-alkanes originating from terrestrial higher plants reveals an expansion of woody plants during both Termination II and the last interglacial with respect to the penultimate glacial. This is consistent with previous reconstructions for the last glacial–interglacial cycle, indicating that water-stress conditions during dry seasons of southern Indonesia should have been relieved during interglacials and deglacials relative to glacials. However, in contrast to an only muted response during Heinrich Stadial 1, the vegetation composition showed a dramatic and prominent change during Heinrich Stadial 11. This was probably caused by a much more significant and longer duration iceberg discharge event to the northern North Atlantic during Heinrich Stadial 11 relative to Heinrich Stadial 1, which exerted a significant influence on the tropical hydrological cycle and monsoon rainfall seasonality. In addition, marine algae-derived biomarkers are used to reconstruct primary productivity variations, which are in agreement with previous studies from the same area. The precessional cyclicity of paleo-productivity variations is possibly induced by changes in the Australian–Indonesian winter monsoon intensity, showing a correlation with boreal winter insolation. However, this phase relationship is in contrast to the inference based on other lines of evidence, which suggests strong winter monsoon strength during boreal summer insolation maxima. New winter monsoon proxies are therefore required to clarify this discrepancy.

Mid- to Late-Holocene Australian–Indonesian summer monsoon variability

The Australian–Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ∼6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ∼3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

Rapid interhemispheric climate links via the Australasian monsoon during the last deglaciation

Recent studies have proposed that millennial-scale reorganization of the ocean-atmosphere circulation drives increased upwelling in the Southern Ocean, leading to rising atmospheric carbon dioxide levels and ice age terminations. Southward migration of the global monsoon is thought to link the hemispheres during deglaciation, but vital evidence from the southern sector of the vast Australasian monsoon system is yet to emerge. Here we present a 230thorium-dated stalagmite oxygen isotope record of millennial-scale changes in Australian-Indonesian monsoon rainfall over the last 31,000 years. The record shows that abrupt southward shifts of the Australian-Indonesian monsoon were synchronous with North Atlantic cold intervals 17,600-11,500 years ago. The most prominent southward shift occurred in lock-step with Heinrich Stadial 1 (17,600-14,600 years ago), and rising atmospheric carbon dioxide. Our findings show that millennial-scale climate change was transmitted rapidly across Australasia and lend support to the idea that the 3,000-year-long Heinrich 1 interval could have been critical in driving the last deglaciation.

Glacial to Holocene swings of the Australian–Indonesian monsoon

The Australian‐Indonesian monsoon is an important component of the climate system in the tropical Indo‐Pacific region 1 . However, its past variability, relation with northern and southern high-latitude climate and connection to the other Asian monsoon systems are poorly understood. Here we present high-resolution records of monsoon-controlled austral winter upwelling during the past 22,000 years, based on planktic foraminiferal oxygen isotopes and faunal composition in a sedimentary archive collected offshore southern Java. We show that glacial‐interglacial variations in the Australian‐Indonesian winter monsoon were in phase with the Indian summer monsoon system, consistent with their modern linkage through cross-equatorial surface winds. Likewise, millennial-scale variability of upwelling shares similar sign and timing with upwelling variability in the Arabian Sea. On the basis of element composition and grain-size distribution as precipitation-sensitive proxies in the same archive, we infer that (austral) summer monsoon rainfall was highest during the Bolling‐Allerod period and the past 2,500 years. Our results indicate drier conditions during Heinrich Stadial 1 due to a southward shift of summer rainfall and a relatively weak Hadley cell south of the Equator. We suggest that the Australian‐Indonesian summer and winter monsoon variability were closely linked to summer insolation and abrupt climate changes in the northern hemisphere. The seasonality of the global monsoon is of central importance to the global hydrologic cycle and its environmental influence on human societies. Palaeoclimate reconstructions generally agree that the strengths of the East Asian monsoon and the Indian monsoon are tightly coupled to the Northern Hemisphere climate through zonal migrations of the monsoon convection centres, also interpreted as Intertropical Convergence Zone (ITCZ) migration 26 . The mechanism of the AustralianIndone

High-resolution stalagmite reconstructions of Australian–Indonesian monsoon rainfall variability during Heinrich stadial 3 and Greenland interstadial 4

Little is known about the possible teleconnections between abrupt climatic changes originating in the North Atlantic and precipitation dynamics in the Australian–Indonesian summer monsoon (AISM) domain. We examine the climatic impacts of Heinrich stadial 3 (HS3) and Greenland interstadials 4 and 3 (GIS4/3) on AISM-associated precipitation through a high-resolution analysis of stable isotope (δ 18O, δ 13C) and trace element (Mg/Ca, P/Ca) ratios in a stalagmite from Liang Luar cave, Flores, Indonesia. Sixteen high precision 230Th dates indicate that stalagmite LR07-E1 grew rapidly (~ 0.3–1.0 mm/yr) in two phases between ~ 31.5–30.1 ka and ~ 27.8–25.6 ka, separated by a ~ 2.3 kyr unconformity. Temporally consistent abrupt responses occur in the Flores record during HS3 and GIS4, which are coherent with changes in stalagmite δ 18O records from China and Brazil. The response of low-latitude precipitation to HS3 cooling and GIS4 warming, as demonstrated by the widely separated sites, comprises three distinct simplified phases: (1) a strong southward migration of the ITCZ during HS3 is associated with a decrease in rainfall at Liang Luar cave and in China, while wetter conditions are reconstructed from Brazil, (2) represents the peak of HS3 impacts and an extended hiatus begins in the Flores record and (3) where suggested dry conditions at Liang Luar throughout GIS4 form part of a coherent north–south anti-phasing in precipitation changes. The reconstructed changes are also broadly consistent with NASA GISS ModelE-R simulations of a Heinrich-like freshwater perturbation in the North Atlantic basin, which produces a southward shift in the ITCZ. The relationship between the palaeoclimate records indicates that atmospheric teleconnections rapidly propagate and synchronise climate change across the hemispheres during periods of abrupt climate change. Our findings augment recent proposals that large-scale atmospheric re-organisations during stadials and interstadials play a key role in driving changes in atmospheric CO 2 concentration, air temperature and global climate change.