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Asian monsoon over mainland Southeast Asia in the past 25 000 years
Stockholm University, Faculty of Science, Department of Geological Sciences.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objective of this research is to interpret high-resolution palaeo-proxy data sets to understand the Asian summer monsoon variability in the past. This was done by synthesizing published palaeo-records from the Asian monsoon region, model simulation comparisons, and analysing new lake sedimentary records from northeast Thailand.

Palaeo-records and climate modeling indicate a strengthened summer monsoon over Mainland Southeast Asia during the Last Glacial Maximum (LGM), compared to dry conditions in other parts of the Asian monsoon region. This can be explained by the LGM sea level low stand, which exposed Sundaland and created a large land-sea thermal contrast. Sea level rise ~19 600 years before present (BP), reorganized the atmospheric circulation in the Pacific Ocean and weakened the summer monsoon between 20 000 and 19 000 years BP.

Both the Mainland Southeast Asia and the East Asian monsoon hydroclimatic records point to an earlier Holocene onset of strengthened summer monsoon, compared to the Indian Ocean monsoon. The asynchronous evolution of the summer monsoon and a time lag of 1500 years between the East Asian and the Indian Ocean monsoon can be explained by the palaeogeography of Mainland Southeast Asia, which acted as a land bridge for the movement of the Intertropical Convergence Zone.

The palaeo-proxy records from Lake Kumphawapi compare well to the other data sets and suggest a strengthened summer monsoon between 10 000 and 7000 years BP and a weakening of the summer monsoon thereafter. The data from Lake Pa Kho provides a picture of summer monsoon variability over 2000 years. A strengthened summer monsoon prevailed between BC 170-AD 370, AD 800-960 and since AD 1450, and was weaker about AD 370-800 and AD 1300-1450. The movement of the mean position of the Intertropical Convergence Zone explains shifts in summer monsoon intensity, but weakening of the summer monsoon between 960 and 1450 AD could be affected by changes in the Walker circulation.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University , 2014. , 61 p.
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper, 353
Keyword [en]
Asian monsoon, ITCZ, palaeo-vegetation, palaeoclimate, lake sediment, Last Glacial Maximum, Holocene
National Category
Climate Research
Research subject
Marine Geoscience
Identifiers
URN: urn:nbn:se:su:diva-107136ISBN: 978-91-7447-969-0 (print)OAI: oai:DiVA.org:su-107136DiVA: diva2:743250
Public defence
2014-10-10, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 621-2008-2855Swedish Research Council, 348-2008-6071Swedish Research Council, 621-2011-4684
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript..

Available from: 2014-09-18 Created: 2014-09-03 Last updated: 2016-01-27Bibliographically approved
List of papers
1. Asian monsoon climate during the Last Glacial Maximum: palaeo-data–model comparisons
Open this publication in new window or tab >>Asian monsoon climate during the Last Glacial Maximum: palaeo-data–model comparisons
2013 (English)In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 43, no 1, 220-242 p.Article in journal (Refereed) Published
Abstract [en]

The Last Glacial Maximum (LGM) (23-19ka BP) in the Asian monsoon region is generally described as cool and dry, due to a strong winter monsoon. More recently, however, palaeo-data and climate model simulations have argued for a more variable LGM Asian monsoon climate with distinct regional differences. We compiled, evaluated, and partly re-assessed proxy records for the Asian monsoon region in terms of wet/dry climatic conditions based on precipitation and effective moisture, and of sea surface temperatures. The comparison of the palaeo-data set to LGM simulations by the Climate Community System Model version 3 (CCSM3) shows fairly good agreement: a dry LGM climate in the western and northern part due to a strengthened winter monsoon and/or strengthened westerly winds and wetter conditions in equatorial areas, due to a stronger summer monsoon. Data-model discrepancies are seen in some areas and are ascribed to the fairly coarse resolution of CCSM3 and/or to uncertainties in the reconstructions. Differences are also observed between the reconstructed and simulated northern boundaries of the Intertropical Convergence Zone (ITCZ). The reconstructions estimate a more southern position over southern India and the Bay of Bengal, whereas CCSM3 simulates a more northern position. In Indochina, the opposite is the case. The palaeo-data indicate that climatic conditions changed around 20-19ka BP, with some regions receiving higher precipitation and some experiencing drier conditions, which would imply a distinct shift in summer monsoon intensity. This shift was probably triggered by the late LGM sea-level rise, which led to changes in atmosphere-ocean interactions in the Indian Ocean. The overall good correspondence between reconstructions and CCSM3 suggests that CCSM3 simulates LGM climate conditions over subtropical and tropical areas fairly well. The few high-resolution qualitative and quantitative palaeo-records available for the large Asian monsoon region make reconstructions however still uncertain

National Category
Climate Research
Research subject
Marine Geoscience
Identifiers
urn:nbn:se:su:diva-98041 (URN)10.1111/bor.12032 (DOI)000328619600013 ()
Funder
Swedish Research Council, 621-2008-2855Swedish Research Council, 348-2008-6071Swedish Research Council, 621-2011-4684
Available from: 2013-12-25 Created: 2013-12-25 Last updated: 2017-12-06Bibliographically approved
2. Climate over mainland Southeast Asia 10.5–5 ka
Open this publication in new window or tab >>Climate over mainland Southeast Asia 10.5–5 ka
2014 (English)In: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 29, no 5, 445-454 p.Article in journal (Refereed) Published
Abstract [en]

We assembled and evaluated Holocene palaeo-vegetation records regarding semi-quantitative precipitation and temperature for mainland Southeast Asia and compared these with precipitation reconstructions for the Indian Ocean (IOM) and East Asian (EAM) monsoon sub-systems. Our results indicate that temperatures and precipitation in mainland Southeast Asia generally exceeded 18 °C and 1100 mm a−1 during the Holocene. Mainland Southeast Asia experienced cool/wet climatic conditions between 10.5 and 10 ka BP, a warmer/drier climate between 10 and 9 ka BP, cooler/wetter conditions between 9 and 7 ka BP, and moderately warmer/drier conditions since 7 ka BP. The reconstructed summer monsoon intensity compares well with the reconstructed hydroclimate for the EAM region, but diverges from that of the IOM region between 10.5 and 9 ka BP and 7–6.5 ka BP. This discrepancy is explained by differences in land–sea configuration, and regional sea-level history. A strengthening/weakening of the Asian summer monsoon between 9 and 7 ka BP and after 6.5 ka BP, respectively, is observed across the whole Asian monsoon region. Our new data sets support an asynchronous onset of the Asian summer monsoon optimum.

Keyword
Asian monsoon, biome, Holocene, hydroclimate, mainland Southeast Asia, palaeovegetation
National Category
Climate Research
Research subject
Marine Geoscience
Identifiers
urn:nbn:se:su:diva-107133 (URN)10.1002/jqs.2715 (DOI)000340596900006 ()
Available from: 2014-09-03 Created: 2014-09-03 Last updated: 2017-12-05Bibliographically approved
3. Lake Kumphawapi - an archive of Holocene palaeoenvironmental and palaeoclimatic changes in northeast Thailand
Open this publication in new window or tab >>Lake Kumphawapi - an archive of Holocene palaeoenvironmental and palaeoclimatic changes in northeast Thailand
Show others...
2013 (English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 68, 59-75 p.Article in journal (Refereed) Published
Abstract [en]

The long-term climatic and environmental history of Southeast Asia, and of Thailand in particular, is still fragmentary. Here we present a new C-14-dated, multi-proxy sediment record (TOC, C/N, CNS isotopes, Si, Zr, K, Ti, Rb, Ca elemental data, biogenic silica) for Lake Kumphawapi, the second largest natural lake in northeast Thailand. The data set provides a reconstruction of changes in lake status, groundwater fluctuations, and catchment run-off during the Holocene. A comparison of multiple sediment sequences and their proxies suggests that the summer monsoon was stronger between c. 9800 and 7000 cal yr BP. Lake status and water level changes around 7000 cal yr BP signify a shift to lower effective moisture. By c. 6500 cal yr BP parts of the lake had been transformed into a peatland, while areas of shallow water still occupied the deeper part of the basin until c. 5400-5200 cal yr BP. The driest interval in Kumphawapi's history occurred between c. 5200 and 3200 cal yr BP, when peat extended over large parts of the basin. After 3200 cal yr BP, the deepest part of the lake again turned into a wetland, which existed until c. 1600 cal yr BP. The observed lake-level rise after 1600 cal yr BP could have been caused by higher moisture availability, although increased human influence in the catchment cannot be ruled out. The present study highlights the use of multiple sediment sequences and proxies to study large lakes, such as Lake Kumphawapi in order to correctly assess the time transgressive response to past changes in hydroclimate conditions. Our new data set from northeast Thailand adds important palaeoclimatic information for a region in Southeast Asia and allows discussing Holocene monsoon variability and ITCZ movement in greater detail.

Keyword
Thailand, Asian monsoon, Lake sediment, Multi-proxy geochemistry, Holocene, Palaeoenvironment, Palaeoclimate, ITCZ, Palaeomonsoon
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-91193 (URN)10.1016/j.quascirev.2013.01.030 (DOI)000318383900004 ()
Note

AuthorCount:10;

Available from: 2013-06-24 Created: 2013-06-24 Last updated: 2017-12-06Bibliographically approved
4. Hydroclimatic shifts in northeast Thailand during the last two millennia - the record of Lake Pa Kho
Open this publication in new window or tab >>Hydroclimatic shifts in northeast Thailand during the last two millennia - the record of Lake Pa Kho
Show others...
2015 (English)In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 111, 62-71 p.Article, review/survey (Refereed) Published
Abstract [en]

The Southeast Asian mainland is located in the central path of the Asian summer monsoon, a region where paleoclimatic data are still sparse. Here we present a multi-proxy (TOC, C/N, delta C-13, biogenic silica, and XRF elemental data) study of a 1.5 m sediment/peat sequence from Lake Pa Kho, northeast Thailand, which is supported by 20 AMS C-14 ages. Hydroclimatic reconstructions for Pa Kho suggest a strengthened summer monsoon between BC 170-AD 370, AD 800-960, and after AD 1450; and a weakening of the summer monsoon between AD 370-800, and AD 1300-1450. Increased run-off and a higher nutrient supply after AD 1700 can be linked to agricultural intensification and land-use changes in the region. This study fills an important gap in data coverage with respect to summer monsoon variability over Southeast Asia during the past 2000 years and enables the mean position of the Intertropical Convergence Zone (ITCZ) to be inferred based on comparisons with other regional studies. Intervals of strengthened/weaker summer monsoon rainfall suggest that the mean position of the ITCZ was located as far north as 35 degrees N between BC 170-AD 370 and AD 800-960, whereas it likely did not reach above 17 degrees N during the drought intervals of AD 370-800 and AD 1300-1450. The spatial pattern of rainfall variation seems to have changed after AD 1450, when the inferred moisture history for Pa Kho indicates a more southerly location of the mean position of the summer ITCZ.

Keyword
Wetland/peatland, Geochemistry, Paleoclimate, Last two millennia, Asian monsoon
National Category
Earth and Related Environmental Sciences
Research subject
Marine Geoscience; Marine Geology
Identifiers
urn:nbn:se:su:diva-117744 (URN)10.1016/j.quascirev.2015.01.007 (DOI)000353431100005 ()
Available from: 2015-06-05 Created: 2015-06-01 Last updated: 2017-12-04Bibliographically approved

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