Hydroclimate variability in central Vietnam: past and present

Wolf, Annabel (2022) Hydroclimate variability in central Vietnam: past and present. Doctoral thesis, Northumbria University.

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Abstract

Central Vietnam is one of only few locations within the Asian Monsoon realm, where the rainy season is associated with the Northeast Winter Monsoon. The winter monsoon in this region is characterised by strong north-easterly winds bringing moisture from the Pacific, leading to intensive rainfall during autumn and winter. This is often associated with natural hazards such as flooding and landslides, threatening the livelihood of millions in this region. Compared with the Asian Summer Monsoon, our knowledge of past and present rainfall variability related to the Northeast Winter Monsoon is more limited, which affects our ability to understand future climate scenarios in this region. This work uses stable isotopes in rainwater and the chemical composition of a stalagmite from central Vietnam to understand the climate in Southeast Asia in the present and past. Cave monitoring and climate simulations indicate that the seasonal cycle in rainwater stable isotopes from central Vietnam does not follow peak rainfall amount, but rather reflects the seasonal shift between the Indian Ocean, providing moisture during summer, and the Pacific Ocean which provides moisture during the rest of the year. This seasonal signal in rainfall oxygen isotopes is partly preserved in cave waters but low values are biased towards the season of recharge in autumn. This work presents the first high-resolution speleothem multi-proxy record from central Vietnam covering the Holocene. By using carbon isotopes and trace elements the history of the Northeast Winter Monsoon was reconstructed. In Southeast Asia, summer and winter monsoons evolved in-phase for most of the Holocene, between 8000 to 3000 years BP (before present). This in-phase relation shifts at 3000 years BP, after which the winter monsoon gets progressively wetter and the summer monsoon progressively drier. Here it is proposed that shifts in the Pacific Walker Circulation controlled the in-phase relation of the monsoons in Southeast Asia until 3000 years BP. Afterwards the summer monsoon was mainly controlled by changes in the Indian Walker Circulation and the winter monsoon by changes in Pacific sea surface temperatures. Investigating central Vietnam’s climate on a seasonal scale during a cool phase between 1600 and 1300 years BP showed that the timing of the ITCZ migration is key in modulating rainfall variability. A cooling of primarily autumn/winter sea surface temperatures in the western Pacific led to a delay of the ITCZ withdrawal during this season, causing enhanced rainfall in central Vietnam. The findings of this work have far-reaching implications for future palaeoclimate studies, such as the interpretation of proxies and the understanding of the monsoonal system in Southeast Asia over the Holocene.

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