Axel Timmermann | Pusan National University (original) (raw)

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Papers by Axel Timmermann

AGUFM, Dec 1, 2010

ABSTRACT The mid- to high-latitude region of the western-central North Pacific---including the Ku... more ABSTRACT The mid- to high-latitude region of the western-central North Pacific---including the Kuroshio-Oyashio transition area and marginal seas such as the Okhotsk, Japan, and Bering Seas---is a key area for understanding climate variability of the eastern Asian continent. Despite the review by the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project, the variations in sea surface and subsurface temperatures in the mid- to high-latitude region of the western-central North Pacific Ocean---including the Kuroshio-Oyashio transition area and marginal seas except for the Japan Sea---during the glacial-interglacial cycles have been insufficiently studied. Therefore, the purpose of this study was to clarify the interaction between atmospheric circulation, the sea-surface environment, and circulation in the intermediate-deep ocean in the western-central North Pacific and its marginal seas in response to global climate changes at the millennial time-scale during the last glacial-deglacial period. We determined sea surface and subsurface temperatures in the Okhotsk Sea during the Last Glacial Maximum (LGM) and the last deglaciation from measurements of alkenones and the Tetra Ether indeX of tetraethers consisting of 86 carbon atoms (TEX86) in piston core sediments, which reveal the climate response of this region to global climate changes. The TEX86-derived temperatures are different from the alkenone-derived temperatures from the same and nearby sediment samples in the Okhotsk Sea. This suggests that the different proxies reflect different aspects of thermal structure changes during the LGM and the last deglaciation. During the LGM, alkenone-derived temperatures in the Okhotsk Sea were relatively warm. In addition, during the last deglaciation, alkenone-derived temperatures changed in response to the millennial-scale climate change; from 19-10 kyr BP the main feature was higher temperatures during Heinrich Event 1 and Younger Dryas and lower during the Bølling-Allerød. The apparent warmer alkenone-derived temperatures during the cold events (LGM, H1 and YD) have also been found at many other sites in the western North Pacific and may reflect the shift in the season and depth of biomarker production from early summer and autumn to midsummer because of an expansion of the season of sea-ice cover. Empirical Orthogonal Function (EOF) analysis also indicated a shift in the alkenone production season as the first principal component. The EOF analysis further implied that the alkenone-derived temperature traced the precessional cycle of fall insolation at 45°N and millennial time-scale variability in the North Atlantic.

2014 AGU Fall Meeting, Dec 17, 2014

Global Biogeochemical Cycles, Nov 1, 2015

AGU Fall Meeting Abstracts, Dec 15, 2014

Japan Geoscience Union, Mar 10, 2017

AGU Fall Meeting Abstracts, Dec 1, 2014

Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last ... more Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000–19,000 years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise.

2015 AGU Fall Meeting, Dec 18, 2015

npj Climate and Atmospheric Science

The increasing frequency of heatwaves over East Asia (EA) is impacting agriculture, water managem... more The increasing frequency of heatwaves over East Asia (EA) is impacting agriculture, water management, and people’s livelihood. However, the effect of humidity on high-temperature events has not yet been fully explored. Using observations and future climate change projections conducted with the latest generation of Earth System models, we examine the mechanisms of dry and moist heatwaves over EA. In the dry heatwave region, anticyclonic circulation has been amplified after the onset of heatwaves under the influence of the convergence of anomalous wave activity flux over northern EA, resulting in surface warming via adiabatic processes. In contrast, the moist heatwaves are triggered by the locally generated anticyclonic anomalies, with the surface warming amplified by cloud and water vapor feedback. Model simulations from phase six of the Coupled Model Intercomparison Project projected display intensification of dry heatwaves and increased moist heatwave days in response to projected ...