Past climate variability: model analysis and proxy intercomparison
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This thesis investigates the climate variability of the late Quaternary (21 000 yrs BP to present day) using model simulations and proxy data. The thesis consists of four manuscripts and one appendix.
In the first two manuscripts and the appendix I, the extratropical Northern Hemisphere atmospheric circulation in different Quaternary time slices (preindustrial, PI, 1750 AD; Mid Holocene, MH, 6 kyrs BP; Last Glacial Maximum, LGM, 21 kyrs BP) is investigated using different climate models. The contributions of greenhouse gases, ice-sheet topography and albedo on the atmospheric mean climate and its variability are analyzed. In general, the models show no major changes in atmospheric circulation nor in its interannual variability in a climate slightly warmer (MH) than the PI one. In the LGM simulations, the models show decreased sea level pressure interannual variability relative to PI; on the other hand, the interannual variability of surface temperature is increased. The leading mode of sea level pressure variability in the North Atlantic is characterized by a NAO-like behavior in all climate states; however, it represents less total variance and the centers of action are weaker at the LGM. The presence of the Laurentide ice-sheet over North America during the LGM accounts for most of the changes observed in the LGM climate. Finally, the models show that the link between atmospheric and surface climate (temperature and precipitation) variability is altered in a glacial climate compared to the PI. Therefore, assuming present-day climate-proxy relationships when interpreting proxy records may well lead to a misinterpretation of past climates.
The results of the first manuscript point out that certain proxies may record seasonal rather than annual climate changes or that they could be tape recorders for climate changes far afield rather than local. These issues are tackled in the second and third manuscripts. In paper III, various marine proxy records from the North Atlantic Ocean that spann the Holocene (10-0 kyrs BP) are compared with each other and with a model simulation of the MH. Sea-surface temperature records based on phytoplankton generally show the existence of a warm early to mid Holocene (9-6 kyrs BP) optimum. In contrast, zooplankton-based temperature records from the North Atlantic and Norwegian Sea show a cool mid Holocene anomaly and a trend towards warmer temperatures in the late Holocene. Model results indicate that while the warming of the sea surface was stronger in summer during the MH compared to the PI due to higher solar radiation at the high latitudes, sub-surface depths experienced a cooling, mirroring the winter sea surface temperatures. These physical changes in the surface and sub-surface characteristics of the water column can explain the discrepancies between the Holocene trends exhibited by phytoplankton- and zooplankton- based temperature proxy records.
Paper III addresses the possibility that cave deposits, specifically in South Asia, record non-local rather than local climate changes. Using an atmospheric climate model with embedded stable water-isotope tracers, we propose a novel conceptual model to explain the oxygen isotopic changes recorded in Asian caves during abrupt climate changes (such as Heinrich events). We show the key role of the Indian Ocean in driving δ18O variations in both Indian and Chinese cave deposits: changes of the Indian Ocean surface temperature affects the Indian summer monsoon, which in turn leads to a change in the δ18O signature of the precipitation falling over the Indian subcontinent. This signal is eventually transferred from the Indian Ocean to Chinese caves via recycling of continental precipitation. Therefore, caves in eastern China (e.g., Hulu) do not record changes in the East Asian summer monsoon, as previously thought, but rather changes in the Indian summer monsoon.