Abstract
The Tibetan Plateau (TP) is primarily influenced by the northern hemispheric middle latitude Westerlies and the Indian summer monsoon (ISM). The extent, long-distance effects and potential long-term changes of these two atmospheric circulations are not yet fully understood. Here, we analyse modern airborne pollen in a transition zone of seasonally alternating dominance of the Westerlies and the ISM to develop a pollen discrimination index (PDI) that allows us to distinguish between the intensities of the two circulation systems. This index is applied to interpret a continuous lacustrine sedimentary record from Lake Nam Co covering the past 24 cal kyr BP to investigate long-term variations in the atmospheric circulation systems. Climatic variations on the central TP widely correspond to those of the North Atlantic (NA) realm, but are controlled through different mechanisms resulting from the changing climatic conditions since the Last Glacial Maximum (LGM). During the LGM, until 16.5 cal kyr BP, the TP was dominated by the Westerlies. After 16.5 cal kyr BP, the climatic conditions were mainly controlled by the ISM. From 11.6 to 9 cal kyr BP, the TP was exposed to enhanced solar radiation at the low latitudes, resulting in greater water availability.
Highlights
Been proposed for the coupling mechanisms and processes between the Tibetan Plateau (TP) and the North Atlantic (NA) regarding the climate signal transfer[12]
During the summer and fall, Pinus and Picea pollen may originate from the south or south-east, but these taxa occupy only a small proportion of the pollen assemblages because dominant local herbaceous taxa bloom during this period
Environmental, climatic and circulation proxies derived from the sedimentary record of Lake Nam Co reveal high variability throughout the examined period (Fig. 3)
Summary
Been proposed for the coupling mechanisms and processes between the TP and the NA regarding the climate signal transfer[12]. Prior to reaching the TP, the Westerlies have to cross large portions of Eurasia, whereas air masses related to the ISM reach the TP from the south after crossing only small areas of the Indian subcontinent These two pathways vary significantly in their general source area (Westerlies, northern hemisphere mid-latitudes; ISM, tropical Indian Ocean) and in their distance to water sources (oceans). 2005 to Aug. 2006) at Lake Nam Co (a large lake on the central TP) and used to establish a pollen discriminant index (PDI) to distinguish between the influences of the Westerlies and ISM. This PDI is interpreted in terms of the wind direction and acts as a proxy for the dominating atmospheric circulation system. The elemental concentrations of Ca and the ratio of Fe and Mn are used to indicate hydrological (lake level) changes to support the pollen-derived proxies
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