A climate threshold at the eastern edgeof the Tibetan plateau

Proxy records of summer monsoon moisture at Lake Qinghai on the northeastern Tibetan Plateau reveal a late Quaternary climate history that is subtly different from that of speleothems from southern and eastern China. Total organic carbon and authigenic carbonate in two independently analyzed and dated cores indicate (1) relative stability and aridity during the glacial interval, (2) small variations during the B&oslash;lling-Aller&oslash;d and the Younger Dryas intervals, (3) comparatively abrupt change at the late Pleistocene/Holocene transition, and (4) relatively high variability during a wet early Holocene. Taken together, the data suggest that a climate threshold exists for penetration of Asian monsoon rainfall onto the Tibetan Plateau, a threshold that was crossed at the beginning of the Holocene. Conceptually, the threshold simply may be related to the topographic barrier that the eastern margin of the Tibetan Plateau presents to the landward penetration of the monsoon, or it may be related to nonlinearities in the climate system itself, such as sudden shifts in the configuration of the Westerly jet stream. Different mechanisms for producing a threshold are not mutually exclusive and may have combined to affect the dynamics of the Asian monsoon. In any case, the threshold is related to the presence of the Tibetan Plateau, which has a profound influence on the Asia monsoon system.</p

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.</p