Variability of East Asian winter monsoon in Quaternary climatic extremes in North China

In order to examine high-frequency variations of East Asian winter monsoon in Quaternary climatic extremes, two typical loess-paleosol sequences in th e Chinese Loess Plateau were investigated. Sandy layers in the loess deposi ts, the "Upper sand" and "Lower sand" (layers L9 and L15, respectively), wh ich represent a high-resolution record of paleomonsoon changes, have been s ampled at intervals of 5-6 cm from sections at Luochuan and Xifeng, The gra in size and magnetic susceptibility was measured for all samples, The grain -size results (a proxy of winter monsoon strength) indicate that the winter monsoon strength fluctuated on a millennial timescale during cold climatic extremes, with climatic events of a few hundred to a few thousand years. H owever, the winter monsoon was relatively stable during warm periods. The m agnetic susceptibility signal (a proxy of summer monsoon intensity) is prac tically constant over the same period. This is tentatively explained by the assumption that the summer monsoon intensity was too low to be recorded in the magnetic susceptibility signal. The intensified winter monsoon events show periodicities in a range of 1000 to 2770 yr, with a dominant cycle of approximately 1450 yr, The detection of this oscillation in older glacial s tages strongly suggests that it may be a pervasive cycle of the cold climat ic phases of the Quaternary, Millennial-scale variations of the winter mons oon may be caused by instability of the westerly jet, which is determined b y temperature differences between the polar and the equatorial regions. (C) 2000 University of Washington.