The low-latitude monsoon climate during Dansgaard-Oeschger cycles and Heinrich Events

During the last 100,000 years Dansgaard-Oeschger cycles (D/O cycles) and Heinrich Events have been the dominant signal of past climate variability over Greenland and the North Atlantic. The succession of stadials (cold) and interstadials (warm) associated with these cycles has been documented in records from the entire northern hemisphere, South America, New Zealand, Antarctica, the South Atlantic and the Southern Ocean. Evidently, climate forcing in the D/O band affects both hemispheres. The origin and cause of these teleconnected patterns is still unknown, even if a large proportion of the cooling in Europe and northern Asia during Heinrich Events is a meteorological response to cold surface water in the North Atlantic resulting from the surge of the Laurentian and Scandinavian ice sheets. But, this does not imply that the D/O cycles are a priori caused by the collapse of these large icesheets. A severe challenge to a primary origin from the northern ice sheet and North Atlantic comes from the observation that warming in Antarctica and the circumpolar current leads the North Atlantic changes by some 1500 years (Charles et al., 1996; Blunier et al., 1998). In the second part of the paper we investigate new deep sea records from the low-latitude monsoon system, mainly corroborating the result of Schulz et al. (1998) that the Indian monsoon showed D/O variability. The abundance of eolian dust (indicating continental humidity/aridity) in core 70KL from the Arabian Sea shows humid intervals which seem to correlate with temperature maxima in the Antarctic Vostok ice core. Apparently, the low-frequency, sub-Milankovitch variability of the monsoon is associated with the southern hemisphere. The D/O-scale component in the monsoonal climate, on the other hand, shows a succession of short humid intervals. The sequence is most closely comparable to the Greenland temperature record and to the stadial/interstadial succession in the Pacific Santa Barbara Basin ODP 893. In order to understand the global forcing of climate change in the D/O band, it appears necessary to quantify the phase relationships not only between D/O cycles in the polar ice cores (Bender et al., 1994; Blunier et al., 1998; Steig et al., 1998), but also between the low latitude Pacific, Indian Ocean and the high-latitude records. (C) 1999 Elsevier Science Ltd. All rights reserved.