Australasian evidence for mid-holocene climate change implies precessionalcontrol of Walker Circulation in the Pacific

Pit its simplest, the Pacific cell of the Walker Circulation may be regarde d as a thermal circulation, driven by the temperature contrast between the west and east Pacific. Evidence from Australasia is summarized, indicating that this circulation was greatly weakened in the early Holocene but was en hanced at about 5000 BP. The initial effect of this enhancement was the int ensification of the Northern Australian Monsoon, within the Australian trop ics, but this was reversed, abruptly, after 3700 BP. The enhancement is att ributed to an increased polar-equator pressure gradient after 5000 BP with consequent intensification of the overall circulation pattern in the southe rn hemisphere including the mid-latitude westerlies and the trade winds. Th is in turn, appears to be a result of increased southern hemisphere seasona lity, driven by the precessional cycle. It is proposed that increased wind speeds off South America after 5000 BP, in both mid- and low-latitudes, enh anced upwelling and reduced sea surface temperatures (SSTs). This created a greater east-west surface temperature contrast across the Pacific, compoun ding a pre-existing trend towards enhanced circulation, and 'flipping' clim ates in the southern hemisphere Pacific Basin from an Early Holocene mode t o a Late Holocene mode. Under reduced circulation conditions, Early Holocen e climates at low latitudes in the Southern Hemisphere may have been domina ted by monsoonal heat transfers. (C) 1999 INQUA/Elsevier Science Ltd. All r ights reserved.