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

Abstract

Abstract At its simplest, the Pacific cell of the Walker Circulation may be regarded 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 enhanced at about 5000 BP. The initial effect of this enhancement was the intensification of the Northern Australian Monsoon, within the Australian tropics, but this was reversed, abruptly, after 3700 BP. The enhancement is attributed to an increased polar-equator pressure gradient after 5000 BP with consequent intensification of the overall circulation pattern in the southern hemisphere including the mid-latitude westerlies and the trade winds. This in turn, appears to be a result of increased southern hemisphere seasonality, driven by the precessional cycle. It is proposed that increased wind speeds off South America after 5000 BP, in both mid- and low-latitudes, enhanced upwelling and reduced sea surface temperatures (SSTs). This created a greater east–west surface temperature contrast across the Pacific, compounding a pre-existing trend towards enhanced circulation, and ‘flipping’ climates in the southern hemisphere Pacific Basin from an Early Holocene mode to a Late Holocene mode. Under reduced circulation conditions, Early Holocene climates at low latitudes in the Southern Hemisphere may have been dominated by monsoonal heat transfers.