Late Quaternary Abrupt Climate Change in the Tropics and Sub-Tropics: The Continental Signal of Tropical Hydroclimatic Events (THEs)

Tropical hydroclimatic events, characterized by extreme regional rainfall anomalies, were a recurrent feature of marine isotope stages 2-4 and involved some of the most abrupt and dramatic climatic changes in the late Quaternary. These anomalies were pervasive throughout the tropics and resulted from the southward displacement of the Hadley circulation and the Intertropical Convergence Zone (ITCZ) and its associated convective rainfall, modulated by regional factors. Lake sediments, stalagmites, and offshore marine sediments that integrate inland continental conditions provide a comprehensive record of these changes over the past similar to 70,000 yr. Vast areas experienced severe drought while other areas recorded greatly increased rainfall. Within the uncertainties of dating, these tropical rainfall anomalies occurred very close in time (+/- 10(2)-10(3) yr) to the deposition of North Atlantic ice-rafted debris (IRD) that defines Heinrich events (HEs). The IRD record is a good proxy for the amount and distribution of additional freshwater forcing which was necessary to bring about a drastic reduction in the Atlantic Meridional Overturning Circulation (AMOC) strength during each HE. As a consequence of this reduction in AMOC and an abrupt expansion in the area of sea-ice, cooling of the North Atlantic and adjacent continents took place, with a rapid atmospheric response involving the southward displacement of the ITCZ and associated rainfall belts. The climatic consequences of this large-scale change in the Hadley circulation, modulated by regional factors, is clearly recorded throughout the tropics as a series of abrupt and extreme hydroclimatic events. Some of the physical mechanisms that may have played a role in those changes are discussed.