ABRUPT CLIMATE SHIFTS IN THE WESTERN TROPICAL TO SUBTROPICAL ATLANTIC REGION DURING THE LAST GLACIAL

Abstract

Millennial-scale climate oscillations of the last glacial, called Dansgaard/Oeschger (D/O) cycles, hold great potential for analysing mechanisms of global climate changes. In the tropics, D/O cycles are primarily expressed as precipitation pattern variations, which are coupled to latitudinal shifts of the Intertropical Convergence Zone (ITCZ). These arise from reorganisations of the atmospheric circulation in the northern hemisphere, which are driven by pronounced temperature changes in the high northern latitudes.This study investigates the coupling mechanisms between high northern latitude temperature changes and ITCZ shifts based on proxy records from sediment cores of the western tropical Atlantic. It evaluates the impact of ITCZ shifts on both the spatial extent and the amplitude of associated precipitation pattern changes. Furthermore, it analyses the relation between changes in North Atlantic Deep Water (NADW) production and shifts in tropical precipitation patterns.The main conclusions of this study may be summarised as follows:1) The tropical South American climate system underwent significant reorganisations during D/O cycles which were driven by ITCZ shifts2) The ITCZ position is dynamically coupled to variations in the North Atlantic meridional temperature gradient3) Precipitation records from equatorial regions are dominated by millennial-scale changes, whereas southern tropical regions primarily show shifts of orbital timescale4) In contrast to the modern climate system, tropical sea surface temperature changes do not alter South American precipitation patterns significantly on millennial timescales5) D/O cycles are associated with variations in western tropical Atlantic deep ocean conditions, which arise from changes in NADW production6) Tropical precipitation pattern changes are coupled to variations in NADW production via changes in high northern latitude temperatures, variations in the North Atlantic meridional temperature gradient and ITCZ shifts