Academic Report on 2017-5-18: Atmosphere-land-ocean interaction across South America along with millennial-scale AMOC slowdown during last glacial period

Subject: Atmosphere-land-ocean interaction across South America along with millennial-scale AMOC slowdown during last glacial period

Reporter：Dr. Yancheng Zhang

Time：14:00, May,8^th, 2016

Venue：Room  1102

Abstract

During the last glacial period, a succession of millennial-scale cold intervals was documented in Greenland ice cores, termed Heinrich stadials (HS) and Dansgaard-Oeschger (DO) stadials. These stadials, which involved significant reductions in the strength of the Atlantic meridional overturning circulation (AMOC),greatly influenced tropical South American precipitation due to reorganizations of the South American monsoon system and the Intertropical Convergence Zone (ITCZ). Detailed knowledge about changes of South American precipitation along with such cold intervals may provide relevant insights about the reaction of Amazonian biodiversity to future AMOC slowdown, e.g., ca. 20-40% over the 21^st century. During my PhD study, we combined geochemical proxies from different paleorecords (e.g., new sediment cores from the western tropical Atlantic and published terrestrial hydroclimate recordsacross tropical South America) and a set of sensitivity climate model experiments. We focused on the HS1 (18-15 ka ago) as a specific case to address: (i) the simulated reversal of the North Brazil Current (NBC); and (ii) the mechanism linking a weakened AMOC and tropical South American precipitation anomalies. Our results suggested that: (i) the NBC was very unlikely to reverse during HS1; and (ii) the eastern equatorial Pacific forcing was crucial to cause anomalous wet conditions over the Amazonian Andes. Besides, we also investigated the difference in spatial patterns of South American precipitation across HS and DO stadials. We found decreased precipitation over northernmost South America and increased precipitation over the Andes during both HS and DO stadials. In contrast, northeastern Brazilian precipitation experienced significant increases during HS, but was characterized by subtle changes during DO stadials. Regarding moderate AMOC slowdown associated with DO stadials relative to HS, we propose that shifts of the ITCZ mean position during DO stadials did not reached as far south as during HS.

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