Long-term variability of the western tropical Atlantic sea surface temperature driven by greenhouse gases and AMOC

Abstract

The long-term orbital-scale sea surface temperature (SST) variability in the tropics is thought to be mainly driven by greenhouse gases (GHG) forcing. However, few studies have investigated the drivers of such variability in the tropical Atlantic. Given the evidence of orbital-scale changes in Atlantic Meridional Overturning Circulation (AMOC) strength, one can hypothesize that AMOC variability also modulates the long-term tropical Atlantic SST through the bipolar seesaw mechanism. According to this mechanism, under weak [strong] AMOC conditions, the Southern Hemisphere is expected to warm up [cool down], while the Northern Hemisphere cools down [warms up]. Here, we investigate the long-term SST variability of the western tropical South Atlantic (WTSA), i.e., along the main pathway of the upper AMOC branch towards the equator, using a new 300 thousand years (kyr)-long Mg/Ca-based SST record. Our SST record shows glacial-interglacial variability superimposed by four remarkable long-term warm events during the three recorded glacial periods. These glacial warm events occurred between ca. 280–260, 160–143, 75–60, and 40–24 ka before present. Our results support the notion that atmospheric GHG plays a leading role in modulating the glacial-interglacial SST variability in the WTSA. However, it does not explain the occurrence of glacial warm events. Our study supports that the glacial warm events were caused by an orbital-scale bipolar seesaw mechanism operating in the Atlantic due to changes in the AMOC strength. These warm events may have been amplified by annual mean insolation driven by obliquity. Finally, we suggest that the long-term bipolar seesaw warmed the western tropical (South) Atlantic during the MIS 5/4 transition when the Earth's climate was cooling off.