Enhanced zonal trade winds warm the western South Atlantic surface during MIS 4 glacial descent

Abstract

Marine Isotope Stage 4 (MIS 4, 70 - 60 ka BP) is commonly characterized by a rapid drop in global temperatures due to the establishment of glacial conditions. However, recent sea surface temperatures (SST) paleoclimatic records based on planktonic foraminiferal Mg/Ca ratios indicate a warming trend at the western margin of the South Atlantic during MIS 4. Here, we provide a compilation of Mg/Ca-based SST records from both the eastern and western margins of the South Atlantic, aiming to understand the main patterns and underlying mechanisms associated with the thermal evolution of the South Atlantic during MIS 4. Our results show that SST on the eastern margin of the South Atlantic followed the global cooling trend during the transition to MIS 4, while the western margin experienced warming during the same interval. We propose that the eastern-western thermal contrast observed in the first half of MIS 4 was caused by changes in the trade wind system. During the onset of MIS 4, a period of minimum Northern Hemisphere summer insolation led to an intensified trade wind zonal component and a weakened west African monsoon. This, in turn, strengthened the South Atlantic westward flowing currents, enhancing the transport of warm surface waters to the western margin. The intensified and more zonal southeastern trade winds increased eastern boundary upwelling, resulting in lower temperatures on the African margin. Our results highlight the importance of considering changes in the trade wind system in order to interpret the thermal responses of the South Atlantic during past glacial periods.