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

Marine Isotope Stage 4 (MIS 4) was punctuated by full glacial conditions between 70-60 ka. The transition into full glacial conditions was characterized by a substantial cooling of the oceans, extensive ice sheets formation expansion in Southern Hemisphere, a significant fall in sea level (about 100 m), a drawdown of atmospheric carbon dioxide (CO_2atm), and changes in ocean circulation. Some studies suggest a sudden cooling of the sea surface temperature (SST) in the southeastern South Atlantic during MIS 4, coinciding with temperature decreases in both Greenland and Antarctica. These findings lend support to the hypothesis of globally synchronous cold conditions during this period. However, recent paleoclimate records based on planktonic foraminiferal Mg/Ca ratios suggest a warming trend at SST along the western margin of the South Atlantic. The observed warming trend likely reflect regional oceanographic variability within the South Atlantic basin and challenges the notion that MIS 4 represented a period of globally in-phase cold conditions. To address this apparent anomaly, we evaluate the influence of (i) CO₂_atm concentration, (ii) Atlantic Meridional Overturning Circulation (AMOC) strength, and (iii) the southeastern trade wind system over SST in order to elucidate the mechanisms underlying the thermal response of the South Atlantic during MIS 4. To achieve this goal, ten records of Mg/Ca-based temperatures from planktonic foraminifera were compiled from sediment cores retrieved on both margins of the South Atlantic. The compiled data covered MIS 4, as well as a portion of MIS 5 and 3 (total time recovery: ~75-30 ka), providing a robust basis for reconstructing past oceanic and climatic conditions. The thermal evolution during MIS 4 manifests two distinct behaviors. During the early phase of this stage, a pronounced thermal contrast between the eastern and western margins is evident. It is proposed that this SST contrast during the early phase results from changes in the trade wind system. The pronounced intensification of both meridional and zonal wind components is likely to have enhanced upwelling along the eastern margin of the South Atlantic, simultaneously strengthening the South Equatorial Current and increasing the transport of warm waters towards the western South Atlantic. In contrast, the so called “late MIS 4” is equivalent to Heinrich Stadial 6, which is characterized by a weak AMOC, an increase in atmospheric CO_2atm and in global temperatures. Thus, the temperature increases over the South Atlantic is linked to the bipolar seesaw mechanism, which served as the primary driver of warming during this period. This pattern is evident in our stacks, along with a corresponding temperature decrease over the North Atlantic. These findings suggest a more prominent role of the trade wind system in modulating upper ocean circulation and driving the thermal evolution of the South Atlantic during the early phase of MIS 4 than previously reported in the literature.