Tropical Atlantic stratification response to late Quaternary precessional forcing

The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward cross-equatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This cross-equatorial transport is influenced by the thermocline circulation and stratification. Although seasonal thermocline stratification in the WTA is precession-driven, the existence of an orbital pacemaker of changes in the entire WTA upper ocean stratification, which comprises the main thermocline, remains elusive. Here, we present a 300 Ica-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (delta O-18) and Mg/Ca of planktonic foraminifera. Our Delta delta O-18 record between Globigerinoides ruber and Globorotalia truncatulinoides, representing upper ocean stratification, shows a robust precession pacing, where strong stratification was linked to high summer insolation in the Northern Hemisphere (precession minima). Mg/Ca-based temperatures support that stratification is dominated by changes in thermocline temperature. We present a new mechanism to explain changes in WTA stratification, where during the Northern Hemisphere summer insolation maxima, the Intertropical Convergence Zone shifts northward, developing a negative wind stress curl anomaly in the tropical Atlantic. This, in turn, pulls the main thermocline up and pushes the South Atlantic Subtropical Gyre southwards, increasing the stratification to the north of the gyre. This mechanism is supported by experiments performed with the Community Earth System Model (CESM1.2). Finally, we hypothesize that the precession-driven WTA stratification may affect the cross-equatorial flow into the North Atlantic. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study revealed the importance of changes in upper ocean circulation in the western tropical Atlantic for the northward heat transport as part of the Atlantic Meridional Overturning Circulation, with seasonal thermocline stratification being driven by precession. Using oxygen isotopes and Mg/Ca, the research confirmed that changes in thermocline temperature predominantly impact stratification.