R.A. Nascimento, I.M. Venancio, C.M. Chiessi, J.M. Ballalai, H. Kuhnert, H. Johnstone, T.P. Santos, M. Prange, A. Govin, S. Crivellari, S. Mulitza, A.L.S. Albuquerque
The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward crossequatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This crossequatorial 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 ka-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (δ18O) and Mg/Ca of planktonic foraminifera. Our δ18O 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.
