On October 29, 2024, Sifan Gu, the tenure-track Associate Professor of School of Oceanography, Shanghai Jiao Tong University published a research article titled “Open ocean convection drives enhanced eastern pathway of the Glacial Atlantic Meridional Overturning Circulation” in Proceedings of the National Academy of Sciences.

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 years ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. This study investigates the three-dimensional AMOC during the LGM by combining sedimentary 231Pa/230Th reconstructions with model simulations, revealing a substantially intensified Eastern Pathway, which transports Glacial North Atlantic Intermediate Water from subpolar to subtropical North Atlantic in the eastern basin (Figure 1). A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite 231Pa/230Th changes between eastern and western basin during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic (Figure 2). Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC.

This work is supported by National Key R&D Program of China 2023YFF0806100, Science and Technology Innovation Project of Laoshan Laboratory LSKJ202203303, Chinese National Science Foundation grant 42106001, Oceanic Interdisciplinary Program of Shanghai Jiao Tong University SL2021PT102, Shanghai Frontiers Science Center of Polar Science.

Link to the article: https://www.pnas.org/doi/10.1073/pnas.2405051121

Figure 1. Difference of deep flow and 231Pa/230Th between LGM and PD. (a) Velocity difference (in cm/s) averaged over 1,500-3,500-m. Velocity vectors are shown for velocity difference larger than 0.2 cm/s (b) 231Pa/230Th difference averaged over 2,500-3,500-m, with observations from this depth range (Supplementary Table S1) overlaid as colored circles.

Figure 2. Eastern Pathway mechanism. (a-e) Potential vorticity (color shading with grey contour lines) and velocity (vector) at specific isopycnal layer in different experiments: (a) PD, isopycnal surface of 27.75 kg/m3; (b) PD_nowind, isopycnal surface of 27.8 kg/m3; (c) LGM, isopycnal surface of 29.2 kg/m3; (d) LGM_nowind, isopycnal surface of 29.2 kg/m3; (e) LGM_nowind_FB, isopycnal surface of 29.2 kg/m3.