Abstract
The spreading pathway of the North Atlantic Deep Water, which is the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), determines how climate change signals are transported throughout the global ocean. The North Atlantic Deep Water is suggested to be transported from the subpolar Atlantic to the subtropics in the western basin by the Deep Western Boundary Current and the eddy-driven interior pathway west of the Mid-Atlantic Ridge. However, much less attention has been paid to AMOC cross-gyre transport in the eastern basin. Here, combining hydrographic observations and reanalysis, we identify a robust mid-depth Eastern Pathway located east of the Mid-Atlantic Ridge, which is further corroborated by model simulations with various resolutions, including eddy-resolving simulations. The Eastern Pathway accounts for half of the North Atlantic Deep Water transport across the intergyre boundary. Sensitivity experiments suggest that the mid-depth Eastern Pathway is formed by basin-scale ocean circulation dynamics due to wind steering on the intergyre communicating window instead of bottom topography. Our results provide a model for the AMOC pathway and call for further investigations on the climate response and variabilities associated with different AMOC pathways.
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Data availability
The CESM POP2 simulation outputs are publicly available via Zenodo (https://zenodo.org/records/10635012).
Code availability
The National Center for Atmospheric Research Command Language (version 6.6.2) was used for all of the analyses and figures in this study and is available from https://doi.org/10.5065/D6WD3XH5.
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Acknowledgements
We thank A. Bower, S. Lozier, J. Pedlosky and C. Wunsch for helpful discussions, as well as O. Wang for processing some of the ECCO output. Acknowledgement is extended to C. Böning and A. Biastoch for providing FLAME output. We acknowledge high-performance computing support from the National Center for Atmospheric Research and Laoshan Laboratory. This study was supported by the National Natural Science Foundation of China (42106001), Science and Technology Innovation Project of Laoshan Laboratory (LSKJ202203303), Oceanic Interdisciplinary Program of Shanghai Jiao Tong University (SL2021PT102) and Shanghai Frontiers Science Center of Polar Research (to S.G.), National Science Foundation (AGS 2321042), National Oceanic and Atmospheric Administration (NA20OAR4310403) and Department of Energy (SciDAC9233218CNA) (to Z.L.), National Natural Science Foundation of China (no. 42376005) (to S. Zou) and Science and Technology Innovation Project of Laoshan Laboratory (LSKJ202300402), Shandong Province’s Taishan Scientist Program (to S. Zhang).
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Z.L. and S.G. conceived of the study and wrote the paper. S.G. performed the analyses on WOA, ECCO, HighRes and POP2 and the experiments on POP2. S. Zou performed the analysis on FLAME. S. Zhang and Y.Y. contributed to the analyses on HighRes. S. Zhang contributed to the HighRes simulation. S.G. and C.H. contributed to analysis of the Coupled Model Intercomparison Project Phase 6 results. All authors discussed the results and contributed to the manuscript.
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Nature Geoscience thanks Marlos Goes and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super, in collaboration with the Nature Geoscience team.
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Extended data
Extended Data Fig. 1 FLAME results.
(a) Isopycnal depth of σ2 = 36.92 kg/m3. (b) meridional velocity (cm/s) at σ2 = 36.92 kg/m3. (c) 1500–2500 m average velocity. Velocities vectors are shown for larger than 0.2 cm/s. FLAME results are regirded into 1° resolution for the vectors. (d) AMOC. (e) The zonally integrated meridional velocity across the basin (\({\int }_{{X}_{W}}^{{X}_{E}}{vdx},\) black), the western basin (\({\int }_{{X}_{W}}^{30W}{vdx},\) blue) and the eastern basin (\({\int }_{30W}^{{X}_{E}}{vdx},\) red), with XW and XE being the western and eastern margins. (f) The southward (dots) and northward (circles) velocity averaged across the basin (black), the western basin (blue) and the eastern basin (red). (g) Meridional velocity at 45°N (color), with the steadiness \({\rm{s}}=\bar{v}/\sigma (v)\) in black dots for 1/4≤s < 1/2 and yellow dots for 1/2 ≤ s (Supplementary Information text); purple line is the isopycal of σ2 = 36.92 kg/m3.
Extended Data Fig. 2 Time evolution of Eastern Pathway and AMOC transports across 45°N.
(a) decadal mean EP transport for geostrophic velocities of WOA18 using the level of no-motion at 4-km (blue dots), and deep reference velocity from HighRes simulation (blue stars, Methods). (b) ECCO transports for monthly (cyan dash) and annual (blue solid) Eastern Pathway, monthly (magenta dash) and annual (red solid) AMOC. (c) Monthly (cyan dash) and annual (blue solid) EP, monthly (magenta dash) and annual (red solid) AMOC in the last 10 years in HighRes. (d) Decadal mean transports of Eastern Pathway (blue) and AMOC (red) in the last 200 years in HighRes.
Extended Data Fig. 3 Intergyre exchange flow and the transport decompositions into western and eastern basins at 45°N in 1° resolution experiments.
The left column is the zonal sections of the meridional velocity (color, in cm/s) and σ2 isopycnals (lines) in the intergyre boundary; the middle column is the zonally integrated meridional velocity across the basin (\({\int }_{{X}_{W}}^{{X}_{E}}{vdx},\) black), the western basin (\({\int }_{{X}_{W}}^{30W}{vdx},\) blue) and the eastern basin (\({\int }_{30W}^{{X}_{E}}{vdx},\) red), with XW and XE being the western and eastern margins; the right column is the southward (dots) and northward (circles) velocity averaged across the basin (black), the western basin (blue) and the eastern basin (red). (a–c) CTRL_R1 in 1° resolution. (d–f) NoWind_R1 in 1° resolution. (g–i) FB_R1. The isopycnal interval is 0.2 kg/m3 for the dashed lines and is 0.05 kg/m3 for the solid lines. The purple lines indicate the mid-depth isopycnal. The yellow lines indicate the intergyre communication window.
Extended Data Fig. 4 Meridional velocity at 47°N in the western Atlantic.
(a) Meridional velocity in HighRes, with the steadiness \(s=\bar{v}/\sigma (v)\) in black dots for 1/2 ≤ s < 1 and yellow dots for 1 ≤ s; green line is the isopycal of σ2 = 37.1 kg/m3. (b) Same as (a) but in FLAME (σ2 = 36.92 kg/m3). The simulated meridional velocity compares well with the observed meridional velocity in Fig. 1 from ref. 38.
Extended Data Fig. 5 Meridional velocity (color, in cm/s) and σ2 isopycnals (lines) at 45°N in different sensitivity experiments.
(a) NA_0.01Wind; (b) 15N-45N; (c) 45N-60N; (d) 40N-60N; (e) 40N-50N; (f) 50N-60N; (g)Wind_NoTilt; (h) FB_NoWind. The isopycnal interval is 0.2 kg/m3 for the dashed lines and is 0.05 kg/m3 for the solid lines.
Extended Data Fig. 6 Barotropic streamfunction (unit: Sv).
(a) ECCO, (b) CTRL, (c) NoWind, (d) FB, and (e) FB_NoWind. Topography is overlaid as grey contours in (b) and (d) with contour interval of 500-m.
Extended Data Fig. 7 Meridional velocity (color, in cm/s) and σ2 isopycnals (lines) at 45°N in different experiments.
(a) CTRL, (b) NA_0.5Wind, (c) NA_0.1Wind, and (d) NA_0.01Wind.
Extended Data Fig. 8 Trace distribution on a mid-depth isopycnal.
(a) Planetary potential vorticity in WOA18, (b) Potential vorticity in ECCO, (c) potential vorticity and (d) ideal age in HighRes, (e) potential vorticity and (f) ideal age in CTRL, (g) potential vorticity and (h) ideal age in NoWind, (i) potential vorticity and (j) ideal age in FB. Vectors are the velocity on the isopycnal surface (regrided to 1° resolution in HighRes for clarity). Potential vorticity unit is 10−12m−1s−1. Ideal age unit is year. σ2 = 37.05 kg/m3\(\,\text{in}\,\text{WOA}18,\text{}\) 37.03 kg/m3 in ECCO, 37.1 kg/m3 in HighRes, 37.0 kg/m3 in CTRL and NoWind, and 36.8 kg/m3 in FB.
Extended Data Fig. 9 Tracer distributions (color) and σ2 (line) at the intergyre boundary (45°N).
(a) Planetary potential vorticity in WOA18, (b) potential vorticity in ECCO, (c) potential vorticity and (d) ideal age in HighRes, (e) potential vorticity and (f) ideal age in CTRL, (g) potential vorticity and (h) ideal age in NoWind, (i) potential vorticity and (j) ideal age in FB. Vorticity is in 10−12m−1s−1 and ideal age is in year. The isopycnal interval is 0.2 kg/m3 for the dashed lines and is 0.05 kg/m3 for the solid lines.
Extended Data Fig. 10 CMIP6 AMOC Pathway.
Isopycnal depth and cross-gyre meridional velocity and transport in CMIP6 pre-industrial control simulations. Isopycnal depth (colour and grey contours) and velocity (vector) with velocity with larger than 0.2 cm/s plotted (a), meridional velocity at 45°N (color) with potential density overlaid (black contours) (b), and the zonally integrated meridional velocity across the basin (\({\int }_{{X}_{W}}^{{X}_{E}}{vdx},\) black), the western basin (\({\int }_{{X}_{W}}^{30W}{vdx},\) blue) and the eastern basin (c) in CanESM5 model (σ2 = 36.85 kg/m3). (d–f) in CNRM (σ2 = 37.0 kg/m3). (g–i) in MIROC (σ2 = 36.75 kg/m3). (j–l) in E3SM (σ2 = 36.95 kg/m3). (m–o) in BCC. (p–r) in GISS (σ2 = 37.1 kg/m3). These models show excessive export of NADW either via the Eastern Pathway (a-l) or close to the western boundary (p-r).
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Supplementary discussions, Figs. 1–7 and Table 1.
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Liu, Z., Gu, S., Zou, S. et al. Wind-steered Eastern Pathway of the Atlantic Meridional Overturning Circulation. Nat. Geosci. 17, 353–360 (2024). https://doi.org/10.1038/s41561-024-01407-3
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DOI: https://doi.org/10.1038/s41561-024-01407-3