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Distinct sources of interannual subtropical and subpolar Atlantic overturning variability

Nature Geoscience volume 14, pages 491–495 (2021)Cite this article

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Abstract

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat and carbon. Establishing the causes of historical variability in AMOC strength on different timescales can tell us how the circulation may respond to natural and anthropogenic changes at the ocean surface. However, understanding observed AMOC variability is challenging because the circulation is influenced by multiple factors that co-vary and whose overlapping impacts persist for years. Here we reconstruct and unambiguously attribute intermonthly and interannual AMOC variability at two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While, on interannual timescales, AMOC variability at 26° N is overwhelmingly dominated by a linear response to local wind stress, overturning variability at subpolar latitudes is generated by the combined effects of wind stress and surface buoyancy anomalies. Our analysis provides a quantitative attribution of subpolar AMOC variability to temperature, salinity and wind anomalies at the ocean surface.

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Fig. 1: Schematic of the large-scale ocean circulation in the subtropical and subpolar North Atlantic.
Fig. 2: Reconstruction of overturning in the North Atlantic.
Fig. 3: Contributions of SSS and SST to variability in overturning.
Fig. 4: Spatial origins of variability in overturning at the RAPID–MOCHA and OSNAP-EAST arrays.

Data availability

The OSNAP data products are publicly available at www.o-snap.org. The derived data including the OSNAP-EAST overturning are furthermore available in Duke Digital Repository, https://research.repository.duke.edu/collections/1z40kt318. The RAPID–MOCHA overturning time series is available at https://www.rapid.ac.uk/rapidmoc/rapid_data/datadl.php.

Code availability

The code for the MITgcm and the scripts for post-processing model output are available at https://github.com/MITgcm/. The ECCO state estimate model configuration can be downloaded from https://github.com/gaelforget/ECCOv4, with initial and boundary conditions available at https://web.corral.tacc.utexas.edu/OceanProjects/ECCO/ECCOv4. The TAF algorithmic differentiation software is proprietary and provided by FastOpt. Code used to process data and produce figures is available from the corresponding author upon reasonable request.

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Acknowledgements

This study used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). In our analysis, we apply the TAF software provided by FastOpt. The maps used in the figures and supplementary material were produced using the freely available software M_Map: A mapping package for MATLAB, provided by R. Pawlowicz. We thank the groups that maintain the OSNAP and RAPID–MOCHA observational networks and the developers of the ECCO version 4 state estimate. Y.K. was funded by the OSNAP project through NERC grant NE/K010948/1 and the TICTOC project through NERC grant NE/P019064/1. H.L.J. and D.P.M. were also funded by NERC grant NE/K010948/1. G.F. acknowledges support from NASA award no. 6937342 and the Simons Foundation award no. 549931. P.H., H.R.P. and T.S. were supported in part by NOAA grant NOAA/NA130AR4310135, NSF grant NSF-OCE-1924546 and a JPL/Caltech subcontract. T.S. received additional funding from an Oden Institute CSEM fellowship. N.P.H. was funded by the OSNAP NERC grant NE/K010875/1. M.S.L. and F.L. were supported by NSF grants OCE-1948335 and OCE-1924456.

Author information

Authors and Affiliations

  1. Geography, University of Exeter, Exeter, UK

    Yavor Kostov

  2. Earth Sciences, University of Oxford, Oxford, UK

    Helen L. Johnson

  3. Physics, University of Oxford, Oxford, UK

    David P. Marshall

  4. Oden Institute for Computational Engineering and Sciences, University of Texas at Austin, Austin, TX, USA

    Patrick Heimbach, Helen R. Pillar & Timothy Smith

  5. Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA

    Patrick Heimbach

  6. Institute for Geophysics, University of Texas at Austin, Austin, TX, USA

    Patrick Heimbach

  7. Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA

    Gael Forget

  8. National Oceanography Centre, Southampton, UK

    N. Penny Holliday

  9. Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

    M. Susan Lozier & Feili Li

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  1. Yavor Kostov
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Contributions

All authors discussed the results and contributed to the preparation of the final manuscript. Y.K. took the lead in writing the text while holding regular discussions with H.L.J., D.P.M., T.S. and H.R.P. Y.K. planned, designed and performed the adjoint sensitivity analysis with the MITgcm. P.H. and G.F. developed and maintained the ECCO version 4 state estimate and the associated tools for post-processing MITgcm output on an irregular grid. T.S. adapted the MITgcm diagnostic package. N.P.H., F.L. and M.S.L. developed and applied the data analysis methodology for OSNAP observations, and F.L. provided the OSNAP-EAST overturning time series.

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Correspondence to Yavor Kostov.

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Peer review information Nature Geoscience thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editor: James Super.

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Extended data

Extended Data Fig. 1 Reconstruction skill for OSNAP-EAST observations.

Comparison between observations (yellow envelope showing ±1 standard deviation of the observational uncertainty) and our two reconstructions (outer gray contours) of OSNAP-EAST overturning [Sv] based on two different sets of sensitivity patterns: one set from objective functions in 2001–2002, and a second set from objective functions in 2006–2007. The reconstructions are interpolated onto the same 30-day windows as the observations. We consider both the mean of our two reconstructions (middle gray contour) and the spread between them (outer gray contours). Note that our reconstruction estimate uses the ECCOv4r3 mean seasonal cycle, since the observational record at OSNAP-EAST is short.

Extended Data Fig. 2 Ekman transport contribution to overturning variability at RAPID-MOCHA in ECCO.

ECCO-based comparison between variability in RAPID-MOCHA overturning (purple) and Ekman transport variability at 26°N (orange) over the time-period of the linear reconstructions in the main text. Anomalies are shown relative to the long-term mean.

Extended Data Fig. 3 Geostrophic component of overturning in the North Atlantic.

Overturning variability (purple, volume transport in Sv) at OSNAP-EAST (a) and RAPID-MOCHA (b) in the ECCO state estimate contrasted against variability in the geostrophic component of overturning (orange). The comparison in a spans the time-period of the linear reconstructions in the main text. Anomalies are shown relative to the long-term mean.

Extended Data Fig. 4 Sensitivity of the OSNAP-EAST overturning to surface heat fluxes.

Sensitivity of the OSNAP-EAST overturning in February 2007 to net surface heat fluxes [Sv per (W m-2 sustained over 1 hour)] at a lead time of nine years. Red shading indicates that heat flux into the ocean contributes to a delayed strengthening of the OSNAP-EAST overturning 9 years later. Blue shading indicates that cooling the ocean surface at that lead time causes a lagged strengthening of the OSNAP-EAST overturning. Notice the pattern tracking the Gulf Stream – North Atlantic Current advective pathway from the Caribbean to the subpolar latitudes. This long memory of past sea surface fluxes motivates the use of AMOC sensitivity to SST and SSS instead.

Extended Data Fig. 5 North Atlantic mixed layer depth and spatial origins of buoyancy-driven variability in RAPID-MOCHA overturning.

a, Climatological March mixed layer depth [m] in ECCO; b–e, Spatial sources of variability in the RAPID-MOCHA AMOC overturning: root-mean-square contribution per unit area [Sv m-2] to the convolutions in equation(1) of the main text over the period 1992–2015 using sensitivity patterns based on (b,c) 2006–2007 and (d,e) 2001–2002 AMOC objective functions. Contributions due to SST (b,d), and SSS (c,e) all relative to the seasonal cycle. The scale in all panels is linear.

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Kostov, Y., Johnson, H.L., Marshall, D.P. et al. Distinct sources of interannual subtropical and subpolar Atlantic overturning variability. Nat. Geosci. 14, 491–495 (2021). https://doi.org/10.1038/s41561-021-00759-4

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