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A recent decline in North Atlantic subtropical mode water formation

A Publisher Correction to this article was published on 22 September 2020

This article has been updated

Abstract

As a manifestation of mixing dynamics in the upper ocean, interannual and decadal variability of subtropical mode water (STMW) properties in the North Atlantic Ocean provides a valuable insight into ocean–atmosphere interaction in a changing climate. Here, we use hydrographic data from the Bermuda Atlantic Time-Series Study and Hydrostation S sites near Bermuda, as well as various ocean reanalysis products, to evaluate the modern variability of STMW properties. Our study finds an 86–93% loss of STMW thickness at these sites between 2010 and 2018 and a comparable loss throughout the western subtropical gyre, culminating in the weakest STMW pentad on record. We correlate this decline with a reduction in the annual outcropping volume and northward excursions of the formation region, suggesting a gyre-wide signal of weakening STMW generation. The outcropping volume of STMW is anti-correlated with surface ocean heat content, foreshadowing future STMW loss in the face of continued warming.

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Fig. 1: Map of the study zone and hydrographic profile information.
Fig. 2: Time-series plots of STMW properties from February 1955 to September 2019.
Fig. 3: Six-month window-averaged conditions from BATS and Hydrostation S.
Fig. 4: Time-series plots of STMW properties from observational and reanalysis datasets.
Fig. 5: Spatiotemporal variability of STMW thickness in the North Atlantic subtropical gyre.

Data availability

The authors declare that all data supporting the findings of this study can be found in the following online repositories. The BATS CTD data can be found at http://batsftp.bios.edu/BATS/. The Hydrostation S CTD and extended bottle datasets are available at http://batsftp.bios.edu/Hydrostation_S/. The ECMWF ORAS4 dataset can be accessed at https://climatedataguide.ucar.edu/climate-data/oras4-ecmwf-ocean-reanalysis-and-derived-ocean-heat-content. The Met Office Hadley Centre EN4g10 dataset can be accessed at https://www.metoffice.gov.uk/hadobs/en4/download-en4-2-0.html. The NAO index (DJFM) data can be accessed at https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-station-based. The Argo data can be accessed at https://doi.org/10.17882/42182. The final Argo data used in this study were accessed on 10 October 2019.

Code availability

All custom code used in this study can be found at https://doi.org/10.5281/zenodo.3620816.

Change history

  • 22 September 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

We acknowledge and thank the numerous principal investigators, researchers and technicians who have contributed to the BATS and Hydrostation S time-series projects since their inception. Additional thanks go to the officers and crew of the RV Weatherbird I, RV Weatherbird II and RV Atlantic Explorer. This work was funded by NSF grant OCE-1633215 and supported by the French National Program LEFE/INSU project SOMOVAR (North-Atlantic subtropical ocean: mechanisms of observed and projected low-frequency variability).

Author information

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Authors

Contributions

N.R.B. and R.J.J. are principal investigators for the BATS/Hydrostation S projects and coordinated the sampling and analysis of the data. S.W.S., N.R.B. and R.J.J. designed the study and performed the BATS/Hydrostation S analyses. S.W.S. and G.M. investigated the reanalysis datasets. S.W.S. wrote the manuscript with contributions from N.R.B., R.J.J. and G.M.

Corresponding author

Correspondence to Samuel W. Stevens.

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The authors declare no competing interests.

Additional information

Peer review information Nature Climate Change thanks Terrence Joyce, Olga Sato and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Time-series plots of STMW core properties.

Time-series of (a) <19C surface outcropping denoted by black vertical lines; (b) absolute salinity; (c) potential density anomaly; and (d) temperature gradient per 100 m from January 1988 to September 2019. Temperature gradient was calculated as the maximum temperature gradient, per metre, found in the identified STMW layer, multiplied by 100. Grey scatter points represent the full resolution time-series, solid black lines with white markers represent annual means, red shading represents the 95% confidence limits of the annual means, and red lines represent 2010–2018 trends (p-values < 0.05). Weak STMW pentad (2014–2018) is indicated by a cyan box. Annual means are expressed with 95% confidence limits calculated as the product of \({\sigma }_{\overline{x}}\) multiplied by the sample t-scores. Slopes are calculated using a Sen-Theil estimator \(\pm 2{\sigma }_{\overline{x}}\) of the slope. See methods section for further information on statistics.

Extended Data Fig. 2 Statistical comparison of STMW parameters for the two weakest STMW periods.

All date ranges are inclusive. The Argo and ORAS4 datasets are incomplete for certain periods. Ranges are expressed as the 95% confidence intervals of the mean properties within a given period, calculated as the product of σx and the sample t-scores. See methods section for further information on statistics.

Extended Data Fig. 3 Map of Argo float profiles.

Heat map denotes the amount of Argo profiles performed in 1 × 1grid cells within the formation zone. Scattered points denote the locations of individual profiles in within the formation zone.

Extended Data Fig. 4 Table detailing different STMW definitions.

Different STMW definitions and associated physical classifications employed in Extended Data Fig. 5.

Extended Data Fig. 5 Time-series plots of STMW properties based on different identification algorithms.

Data were averaged into annual means and the resulting signal was smoothed using a three-year moving mean. Red lines represent LS definition, blue lines represent MS definition, green lines represent DO definition, orange lines represent DI definition, olive lines represent BT definition, and brown lines represent BH definition. See Extended Data Fig. 4 for details of STMW definitions.

Extended Data Fig. 6 NAO-STMW correlation time-series.

Lag correlation plot for three-year centred moving average of the NAO DJFM index and the three primary measures of STMW strength. Negative lag occurs when NAO DJFM index leads STMW properties, positive lag occurs when NAO DJFM index lags STMW properties. Filled markers denote lags at which correlations are significant with p-values < 0.05.

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Stevens, S.W., Johnson, R.J., Maze, G. et al. A recent decline in North Atlantic subtropical mode water formation. Nat. Clim. Chang. 10, 335–341 (2020). https://doi.org/10.1038/s41558-020-0722-3

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