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Drivers of ocean warming in the western boundary currents of the Southern Hemisphere

Nature Climate Change volume 12, pages 901–909 (2022)Cite this article

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Abstract

Western boundary currents (WBCs) of the Southern Hemisphere transport heat poleward and are regions of rapid ocean warming. However, the mechanisms responsible for the enhanced warming over the Southern Hemisphere WBC extensions are still debated. Here we show that enhanced eddy generation in the WBC extensions through changes in barotropic and baroclinic instabilities results in enhanced ocean warming as the eddies propagate. This results from a poleward shift of the WBCs, associated with changes in the mid-latitude easterly winds. Consequently, the WBCs have penetrated poleward but not strengthened and are now transporting more heat into their extensions. Our study clearly elucidates the dynamic processes driving increased eddying and warming in the Southern Hemisphere WBC extensions and has implications for understanding and predicting ocean warming, marine heatwaves and the impact on the marine ecosystem in the WBC extensions under climate change.

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Fig. 1: Linear SST, SSH and surface EKE trends from observations in the SH.
Fig. 2: Linear SST, surface EKE, KmKe and PeKe trends in the SH WBCs.
Fig. 3: Mean and linear trends of meridional transport in the SH WBCs.
Fig. 4: Illustration and trends of subtropical ocean gyres in the SH.
Fig. 5: Large-scale wind pattern and trends associated with SAM.
Fig. 6: Schematic diagram of the mechanisms driving ocean warming in the SH WBC extensions.

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Data availability

The satellite altimetry products from AVISO were produced by Ssalto/Duacs and distributed by EU Copernicus Marine and Environment Monitoring Service and can be found at https://resources.marine.copernicus.eu/product-detail/SEALEVEL_GLO_PHY_L4_MY_008_047. The SST products OISST v.2.1 can be downloaded from https://www.ncei.noaa.gov/products/optimum-interpolation-sst. The BRAN2016 and BRAN2020 reanalysis are provided by CSIRO Australia and available at https://research.csiro.au/bluelink/outputs/data-access/. Ocean surface winds were taken from ECMWF’s ERA5 reanalysis product and can be accessed at https://doi.org/10.24381/cds.f17050d7. The SAM index56 was downloaded from http://lijianping.cn/dct/attach/Y2xiOmNsYjpBU0NJSTo4NjQ=.

Code availability

The SAM index and all Jupyter Notebook scripts used for producing the figures will be available in the github repository (https://github.com/lijunde/WBCs_SST_EKE) and publicly available in the figshare57 (https://doi.org/10.6084/m9.figshare.20473941.v1).

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Acknowledgements

M.R. acknowledges fundings from the Australian Research Council grant LP170100498. M.R. is an associate investigator at the Australian Research Council, Centre of Excellence for Climate Extremes (CE170100023). This research was undertaken with the assistance of resources and services from the National Computational Infrastructure, which is supported by the Australian Government. This research also includes computations using the computational cluster Katana (https://doi.org/10.26190/669x-a286) supported by Research Technology Services at UNSW Sydney.

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  1. School of Biological Earth and Environmental Sciences, University of New South Wales, Sydney, New South Wales, Australia

    Junde Li, Moninya Roughan & Colette Kerry

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M.R. and J.L. conceived the study and developed the conceptual framework. J.L. conducted the analysis and wrote the first draft. M.R., J.L. and C.K. contributed to interpreting the results, writing and editing the manuscript.

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Correspondence to Junde Li or Moninya Roughan.

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Extended data

Extended Data Fig. 1 Surface MKE and mean surface EKE over the 28-year (1993–2020) from AVISO and BRAN in the SH WBCs.

a, Spatial distribution of surface MKE in the AC system. The grey vectors indicate surface geostrophic velocities. The black line indicates the 0.9 m contour of climatological mean SSH (1993–2020) from AVISO. b,c, Same as a, but for the EAC and BC, respectively. The black line in c indicates the 0.6 m contour of climatological mean SSH from AVISO. d–f, Same as a–c, but for the surface MKE from BRAN. g–l, Same as a–c, but for the mean surface EKE from AVISO (g–i) and BRAN (j–l), respectively.

Extended Data Fig. 2 Observed mean SST, mean SST gradient magnitude and trends of SST gradient magnitude in the SH WBCs.

a, Spatial distribution of mean SST in the AC system. The black line indicates the 0.9 m contour of climatological mean SSH (1993–2020) from AVISO. b–c, Same as a, but for the EAC and BC, respectively. The black line in c indicates the 0.6 m contour of climatological mean SSH from AVISO. d–f, Same as a–c, but for the mean SST gradient magnitude. g–i, Same as a–c, but for the trends of SST gradient magnitude.

Extended Data Fig. 3 Mean KmKe and PeKe from BRAN over the upper 1000 m in the SH WBCs.

a, Spatial distribution of mean KmKe in the AC system. b–c, Same as a, but for the EAC and BC system, respectively. d–f, Same as a–c, but for the mean PeKe.

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Li, J., Roughan, M. & Kerry, C. Drivers of ocean warming in the western boundary currents of the Southern Hemisphere. Nat. Clim. Chang. 12, 901–909 (2022). https://doi.org/10.1038/s41558-022-01473-8

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