Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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

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.

This is a preview of subscription content, access via your institution

Access options

Buy article

Get time limited or full article access on ReadCube.

$32.00

All prices are NET prices.

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.

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).

References

  1. Hu, D. et al. Pacific western boundary currents and their roles in climate. Nature 522, 299–308 (2015).

    Article  CAS  Google Scholar 

  2. Shi, G., Ribbe, J., Cai, W. & Cowan, T. An interpretation of Australian rainfall projections. Geophys. Res. Lett. 35, L02702 (2008).

    Article  Google Scholar 

  3. Takahashi, T. et al. Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans. Deep-Sea Res. II 56, 554–577 (2009).

    Article  CAS  Google Scholar 

  4. Behrens, E., Fernandez, D. & Sutton, P. Meridional oceanic heat transport influences marine heatwaves in the Tasman Sea on interannual to decadal timescales. Front. Mar. Sci. 6, 228 (2019).

    Article  Google Scholar 

  5. Kwon, Y.-O. et al. Role of the Gulf Stream and Kuroshio–Oyashio systems in large-scale atmosphere–ocean interaction: a review. J. Clim. 23, 3249–3281 (2010).

    Article  Google Scholar 

  6. Wu, L. et al. Enhanced warming over the global subtropical western boundary currents. Nat. Clim. Change 2, 161–166 (2012).

    Article  CAS  Google Scholar 

  7. Yang, H. et al. Intensification and poleward shift of subtropical western boundary currents in a warming climate. J. Geophys. Res. Oceans 121, 4928–4945 (2016).

    Article  Google Scholar 

  8. Holbrook, N. J., Goodwin, I. D., McGregor, S., Molina, E. & Power, S. B. ENSO to multi-decadal time scale changes in East Australian Current transports and Fort Denison sea level: oceanic Rossby waves as the connecting mechanism. Deep-Sea Res. II 58, 547–558 (2011).

    Article  Google Scholar 

  9. Lumpkin, R. & Garzoli, S. Interannual to decadal changes in the western South Atlantic’s surface circulation. J. Geophys. Res. Oceans 116, C01014 (2011).

    Article  Google Scholar 

  10. Roemmich, D. et al. Decadal spinup of the South Pacific subtropical gyre. J. Phys. Oceanogr. 37, 162–173 (2007).

    Article  Google Scholar 

  11. Martínez-Moreno, J. et al. Global changes in oceanic mesoscale currents over the satellite altimetry record. Nat. Clim. Change 11, 397–403 (2021).

    Article  Google Scholar 

  12. Beal, L. M. & Elipot, S. Broadening not strengthening of the Agulhas Current since the early 1990s. Nature 540, 570–573 (2016).

    Article  CAS  Google Scholar 

  13. Imawaki, S., Bower, A. S., Beal, L. & Qiu, B. in Ocean Circulation and Climate: A 21st Century Perspective 2nd edn (eds Siedler, G. et al.) 305–338 (Academic Press, 2013).

  14. Todd, R. E. et al. Global perspectives on observing ocean boundary current systems. Front. Mar. Sci. 6, 423 (2019).

    Article  Google Scholar 

  15. Marshall, G. J. Trends in the Southern Annular Mode from observations and reanalyses. J. Clim. 16, 4134–4143 (2003).

    Article  Google Scholar 

  16. Thompson, D. W. J. & Wallace, J. M. Annular modes in the extratropical circulation. Part I: month-to-month variability. J. Clim. 13, 1000–1016 (2000).

    Article  Google Scholar 

  17. Cai, W. Antarctic ozone depletion causes an intensification of the Southern Ocean super-gyre circulation. Geophys. Res. Lett. 33, L03712 (2006).

    Article  Google Scholar 

  18. Beal, L. M., Ruijter, W. P. M. D., Biastoch, A., Zahn, R. & SCOR/WCRP/IAPSO Working Group 136. On the role of the Agulhas system in ocean circulation and climate. Nature 472, 429-436 (2011).

  19. Cai, W., Whetton, P. H. & Karoly, D. J. The response of the Antarctic Oscillation to increasing and stabilized atmospheric CO2. J. Clim. 16, 1525–1538 (2003).

    Article  Google Scholar 

  20. Fogt, R. L. & Marshall, G. J. The Southern Annular Mode: variability, trends, and climate impacts across the Southern Hemisphere.WIREs Clim. Change 11, e652 (2020).

    Article  Google Scholar 

  21. Qu, T., Fukumori, I. & Fine, R. A. Spin-up of the Southern Hemisphere super gyre. J. Geophys. Res. Oceans 124, 154–170 (2019).

    Article  Google Scholar 

  22. Cai, W., Shi, G., Cowan, T., Bi, D. & Ribbe, J. The response of the Southern Annular Mode, the East Australian Current, and the southern mid-latitude ocean circulation to global warming. Geophys. Res. Lett. 32, L23706 (2005).

    Article  Google Scholar 

  23. Yang, H. et al. Poleward shift of the major ocean gyres detected in a warming climate. Geophys. Res. Lett. 47, e2019GL085868 (2020).

    Article  Google Scholar 

  24. Zilberman, N. V., Roemmich, D. H. & Gille, S. T. Meridional volume transport in the South Pacific: mean and SAM-related variability. J. Geophys. Res. Oceans 119, 2658–2678 (2014).

    Article  Google Scholar 

  25. Biastoch, A., Böning, C. W., Schwarzkopf, F. U. & Lutjeharms, J. R. E. Increase in Agulhas leakage due to poleward shift of Southern Hemisphere westerlies. Nature 462, 495–498 (2009).

    Article  CAS  Google Scholar 

  26. Backeberg, B. C., Penven, P. & Rouault, M. Impact of intensified Indian Ocean winds on mesoscale variability in the Agulhas system. Nat. Clim. Change 2, 608–612 (2012).

  27. Malan, N., Roughan, M. & Kerry, C. The rate of coastal temperature rise adjacent to a warming western boundary current is nonuniform with latitude. Geophys. Res. Lett. 48, e2020GL090751 (2021).

    Article  Google Scholar 

  28. Li, J., Roughan, M. & Kerry, C. Variability and drivers of ocean temperature extremes in a warming western boundary current. J. Clim. 35, 1097–1111 (2022).

    Article  Google Scholar 

  29. Artana, C., Provost, C., Poli, L., Ferrari, R. & Lellouche, J.-M. Revisiting the Malvinas Current upper circulation and water masses using a high-resolution ocean reanalysis. J. Geophys. Res. Oceans 126, e2021JC017271 (2021).

    Article  Google Scholar 

  30. van Sebille, E. et al. Relating Agulhas leakage to the Agulhas Current retroflection location. Ocean Sci. 5, 511–521 (2009).

    Article  Google Scholar 

  31. Pilo, G. S., Oke, P. R., Rykova, T., Coleman, R. & Ridgway, K. Do East Australian Current anticyclonic eddies leave the Tasman Sea? J. Geophys. Res. Oceans 120, 8099–8114 (2015).

    Article  Google Scholar 

  32. Oke, P. R. et al. Revisiting the circulation of the East Australian Current: its path, separation, and eddy field. Prog. Oceanogr. 176, 102139 (2019).

    Article  Google Scholar 

  33. Schmidt, C., Schwarzkopf, F. U., Rühs, S. & Biastoch, A. Characteristics and robustness of Agulhas leakage estimates: an inter-comparison study of Lagrangian methods. Ocean Sci. 17, 1067–1080 (2021).

    Article  Google Scholar 

  34. Cushman-Roisin, B. & Beckers, J.-M. Introduction to Geophysical Fluid Dynamics: Physical and Numerical Aspects 2nd edn (Academic Press, 2011).

  35. Chamberlain, M. A. et al. Next generation of Bluelink ocean reanalysis with multiscale data assimilation: BRAN2020. Earth Syst. Sci. Data 13, 5663–5688 (2021).

    Article  Google Scholar 

  36. Sen Gupta, A. et al. Future changes to the upper ocean western boundary currents across two generations of climate models. Sci. Rep. 11, 9538 (2021).

    Article  Google Scholar 

  37. Zhu, Y., Qiu, B., Lin, X. & Wang, F. Interannual eddy kinetic energy modulations in the Agulhas Return Current. J. Geophys. Res. Oceans 123, 6449–6462 (2018).

    Article  Google Scholar 

  38. Zhu, Y., Li, Y., Zhang, Z., Qiu, B. & Wang, F. The observed Agulhas Retroflection behaviors during 1993–2018. J. Geophys. Res. Oceans 126, e2021JC017995 (2021).

    Article  Google Scholar 

  39. Li, J., Roughan, M. & Kerry, C. Dynamics of interannual eddy kinetic energy modulations in a western boundary current. Geophys. Res. Lett. 48, e2021GL094115 (2021).

    Article  Google Scholar 

  40. Magalhães, F. C., Azevedo, J. L. L. & Oliveira, L. R. Energetics of eddy-mean flow interactions in the Brazil Current between 20 S and 36 S. J. Geophys. Res. Oceans 122, 6129–6146 (2017).

    Article  Google Scholar 

  41. Brum, A. L., de Azevedo, J. L. L., de Oliveira, L. R. & Calil, P. H. R. Energetics of the Brazil Current in the Rio Grande Cone region. Deep-Sea Res. I 128, 67–81 (2017).

  42. Sloyan, B. M. & O’Kane, T. J. Drivers of decadal variability in the Tasman Sea. J. Geophys. Res. Oceans 120, 3193–3210 (2015).

    Article  Google Scholar 

  43. Goes, M., Cirano, M., Mata, M. M. & Majumder, S. Long-term monitoring of the Brazil Current transport at 22S from XBT and altimetry data: seasonal, interannual, and extreme variability. J. Geophys. Res. Oceans 124, 3645–3663 (2019).

    Article  Google Scholar 

  44. Chidichimo, M. P. et al. Brazil Current volume transport variability during 2009–2015 from a long-term moored array at 34.5 S. S. J. Geophys. Res. Oceans 126, e2020JC017146 (2021).

    Google Scholar 

  45. Drouin, K. L., Lozier, M. S. & Johns, W. E. Variability and trends of the South Atlantic subtropical gyre. J. Geophys. Res. Oceans 126, e2020JC016405 (2021).

    Article  Google Scholar 

  46. Fadida, Y., Malan, N., Cronin, M. F. & Hermes, J. Trends in the Agulhas Return Current. Deep-Sea Res. I 175, 103573 (2021).

    Article  Google Scholar 

  47. Shi, J.-R., Talley, L. D., Xie, S.-P., Peng, Q. & Liu, W. Ocean warming and accelerating Southern Ocean zonal flow. Nat. Clim. Change 11, 1090–1097 (2021).

    Article  Google Scholar 

  48. Ridgway, K. R. Long-term trend and decadal variability of the southward penetration of the East Australian Current. Geophys. Res. Lett. 34, L13613 (2007).

    Article  Google Scholar 

  49. Hill, K. L., Rintoul, S. R., Ridgway, K. R. & Oke, P. R. Decadal changes in the South Pacific western boundary current system revealed in observations and ocean state estimates. J. Geophys. Res. Oceans 116, C01009 (2011).

    Article  Google Scholar 

  50. Cetina-Heredia, P., Roughan, M., Sebille, E. V. & Coleman, M. A. Long-term trends in the East Australian Current separation latitude and eddy driven transport. J. Geophys. Res. Oceans 119, 4351–4366 (2014).

    Article  Google Scholar 

  51. Combes, V. & Matano, R. P. Trends in the Brazil/Malvinas Confluence region. Geophys. Res. Lett. 41, 8971–8977 (2014).

    Article  Google Scholar 

  52. Chelton, D. B., Schlax, M. G., Freilich, M. H. & Milliff, R. F. Satellite measurements reveal persistent small-scale features in ocean winds. Science 303, 978–983 (2004).

    Article  CAS  Google Scholar 

  53. Kang, D. & Curchitser, E. N. Energetics of eddy-mean flow interactions in the Gulf Stream region. J. Phys. Oceanogr. 45, 1103–1120 (2015).

    Article  Google Scholar 

  54. Halo, I. et al. Mesoscale eddy variability in the southern extension of the East Madagascar Current: seasonal cycle, energy conversion terms, and eddy mean properties. J. Geophys. Res. Oceans 119, 7324–7356 (2014).

    Article  Google Scholar 

  55. Yue, S. & Wang, C. The Mann–Kendall test modified by effective sample size to detect trend in serially correlated hydrological series. Water Resour. Manag. 18, 201–218 (2004).

    Article  Google Scholar 

  56. Nan, S. & Li, J. The relationship between the summer precipitation in the Yangtze River valley and the boreal spring Southern Hemisphere annular mode. Geophys. Res. Lett. 30, 2266 (2003).

    Article  Google Scholar 

  57. Li, J., Roughan, M. & Kerry, C. WBCs_SST_EKE. Software. figshare https://doi.org/10.6084/m9.figshare.20473941.v1 (2022).

Download references

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.

Author information

Authors and Affiliations

Authors

Contributions

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.

Corresponding authors

Correspondence to Junde Li or Moninya Roughan.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Climate Change thanks Helene Hewitt, Dujuan Kang and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Additional information

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

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. df, Same as ac, but for the surface MKE from BRAN. gl, Same as ac, but for the mean surface EKE from AVISO (gi) and BRAN (jl), 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. bc, 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. df, Same as ac, but for the mean SST gradient magnitude. gi, Same as ac, 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. bc, Same as a, but for the EAC and BC system, respectively. df, Same as ac, but for the mean PeKe.

Rights and permissions

Springer Nature or its licensor holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41558-022-01473-8

This article is cited by

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing