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Sea-ice transport driving Southern Ocean salinity and its recent trends

Nature volume 537, pages 89–92 (2016)Cite this article

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Abstract

Recent salinity changes in the Southern Ocean1,2,3,4,5,6,7 are among the most prominent signals of climate change in the global ocean, yet their underlying causes have not been firmly established1,3,4,6. Here we propose that trends in the northward transport of Antarctic sea ice are a major contributor to these changes. Using satellite observations supplemented by sea-ice reconstructions, we estimate that wind-driven8,9 northward freshwater transport by sea ice increased by 20 ± 10 per cent between 1982 and 2008. The strongest and most robust increase occurred in the Pacific sector, coinciding with the largest observed salinity changes4,5. We estimate that the additional freshwater for the entire northern sea-ice edge entails a freshening rate of −0.02 ± 0.01 grams per kilogram per decade in the surface and intermediate waters of the open ocean, similar to the observed freshening1,2,3,4,5. The enhanced rejection of salt near the coast of Antarctica associated with stronger sea-ice export counteracts the freshening of both continental shelf2,10,11 and newly formed bottom waters6 due to increases in glacial meltwater12. Although the data sources underlying our results have substantial uncertainties, regional analyses13 and independent data from an atmospheric reanalysis support our conclusions. Our finding that northward sea-ice freshwater transport is also a key determinant of the mean salinity distribution in the Southern Ocean further underpins the importance of the sea-ice-induced freshwater flux. Through its influence on the density structure of the ocean, this process has critical consequences for the global climate by affecting the exchange of heat, carbon and nutrients between the deep ocean and surface waters14,15,16,17.

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Figure 1: Effect of northward sea-ice freshwater transport on Southern Ocean salinity.
Figure 2: Mean state and trends of net annual freshwater fluxes associated with sea ice over the period 1982–2008.
Figure 3: Time series of annual northward sea-ice freshwater transport anomalies across latitude bands.
Figure 4: Mean annual sea-ice-related freshwater fluxes associated with melting, freezing and transport over the period 1982–2008.

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Acknowledgements

This work was supported by ETH Research Grant CH2-01 11-1 and by European Union (EU) grant 264879 (CARBOCHANGE). I.F. was supported by C2SM at ETH Zürich and the Swiss National Science Foundation Grant P2EZP2-152133. S.K. was supported by the Center of Excellence for Climate System Analysis and Prediction (CliSAP), University of Hamburg, Germany. F.A.H. and S.K. acknowledge support from the International Space Science Institute (ISSI), Bern, Switzerland, under project #245. We are thankful to F. Massonnet for providing the sea-ice thickness reconstruction and discussion. The ICESat-1 sea-ice thickness data were provided by the NASA Goddard Space Flight Center. The ship-based sea-ice thickness data were provided by the SCAR Antarctic Sea Ice Processes and Climate (ASPeCt) programme. We appreciate the provision of sea-ice concentration and motion data by the National Snow and Ice Data Center, the Integrated Climate Data Center at the University of Hamburg and R. Kwok. We thank T. Frölicher, S. Yang, A. Stössel, M. Frischknecht, L. Papritz, P. Durack, M. van den Broecke, J. Lenaerts, J. van Angelen and M. Meredith for discussion, comments, and ideas.

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Authors and Affiliations

  1. Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, Universitätstrasse 16, Zürich, 8092, Switzerland

    F. Alexander Haumann, Nicolas Gruber, Matthias Münnich & Ivy Frenger

  2. Center for Climate Systems Modeling, ETH Zürich, Universitätstrasse 16, Zürich, 8092, Switzerland

    F. Alexander Haumann & Nicolas Gruber

  3. Biogeochemical Modelling, GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, Kiel, 24105, Germany

    Ivy Frenger

  4. Integrated Climate Data Center (ICDC), Center for Earth System Research and Sustainability, University of Hamburg, Hamburg, Germany

    Stefan Kern

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  1. F. Alexander Haumann
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Contributions

F.A.H., M.M. and I.F. conceived the study. F.A.H. collated the data and performed the analyses. F.A.H. and N.G. wrote the manuscript. M.M., I.F. and S.K. assisted during the writing process. S.K. assisted in the quality and uncertainty assessment. All authors developed the methods and interpreted the results. N.G. and M.M. supervised this study.

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Correspondence to F. Alexander Haumann.

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Reviewer Information Nature thanks K. Ohshima and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Uncertainties and trends in Antarctic sea-ice concentration over the period 1982–2008.

a, BA minus CDR merged data. b, NTA minus CDR merged data. c, Decadal trends of the BA sea-ice concentration. Stippled trends are statistically significant (at a 90% confidence level or higher using Student’s t-test). d, Decadal trends of the BA minus NTA data. The thick grey line marks the mean sea-ice edge (1% sea-ice concentration). See Methods for details.

Extended Data Figure 2 Mean, trend and uncertainty of the Antarctic sea-ice thickness.

a, Decadal trends of the corrected reconstruction (1982–2008). Stippled trends are statistically significant (at a 90% confidence level or higher using Student’s t-test). b, Mean of the reconstruction (1982–2008). c, Mean of the corrected reconstruction (1982–2008). d, Mean of the non-gridded ICESat-1 data (2003–2008, 13 campaigns). e, Reconstruction minus non-gridded ICESat-1 data (2003–2008). f, Corrected reconstruction minus non-gridded ICESat-1 data (2003–2008). g, Mean of the ASPeCt data (1980–2005). h, Reconstruction minus ASPeCt data (1980–2005). i, Corrected reconstructions minus ASPeCt data (1980–2005). The thick grey line marks the mean sea-ice edge (1% sea-ice concentration). Differences are based on data points when both respective products were available. Data points without data in the sea-ice-covered region are shaded in grey in d–i. See Methods for details.

Extended Data Figure 3 Sea-ice drift speed comparison between the NSIDC and Kwok et al. data for the period 1992–2003.

a, b, Low-pass filtered, 21-d running mean for the original (a) and bias-corrected (b) daily meridional NSIDC sea-ice drift speed compared with the low-pass filtered daily meridional Kwok et al. data. Contours mark the number of grid boxes and the blue line marks the fitted least squares linear regression line. c–e, Mean sea-ice drift speed of the original (c) and bias-corrected NSIDC (d) and Kwok et al. (e) sea-ice drift speed. The arrows denote the drift vectors. f, R.m.s. differences between the annual mean bias-corrected NSIDC and Kwok et al. sea-ice drift speed. The thick grey line in c–f marks the mean sea-ice edge (1% sea-ice concentration). Data points were compared when both data sets were available. See Methods for details.

Extended Data Figure 4 Temporal inhomogeneities in the NSIDC satellite sea-ice drift data.

a, Annual mean meridional sea-ice drift speed averaged over the entire sea-ice area (sea-ice concentration >50%). The thick orange lines show the spurious trends due to changes in the underlying data. The black lines shown the data corrected for inconsistencies and used in this study (1982–2008). b, Low-pass filtered (91 d running mean) sea-ice drift speed averaged over the entire sea-ice area (sea-ice concentration >50%). The grey lines show the reduced wind speed from ERA-Interim using a reduction factor from the period 1988–2008. The uncorrected data for each satellite instrument combination are shown in colour (dashed lines show the mean over the respective period). The black vertical lines show the periods of the channels. The coloured text denotes the sensors and the frequency of the microwave radiometer channels used. c, The fraction of sea-ice covered grid boxes with at least one drift vector observation in a 21-d window and a 75 km × 75 km grid box using the non-gridded NSIDC drift data. The colours indicate the contribution of each sensor and channel. d, Different combinations of instruments and passive microwave sensor channels and the related periods underlying the NSIDC sea-ice drift data. See Methods for details.

Extended Data Figure 5 Time series and regions of annual northward sea-ice freshwater transport.

a–c, Transport from the coastal ocean to the open ocean region in the Southern Ocean (a), Atlantic sector (b) and Pacific sector (c). d, Transport across latitude bands in the Atlantic (69.5° S) and Pacific (71° S) sectors. Orange indicates transport estimates if temporal inhomogeneities were not accounted for. Blue shows homogeneous years only. Green represents homogenized time series. Years that have been corrected or removed are shaded in grey. Straight lines show the linear regressions for the periods 1982–2008 (dashed orange and green), 1982–1986 (solid orange) and 1988–2008 (homogeneous years only; solid blue). See Methods for details. e, Regions used for the evaluation of the sea-ice freshwater fluxes. Turquoise shading indicates the area south of the coastal Ross Sea flux gate13,36,66. Dark blue shading highlights the area south of the coastal Weddell Sea flux gate13. Purple lines are the 69.5° S latitude band in the Atlantic sector and the 71° S latitude band in the Pacific sector. The black line shows the smoothed mean zero sea-ice-ocean freshwater flux line that divides the coastal and open ocean regions (see Methods). The thick grey line shows the mean sea-ice edge (1% sea-ice concentration) and the green lines mark basin boundaries.

Extended Data Figure 6 Trends of the net annual freshwater fluxes associated with sea ice over the period 1982–2008 if temporal inhomogeneities in the sea-ice drift data were not considered.

a, b, Linear trends in the meridional sea-ice freshwater transport (a) and the net sea-ice–ocean freshwater flux from freezing and melting (b). The arrows in a denote the trend of the annual transport vectors. Stippled trends are significant at the 90% confidence level using Student’s t-test (Methods). Thick black lines show the zero sea-ice–ocean freshwater flux line used to divide the coastal from the open ocean regions; the thin black lines mark the continental shelf (1,000 m isobath) the grey lines show the sea-ice edge (1% sea-ice concentration) and the green lines indicate the basin boundaries.

Extended Data Figure 7 Contribution of sea-ice freshwater flux trends to ocean salinity.

a, Map showing the regions used for the estimation of salinity changes due to sea-ice freshwater fluxes. The blue lines show the sector important for AAIW formation (167° E to 23° W). The purple line is the Subantarctic Front79. The black line indicates the smoothed mean zero freshwater flux line that divides the coastal and open ocean regions. The thick grey line is the mean sea-ice edge (1% sea-ice concentration). b, The salinity response to a freshwater flux perturbation using the long-term equilibrium response (green) and using a delayed response starting in 1982 for a circumpolar reference volume (5 × 106 km3; purple) or for the region of most AAIW formation (2 × 106 km3; blue). See Methods for details. Dashed lines show the respective asymptotic equilibrium response. The black lines are the respective current trends. The grey shading shows the approximate observed long-term trend in the AAIW1,3,4. c, Observed long-term sea-surface salinity trends (data from ref. 85).

Extended Data Table 1 Mean and uncertainties of the annual sea-ice freshwater fluxes over the period 1982–2008
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Extended Data Table 2 Decadal trends of the annual sea-ice freshwater fluxes and their uncertainties over the period 1982–2008
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Extended Data Table 3 Sensitivity of the northward sea-ice freshwater transport trend to time periods and homogenization
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Haumann, F., Gruber, N., Münnich, M. et al. Sea-ice transport driving Southern Ocean salinity and its recent trends. Nature 537, 89–92 (2016). https://doi.org/10.1038/nature19101

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