Abstract
The loss of oxygen from the ocean due to warming is not ubiquitous. In the Atlantic Ocean above 1 km depth, there is oxygen loss at subpolar latitudes, but there has been no oxygen loss or gain in the subtropics over the past six decades. Here we show that the amplification of the hydrological cycle, a response to climate change that results in a ‘salty-get-saltier, fresh-get-fresher’ sea surface salinity pattern, influences ocean ventilation and introduces a spatial pattern in the rate of climate change-driven oxygen loss in an Earth system model. A salinification enhances ventilation of (already salty) mode waters that outcrop in the subtropics and opposes warming-driven oxygen loss, while a freshening reduces ventilation of (already fresh) deep waters that outcrop at subpolar latitudes and accelerates oxygen loss. These results suggest that climate change introduces patterns of oxygenation through surface salinity changes, key to understanding observed and future regional changes.
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Data availability
The GFDL ESM2M data used in this study, after pre-processing, are hosted by Dataspace: https://doi.org/10.34770/jw96-r404. The ECMWF Ocean Re-Analysis 5 (ORAS5) data were accessed from the Copernicus Climate Data Store on 5 April 2022 (ref. 58). World Ocean Atlas 2018 (WOA18) oxygen data were accessed from the WOA18 webpage hosted by the NOAA National Centers for Environmental Information on 30 July 2019 (ref. 74).
Code availability
Code for producing figures is available with the data on Dataspace at https://doi.org/10.34770/jw96-r404.
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Acknowledgements
This study was supported by the National Science Foundation Career award 2042672. A.H. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant no. DGE-2039656. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. A.H. also acknowledges support from the Mary and Randall Hack Award at the Princeton University High Meadows Environmental Institute. G.V. and M.L. were supported by award 80NSSC20K0879 from the National Aeronautics and Space Administration. We thank S. Fueglistaler for insightful conversations, which greatly improved the manuscript and inspired the addition of Fig. 4.
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A.H., L.R., G.V. and M.L. conceived of the work and experiments; A.H. and L.R. led the analysis and writing.
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Extended data
Extended Data Fig. 1 Warming along mid-Atlantic Ocean section.
(a) ESM2M temperature trend due to total climate effect (200-year trend in the Standard experiment, averaged across 5 members), (b) ESM2M temperature trend due to hydrological effect (Difference in 200-year trends between the Standard and Fix- SSS experiments, averaged over 5 members for each experiment), (c) ORAS5 historical temperature trend (1958-2017). SST trend in (d) climate effect, (e) hydrological effect, (f) ORAS5 historical period.
Extended Data Fig. 2 Sea surface salinity trend in observation-based datasets and climate model.
Stippling in each panel indicates that the trend is not significant at the 95% confidence level. Datasets are as follows: ECMWF’s reanalysis ORAS54, National Center for Environmental Information’s objective analysis Levitus5, Japanese Meteorological Agency’s objective analysis Ishii6, UK-Hadley Centre’s objective analysis EN47, and Institute for Atmospheric Physics’ objective analysis CZ168. The final, lowest right, panel shows the 200-year trend in the ESM2M 1%-to-doubling experiment used in this study. Black contours show water masses according to density and salinity criteria outlined in Methods section, using the mean density and salinity in each dataset (for Levitus, the World Ocean Atlas 2018 dataset is used). Area south of 60◦S has been removed.
Extended Data Fig. 3 Salinity trend in observation-based datasets and climate model, following the trajectory along 25∘W shown in Figure 1.
Stippling in each panel indicates that the trend is not significant at the 95% confidence level. Datasets are as follows: ECMWF’s reanalysis ORAS54, National Center for Environmental Information’s objective analysis Levitus5, Japanese Meteorological Agency’s objective analysis Ishii6, UK-Hadley Centre’s objective analysis EN47, and Institute for Atmospheric Physics’ objective analysis CZ168. The final, lowest right, panel shows the 200-year trend in the ESM2M 1%-to-doubling experiment used in this study. Grey areas are bathymetry and white areas are missing data. Black contours show water masses according to density and salinity criteria outlined in Methods section, using the mean density and salinity in each dataset (for Levitus, the World Ocean Atlas 2018 dataset is used).
Extended Data Fig. 4 Hydrological effect on potential density, and contributions from temperature and salinity changes.
(a) Hydrological effect (200-year linear trends in Standard - Fix-SSS) on potential density referenced to the surface σθ, along 25◦W, as in Figure 4. (b) Potential density trend associated with the hydrological effect but due to (b) changes in temperature alone (temperature from Standard trajectory but salinity from Fix-SSS) and (c) changes in salinity alone (salinity from Standard trajectory but temperature from Fix-SSS). (d) Average difference in potential density, and contributions from salinity and temperature in each water mass. Densities computed with the gsw toolbox9. The section in panels a-c follows the black line shown on maps in Extended Dataset Figure 1. All for five-member ensemble mean.
Extended Data Fig. 5 Freshwater flux trends in response to CO2 forcing.
200-year freshwater flux trends in the ensemble-mean Standard 1%-to-doubling experiment. (a) Precipitation, (b) evaporation, (c) the net of precipitation - evaporation, and (d) the remaining fluxes from ice melt, ice calving, and runoff. (e) Zonal mean trend due to precipitation - evaporation (blue), which broadly follows the dry-get-drier, wet-get-wetter pattern, and the zonal mean of other components (grey).
Extended Data Fig. 6 Mean trends in (de)oxygenation and drivers.
(a) Model contributions to climate effect on oxygen changes (∆O2Hydro in black contours) attributed to changes in ventilation (∆O2Hydro,vent, dark blue), solubility (∆O2Hydro,sat, light blue) and biological activity (∆O2Hydro,bio, green). ∆O2Hydro,bio is only visible in Tropical Waters because it is 2-4 orders of magnitude smaller than the other terms in other water masses. (b) Ensemble mean change in oxygen in Atlantic Ocean water masses due to hydrological effect, reproduced from Figure 3c. Error bars represent the range across five ensemble members.
Extended Data Fig. 7 Trend difference in primary productivity.
Vertically integrated 200-year trend in primary productivity, Standard 1% minus Fix-SSS experiment. Note that primary productivity is limited to the sunlit region of the ocean, thus the vertical integral represents production in near-surface waters.
Extended Data Fig. 8 Mask of water masses in Atlantic Ocean.
Defined with 100-year annual average fields in the ESM2M pre-industrial control experiment, as described in Methods. Black contours are the smoothed water mass limits visualized in all other figures.
Extended Data Fig. 9 Mask of SSS restoring location, and Atlantic Ocean.
Global restoring of sea surface salinity is only applied in the light and dark blue locations, but is not applied where there is seasonal sea ice in pre-industrial control run (white regions). Dark blue mask based on ‘regionmask’ Python package is used to isolate Atlantic Ocean water masses in Figures 2, 4, and Extended Data Figure 3.
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Hogikyan, A., Resplandy, L., Liu, M. et al. Hydrological cycle amplification reshapes warming-driven oxygen loss in the Atlantic Ocean. Nat. Clim. Chang. 14, 82–90 (2024). https://doi.org/10.1038/s41558-023-01897-w
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DOI: https://doi.org/10.1038/s41558-023-01897-w