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Summertime increases in upper-ocean stratification and mixed-layer depth

An Author Correction to this article was published on 02 August 2021

This article has been updated

Abstract

The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchange between the atmosphere and the oceanic interior1,2,3. The mixed layer also shapes marine ecosystems by hosting most of the ocean’s primary production4 and providing the conduit for oxygenation of deep oceanic layers. Despite these important climatic and life-supporting roles, possible changes in the mixed layer during an era of global climate change remain uncertain. Here we use oceanographic observations to show that from 1970 to 2018 the density contrast across the base of the mixed layer increased and that the mixed layer itself became deeper. Using a physically based definition of upper-ocean stability that follows different dynamical regimes across the global ocean, we find that the summertime density contrast increased by 8.9 ± 2.7 per cent per decade (10−5–10−4 per second squared per decade, depending on region), more than six times greater than previous estimates. Whereas prior work has suggested that a thinner mixed layer should accompany a more stratified upper ocean5,6,7, we find instead that the summertime mixed layer deepened by 2.9 ± 0.5 per cent per decade, or several metres per decade (typically 5–10 metres per decade, depending on region). A detailed mechanistic interpretation is challenging, but the concurrent stratification and deepening of the mixed layer are related to an increase in stability associated with surface warming and high-latitude surface freshening8,9, accompanied by a wind-driven intensification of upper-ocean turbulence10,11. Our findings are based on a complex dataset with incomplete coverage of a vast area. Although our results are robust within a wide range of sensitivity analyses, important uncertainties remain, such as those related to sparse coverage in the early years of the 1970–2018 period. Nonetheless, our work calls for reconsideration of the drivers of ongoing shifts in marine primary production, and reveals stark changes in the world’s upper ocean over the past five decades.

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Fig. 1: The three-layer structure of the world ocean.
Fig. 2: Climatological upper-ocean stratification and mixed-layer depth.
Fig. 3: 1970–2018 trends in summer upper-ocean stratification and mixed-layer depth.
Fig. 4: Temperature and salinity contributions to pycnocline stratification and its change.
Fig. 5: Regional time series of summer pycnocline stratification and mixed-layer depth anomaly.

Data availability

All information about the source database used in the paper is available at https://github.com/jbsallee-ocean/GlobalMLDchange/tree/main/Databases. The resulting global maps of trends and climatological fields presented here are available at https://zenodo.org/record/4073174#.YA_jsC2S3XQ (https://doi.org/10.5281/zenodo.4073174.

Code availability

The code used to generate the analysis presented in the paper and its Supplementary Information is available at https://github.com/jbsallee-ocean/GlobalMLDchange.

Change history

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Acknowledgements

This project received funding from the European Union’s Horizon 2020 Research and Innovation programme under grant agreement number 821001. V.P., C.A., E.P. and L.V. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation programme (grant agreement 637770). A.N.G. acknowledges the support of the Royal Society and the Wolfson Foundation. P.S. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation programme (grant agreement No 805186). We thank I. Young for providing the percentage rate change of 10-m wind speed from recently published analysis. We thank L. Sigelman and G. Madec for comments and discussions that greatly helped us to refine our study.

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Contributions

J.-B.S. designed the experiment and performed the computations and data analyses; V.P., C.A., E.P. and L.V. helped with the development of the global database and its analysis, and evaluated the analysis; S.S. developed the mapping method; and A.N.G. and P.S. provided expertise on surface ocean turbulence and associated scaling arguments. M.K. provided expertise on the statistical methods used in this study. All authors discussed the results and wrote the manuscript.

Corresponding author

Correspondence to Jean-Baptiste Sallée.

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The authors declare no competing interests.

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Peer review information Nature thanks Paul Durack, Stephen Riser and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Trends associated with the vertical structure of the upper ocean.

a, b, Schematics showing idealized density profiles in the upper ocean for the cases in which the mixed layer and pycnocline are shallower (a) and deeper (b) than 200 m. The black line shows the typical shape of the density profile with a total density increase of ∂ρ across the pycnocline (thickness h) and the mixed layer (thickness H). The dashed red lines show the density profiles after lightening of the mixed layer with no change of mixed-layer depth, and the dotted red lines show the density profiles after lightening of the mixed layer concomitant with a deepening of the mixed layer.

Extended Data Fig. 2 Geographical distribution of available observations.

ad, Number of mixed layer estimates in 1° × 1° longitude–latitude bins: from all available observation sources (a), from ship profiles (b), from Argo profiles (c) and from instrumented marine mammal observations (d). e, f, Maximum time span (in years) covered by the combined dataset in 1° × 1° longitude–latitude bins in summer (e) and in winter (f).

Extended Data Fig. 3 Impact of linear-regression choices on mean mixed layer.

ad, Winter (a, b) and summer (c, d) mean mixed-layer depth computed using slightly different linear-regression models: Choice 1 (a, c; covariance between observations) and Choice 2 (b, d; no covariance between observations). See Methods for more details.

Extended Data Fig. 4 Impact of linear-regression choices on summer mixed-layer depth, stratification trends and their associated standard errors.

ah, 1970–2018 summer trend for mixed-layer depth (a, c) and summer pycnocline stratification (e, g) and their associated standard error: standard error of mixed-layer depth trend (b, d) and standard error of summer pycnocline stratification trend (f, h). a, b, e, f show the solution computed with the linear-regression model Choice 1 (covariance between observations); c, d, g, h show the solution computed with the linear-regression model Choice 2 (no covariance between observations).

Extended Data Fig. 5 1970–2018 trends in winter upper-ocean stratification and mixed-layer depth.

a, b, Map of the 1970–2018 winter 0–200 m trend (a; \({N}_{200}^{2}\) trend in s−2 dec−1) and pycnocline stratification trend (b; N2 trend in s−2 dec−1); thick black lines show the zonal-median value and thin black lines show the 33th–66th percentile. Regions with no significant trend (see Methods) are shaded in grey on the map. c, As in a, b, but for winter mixed-layer trend, in m dec−1 (mixed-layer deepening is shown as a negative trend).

Extended Data Fig. 6 Regional time series of winter pycnocline stratification and mixed-layer depth anomaly.

a, Winter climatological mixed-layer depth (same as Fig. 2f) with three specific regions of interest outlined by red contours: North Atlantic subpolar convection region (A); Southern Ocean Indian sector convection region (B); and Southern Ocean Pacific sector convection region (C). b, d, f, Winter stratification anomaly time series and associated trends for regions A (b), B (d) and C (f). c, e, g, Winter mixed-layer depth anomaly times series and associated trends for regions A (c), B (e) and C (g). A negative depth anomaly refers to a deepening. Each time series shows: thin grey line, the annual median percentage anomaly (from the local climatological seasonal cycle), computed for each individual observation; error bars referring to the 33th–66th percentile range of the percentage anomaly (error bars are shown in black (grey) when more (fewer) than 50 data points are used in the annual statistics); the associated five-year smoothed median time series superimposed in blue; and a linear trend from 1970–2018, shown by the red line, if greater than twice its standard error.

Extended Data Fig. 7 Comparison between mixed-layer temperature trend and SST trends.

a, Summer mixed-layer mean temperature trend from 1970 to 2018, as estimated in this study. b, Summer SST trend from 1982 to 2018, as estimated from the satellite-based product GHRSSTv2. c, Box plot showing the median (red) and interquartile range (blue box) of local summer SST trend estimates from this study (mixed-layer mean temperature from 1970–2018), from the satellite-based product GHRSSTv2 (SST from 1982–2018) and from the in situ observation reconstruction product HadSSTv4 (SST from 1970–2018). The whiskers extend to the most extreme data points.

Extended Data Fig. 8 Difference between Argo- and ship-based derived mixed-layer depth.

a, b, Difference between mixed-layer depth (MLD) estimates coming from nearby (sampled within 330 km and 1.5 day) Argo and ship-based observation profiles (that is, co-located in time and space) for all instances for which we derived a smaller (a) or a greater (b) mixed-layer depth from the Argo profile than from the ship-based profile. c, Histogram of all differences. Because Argo started in the 2000s, the co-located profiles cover only the years 2000–2018.

Extended Data Fig. 9 1970–2018 trends in summer pycnocline stratification and mixed-layer depth when using only ship-based profiles (removing all Argo and MEOP programme observations).

a, Map of the 1970–2018 summer pycnocline stratification trend (N2 trend, in s−2 dec−1) along with the zonal-median value: median (thick black line) and 33th–66th percentile (thin black line). The red shading shows the global 33th–66th percentile range of the local trend estimates. Regions with no significant trend (see Methods) are shaded in grey on the map. b, As in a but for the summer mixed-layer trend, in m dec−1 (mixed-layer deepening is shown as a negative trend).

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Sallée, JB., Pellichero, V., Akhoudas, C. et al. Summertime increases in upper-ocean stratification and mixed-layer depth. Nature 591, 592–598 (2021). https://doi.org/10.1038/s41586-021-03303-x

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