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Southern Ocean warming delayed by circumpolar upwelling and equatorward transport

Nature Geoscience volume 9, pages 549–554 (2016)Cite this article

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Abstract

The Southern Ocean has shown little warming over recent decades, in stark contrast to the rapid warming observed in the Arctic. Along the northern flank of the Antarctic Circumpolar Current, however, the upper ocean has warmed substantially. Here we present analyses of oceanographic observations and general circulation model simulations showing that these patterns—of delayed warming south of the Antarctic Circumpolar Current and enhanced warming to the north—are fundamentally shaped by the Southern Ocean’s meridional overturning circulation: wind-driven upwelling of unmodified water from depth damps warming around Antarctica; greenhouse gas-induced surface heat uptake is largely balanced by anomalous northward heat transport associated with the equatorward flow of surface waters; and heat is preferentially stored where surface waters are subducted to the north. Further, these processes are primarily due to passive advection of the anomalous warming signal by climatological ocean currents; changes in ocean circulation are secondary. These findings suggest the Southern Ocean responds to greenhouse gas forcing on the centennial, or longer, timescale over which the deep ocean waters that are upwelled to the surface are warmed themselves. It is against this background of gradual warming that multidecadal Southern Ocean temperature trends must be understood.

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Figure 1: Observed trends over 1982–2012.
Figure 2: CMIP5-mean trends over 1982–2012 (left) and response to CO2 forcing (right).
Figure 3: MITgcm response to uniform GHG forcing (left) and passive-tracer forcing (right).

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Acknowledgements

The authors thank R. Abernathey, C. Bitz, S. Emerson, Y. Kostov, L.-P. Nadeau, L. Polvani, P. Rhines, G. Roe, L. Thompson and L. Zanna for enlightening feedback; and J.-M. Campin and G. Forget for technical help. The authors are grateful for support from the National Science Foundation through grants OCE-1259388 (J.R.S.), OCE-1338814 (J.M.), OCE-1523641 (K.C.A.) and PLR-1341497 (E.R.N.); from the National Aeronautics and Space Administration through award NNX11AL79G (K.C.A.); and from the Joint Program on the Science and Policy of Global Change, which is funded by a number of federal agencies and a consortium of 40 industrial and foundation sponsors (J.R.S.).

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Authors and Affiliations

  1. School of Oceanography and Department of Atmospheric Sciences, University of Washington, Seattle, Washington 98195, USA

    Kyle C. Armour

  2. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    John Marshall & Jeffery R. Scott

  3. Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    Jeffery R. Scott

  4. Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington 98195, USA

    Aaron Donohoe

  5. Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA

    Emily R. Newsom

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  1. Kyle C. Armour
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Contributions

K.C.A. performed the analyses and wrote the manuscript. J.R.S. performed the ocean-only simulations and associated diagnostics. All authors contributed to the design of the study and interpretation of the results.

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Correspondence to Kyle C. Armour.

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Armour, K., Marshall, J., Scott, J. et al. Southern Ocean warming delayed by circumpolar upwelling and equatorward transport. Nature Geosci 9, 549–554 (2016). https://doi.org/10.1038/ngeo2731

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