Southern Ocean warming delayed by circumpolar upwelling and equatorward transport

Journal name:
Nature Geoscience
Volume:
9,
Pages:
549–554
Year published:
DOI:
doi:10.1038/ngeo2731
Received
Accepted
Published online

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.

At a glance

Figures

  1. Observed trends over 1982-2012.
    Figure 1: Observed trends over 1982–2012.

    a, Annual-mean SST trend. b, Net SHF trend (positive into ocean). c, Zonally and depth-integrated ocean heat content trends from two different subsurface temperature data sets: EN4 (solid; ref. 25) and Ishii (dashed; ref. 26). d, Zonal-mean ocean potential temperature trend from EN4, with contours of climatological ocean salinity in intervals of 0.15 practical salinity units (psu) (grey lines). Arrows indicate the orientation of the residual-mean MOC following ref. 22, along 34.4 and 34.7psu contours (black lines). Grey line in a and b shows maximum winter sea-ice extent from ref. 24.

  2. CMIP5-mean trends over 1982-2012 (left) and response to CO2 forcing (right).
    Figure 2: CMIP5-mean trends over 1982–2012 (left) and response to CO2 forcing (right).

    a, Annual-mean SST trend. b, Net SHF trend (positive into ocean). c, Zonally integrated average SHF (blue) and full-depth ocean heat content trend (red). d, Anomalous OHT for CMIP5-mean (blue) and CCSM4 (black; solid, dashed and dotted lines show total, residual-mean advection and diffusion, respectively). e, Zonal-mean ocean potential temperature trend, with contours showing the MOC from CCSM4 (black contours show positive circulation in 4 Sv increments, grey contours show negative circulation in −4Sv increments). fj, As in ae, but anomalies over 100yr in response to abrupt CO2 quadrupling. Grey line in a,b,f and g shows maximum winter sea-ice extent, as in Fig. 1. Shading in c,d,h and i shows the ±1 s.d. range across the CMIP5 models; these ranges are broader than those from internal variability alone (Supplementary Fig. 9).

  3. MITgcm response to uniform GHG forcing (left) and passive-tracer forcing (right).
    Figure 3: MITgcm response to uniform GHG forcing (left) and passive-tracer forcing (right).

    a, Annual-mean SST anomaly. b, Net SHF anomaly (positive into ocean). c, Zonally integrated average SHF anomaly (blue) and full-depth ocean heat content anomaly (red). d, Anomalous OHT (solid, dashed and dotted lines show total, residual-mean advection and diffusion, respectively). e, Zonal-mean ocean potential temperature anomaly, with contours showing the MOC from the control simulation (black contours show positive circulation in 2Sv increments, grey contours show negative circulation in −4Sv increments). fj, As in ae, but for the passive-tracer simulation. Grey line in a,b,f and g shows maximum winter sea-ice extent, as in Fig. 1.

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Author information

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

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.

Competing financial interests

The authors declare no competing financial interests.

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