Reevaluating the Causes of Observed Changes in Indian Ocean Water Masses

Sheila Stark Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, United Kingdom

Search for other papers by Sheila Stark in
Current site
Google Scholar
PubMed
Close
,
Richard A. Wood Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, United Kingdom

Search for other papers by Richard A. Wood in
Current site
Google Scholar
PubMed
Close
, and
Helene T. Banks Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon, United Kingdom

Search for other papers by Helene T. Banks in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

The consistency between observed changes in Subantarctic Mode Water (SAMW) properties at 32°S in the Indian Ocean and model simulations is explored using the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3). Hydrographic data collected in 2002 show that the water mass is warmer and saltier on isopycnals than in 1987, in contrast to the isopycnal freshening observed between 1962 and 1987. The response of HadCM3 under a range of forcing scenarios is explored and the observed freshening is only seen in experiments that include greenhouse gas forcing; however, there is no subsequent return to more saline conditions in 2002. The response of the model to greenhouse gas forcing is dominated by a persistent freshening trend, the simulated water mass variability agrees well with that suggested by the limited observations. Comparing model isopycnal changes from the forced experiments with a control run shows that the changes from the 1960s to 2002 are best explained by internal variability. This is in contrast to earlier work, which attributed the observed isopycnal freshening to anthropogenic forcing. Although the model shows that at present an anthropogenic climate change signal is not detectable in SAMW, the model water mass freshens on isopycnals during the twenty-first century under increased greenhouse gas forcing. This is consistent with recent heat content observations, which suggest that the salting is unlikely to persist. In HadCM3, this freshening is due to an increasing surface heat flux and Ekman heat and freshwater flux into the water mass formation region. This paper emphasizes the importance of higher-frequency observations of SAMW if detection and attribution statements are to be made.

Corresponding author address: Sheila Stark, Hadley Centre for Climate Prediction and Research, Met Office, Fitzroy Road, Exeter, Devon EX1 3PB, United Kingdom. Email: sheila.stark@metoffice.gov.uk

Abstract

The consistency between observed changes in Subantarctic Mode Water (SAMW) properties at 32°S in the Indian Ocean and model simulations is explored using the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3). Hydrographic data collected in 2002 show that the water mass is warmer and saltier on isopycnals than in 1987, in contrast to the isopycnal freshening observed between 1962 and 1987. The response of HadCM3 under a range of forcing scenarios is explored and the observed freshening is only seen in experiments that include greenhouse gas forcing; however, there is no subsequent return to more saline conditions in 2002. The response of the model to greenhouse gas forcing is dominated by a persistent freshening trend, the simulated water mass variability agrees well with that suggested by the limited observations. Comparing model isopycnal changes from the forced experiments with a control run shows that the changes from the 1960s to 2002 are best explained by internal variability. This is in contrast to earlier work, which attributed the observed isopycnal freshening to anthropogenic forcing. Although the model shows that at present an anthropogenic climate change signal is not detectable in SAMW, the model water mass freshens on isopycnals during the twenty-first century under increased greenhouse gas forcing. This is consistent with recent heat content observations, which suggest that the salting is unlikely to persist. In HadCM3, this freshening is due to an increasing surface heat flux and Ekman heat and freshwater flux into the water mass formation region. This paper emphasizes the importance of higher-frequency observations of SAMW if detection and attribution statements are to be made.

Corresponding author address: Sheila Stark, Hadley Centre for Climate Prediction and Research, Met Office, Fitzroy Road, Exeter, Devon EX1 3PB, United Kingdom. Email: sheila.stark@metoffice.gov.uk

Save
  • Banks, H. T. and R. Wood, 2002: Where to look for anthropogenic climate change in the ocean. J. Climate, 15:879891.

  • Banks, H. T. and N. L. Bindoff, 2003: Comparison of observed temperature and salinity changes in the Indo-Pacific with results from the coupled climate model HadCM3: Processes and mechanisms. J. Climate, 16:156166.

    • Search Google Scholar
    • Export Citation
  • Banks, H. T., R. A. Wood, J. M. Gregory, T. C. Johns, and G. S. Jones, 2000: Are observed decadal changes in intermediate water masses a signature of anthropogenic climate changes? Geophys. Res. Lett., 27:29612964.

    • Search Google Scholar
    • Export Citation
  • Banks, H. T., R. Wood, and J. Gregory, 2002: Changes to Indian Ocean subantarctic mode water in a coupled climate model as CO2 forcing increases. J. Phys. Oceanogr., 32:28162827.

    • Search Google Scholar
    • Export Citation
  • Baumgartner, A. and E. Reichel, 1975: The World Water Balance. Elsevier, 179 pp.

  • Bindoff, N. L. and T. J. McDougall, 2000: Decadal changes along an Indian Ocean section at 32°S and their interpretation. J. Phys. Oceanogr., 30:12071222.

    • Search Google Scholar
    • Export Citation
  • Bryden, H. L., E. McDonagh, and B. A. King, 2003: Changes in ocean water mass properties: Oscillations or trends? Science, 300:20862088.

    • Search Google Scholar
    • Export Citation
  • Church, J. A., S. Godfrey, D. R. Jackett, and T. J. McDougall, 1991: A model of sea level rise caused by ocean thermal expansion. J. Climate, 4:438456.

    • Search Google Scholar
    • Export Citation
  • Curry, R., B. Dickson, and I. Yashayaev, 2003: A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 426:826829.

    • Search Google Scholar
    • Export Citation
  • da Silva, A. M., C. C. Young, and S. Levitus, 1994: Algorithms and Procedures. Vol. 1, Atlas of Surface Marine Data, NOAA Atlas Series 6, 74 pp.

  • England, M. H., J. S. Godfrey, A. C. Hirst, and M. C. Tomczak, 1993: The mechanisms for Antarctic Intermediate Water renewal in a World Ocean model. J. Phys. Oceanogr., 23:15531560.

    • Search Google Scholar
    • Export Citation
  • Gordon, C., C. Cooper, C. A. Senior, H. Banks, J. M. Gregory, T. C. Johns, J. F. B. Mitchell, and R. A. Wood, 2000: The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre Couple Model without flux adjustments. Climate Dyn., 16:147168.

    • Search Google Scholar
    • Export Citation
  • Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, and D. Xiaosu, Eds. 2001: Climate Change 2001: The Scientific Basis. Cambridge University Press, 944 pp.

    • Search Google Scholar
    • Export Citation
  • Johns, T. C., Coauthors 2003: Anthropogenic climate change for 1860 to 2100 simulated with the HadCM3 model under updated emissions scenarios. Climate Dyn., 20.doi:10.1007/s00382-002-0296-y.

    • Search Google Scholar
    • Export Citation
  • Johnson, G. C. and A. H. Orsi, 1997: Southwest Pacific Ocean water-mass changes between 1968/69 and 1990/91. J. Climate, 10:306316.

  • Josey, S. A., E. C. Kent, D. Oakley, and P. K. Taylor, 1996: A new global air-sea heat and momentum climatology. International WOCE Newsletter, No. 24, WOCE International Project Office, Southampton, United Kingdom, 3–5.

  • Levitus, S. and T. P. Boyer, 1994: Temperature. Vol 4, World Ocean Atlas 1994, NOAA Atlas NESDIS 4, 117 pp.

  • McCartney, M. S., 1977: Subantarctic Mode Water. Deep-Sea Res., 24:(Suppl.). 103119.

  • McCartney, M. S., 1982: The subtropical recirculation of Mode Waters. J. Mar. Res., 40:(Suppl.). 427464.

  • McDonagh, E. L., H. L. Bryden, B. A. King, R. J. Sanders, S. A. Cunningham, and R. Marsh, 2005: Decadal changes in the South Indian Ocean thermocline. J. Climate, 18:15751590.

    • Search Google Scholar
    • Export Citation
  • Pardaens, A. K., H. T. Banks, J. M. Gregory, and P. R. Rowntree, 2003: Freshwater transports in HadCM3. Climate Dyn., 21:177195.

  • Ribbe, J., 1999: On wind driven mid-latitude convection in ocean general circulation models. Tellus, 51A:505516.

  • Rintoul, S. R. and M. H. England, 2002: Ekman transport dominates local air–sea fluxes in driving variability of Subantarctic Mode Water. J. Phys. Oceanogr., 32:13081321.

    • Search Google Scholar
    • Export Citation
  • Sloyan, B. M. and S. Rintoul, 2001: Circulation, renewal and modification of Antarctic Mode and Intermediate Water. J. Phys. Oceanogr., 31:10051030.

    • Search Google Scholar
    • Export Citation
  • Stramma, L. and J. R. E. Lutjeharms, 1997: The flow field of the subtropical gyre of the South Indian Ocean. J. Geophys. Res., 102:55135530.

    • Search Google Scholar
    • Export Citation
  • Toole, J. M. and B. A. Warren, 1993: A hydrographic section across the subtropical South Indian Ocean. Deep-Sea Res., 40A:19732019.

  • Warren, B. A., 1972: Insensitivity of subtropical mode water characteristics to meteorological fluctuations. Deep-Sea Res., 19:119.

  • Willis, J. K., D. Roemmich, and B. Cornuelle, 2004: Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales. J. Geophys. Res., 109.C12036, doi:10.1029/2003JC002260.

    • Search Google Scholar
    • Export Citation
  • Wong, A. P. S., N. L. Bindoff, and J. A. Church, 1999: Large-scale freshening of intermediate waters in the Pacific and Indian Oceans. Nature, 400:440443.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 185 42 4
PDF Downloads 94 21 1