• Barnier, B., L. Siefridt, and P. Marchesiello, 1995: Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses. J. Mar. Syst.,6, 363–380.

  • Bindoff, N. L., and J. A. Church, 1992: Warming of the water column in the Southwest Pacific Ocean. Nature,357, 59–62.

  • ——, and C. Wunsch, 1992: Comparison of synoptic and climatologically mapped sections in the South Pacific Ocean. J. Climate,5, 631–645.

  • ——, and T. J. McDougall, 1994: Diagnosing climate change and ocean ventilation using hydrographic data. J. Phys. Oceanogr.,24, 1137–1152.

  • Bottomley, M., C. K. Folland, J. Hsiung, R. E. Newell, and D. E. Parker, 1990: Global ocean surface temperature atlas: GOSTA, Tech. Rep., Meteorological Office, Bracknell, United Kingdom 1–333.

  • Bourke, W., T. Hart, R. Seaman, L. Rikus, P. Steinle, M. Naughton, P. Mullenmeister, and G. Emberg, 1991: Operational global assimilation and prediction in the Bureau of Meteorology. Tech. Rep., Bureau of Meteorology Research Centre, Melbourne, Australia, 54–72.

  • Church, J. A., J. S. Godfrey, D. R. Jackett, and T. J. McDougall, 1991: A model of sea level rise caused by ocean thermal expansion. J. Climate,4, 438–456.

  • Cook, M. F., J. M. Toole, G. P. Knapp, R. A. Fine, Z. Top, and J. C. Jennings Jr., 1992: A trans-Indian Ocean hydrographic section at latitude 32°S. Data report of RRS Charles Darwin Cruise #29. Woods Hole Oceanographic Institution Tech. Rep., WHOI-92-07, 190 pp.

  • Douglas, B. C., 1991: Global sea level rise. J. Geophys. Res.,96 (C4), 6981–6992.

  • England, M. H., 1992: On the formation of Antarctic Intermediate and Bottom Water in ocean general circulation models. J. Phys. Oceanogr.,22, 918–926.

  • Fine, R. A., 1993: Circulation of Antarctic Intermediate Water in the South Indian Ocean. Deep-Sea Res.,40, 2021–2042.

  • Fukumori, I., and C. Wunsch, 1991: Efficient representation of the North Atlantic hydrographic and chemical distributions. Progress in Oceanography, Vol. 27, Pergamon, 111–124.

  • Gordon, A., E. J. Molinelli, and T. N. Baker, 1982: Southern Ocean Atlas. Alfred Wegener Institute, 11 pp. and 233 plates.

  • Hansen, J., and S. Lebedeff, 1987: Global trends of measured surface air temperature. J. Geophys. Res.,92, 13 345–13 372.

  • ——, and ——, 1988: Global surface air temperatures: Update through 1987. J. Geophys. Res. Lett.,15, 323–326.

  • Jacka, T. H., and W. F. Budd, 1991: Detection of temperature and sea-ice extent changes in the Antarctic and Southern Ocean. Int. Conf. on the Role of Polar Regions in Global Change, G. Weller, C. L. Wilson, and B. A. B. Severin, Eds. University of Alaska, 63–70.

  • Jackett, D., and T. J. McDougall, 1997: A neutral density variable for the world’s ocean. J. Phys. Oceanogr.,27, 237–263.

  • Jacobs, G. A., H. E. Hurlburt, J. C. Kindle, E. J. Metzger, J. L. Mitchell, W. J. Teague, and A. J. Wallcraft, 1996: Decade-scale trans-Pacific propagation and warming effects of El Niño anomaly. Nature,370, 360–363.

  • Johnson, G. C., and A. H. Orsi, 1997: Southwest Pacific Ocean water-mass changes between 1968/69 and 1990/91. J. Climate,10, 306–316.

  • Jones, P. D., 1988: Hemispheric surface air temperature variations: Recent trends and an update to 1987. J. Climate,1, 654–660.

  • Latif, M., and T. P. Barnett, 1994: Causes of decadal climate variability over the North Pacific and North America. Science,266, 634–637.

  • Lozier, M., M. McCartney, and W. Owens, 1994: Anomalous anomalies in averaged hydrographic data. J. Phys. Oceanogr.,24, 2624–2638.

  • Manabe, S., K. Bryan, and M. J. Spelman, 1990: Transient response of a global ocean–atmosphere model to a doubling of atmospheric carbon dioxide. J. Phys. Oceanogr.,20, 722–749.

  • ——, R. J. Stouffer, M. J. Spelman, and K. Bryan, 1991: Transient responses of a coupled ocean–atmosphere model to gradual changes of atmospheric CO2. Part I: Annual mean response. J. Climate,4, 785–818.

  • Mantyla, A., 1987: Standard seawater comparisons updated. J. Phys. Oceanogr.,17, 543–548.

  • McCartney, M. S., 1977: Subantarctic mode water. A Voyage of Discovery, M. Angel, Ed., Pergamon, 103–119.

  • McDougall, T. J., 1987: Neutral surfaces. J. Phys. Oceanogr.,17, 1950–1964.

  • Menke, W., 1984: Geophysical Data Analysis: Discrete Inverse Theory. Academic Press, 289 pp.

  • Robbins, P. E., and J. M. Toole, 1997: The dissolved silica budget as a constraint on the meridional overturning circulation of the Indian Ocean. Deep-Sea Res.,44 (5), 879–906.

  • Toole, J. M., and B. A. Warren, 1993: A hydrographic section across the subtropical South Indian Ocean. Deep-Sea Res.,40 (10), 1973–2019.

  • Vaughan, S. L., and R. L. Molinari, 1997: Temperature and salinity variability in the deep western boundary current. J. Phys. Oceanogr.,27, 749–761.

All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 9 9 9
PDF Downloads 6 6 6

Decadal Changes along an Indian Ocean Section at 32°S and Their Interpretation

View More View Less
  • 1 Antarctic CRC, University of Tasmania, Hobart, Tasmania, Australia
  • | 2 Antarctic CRC, University of Tasmania, and CSIRO Division of Marine Research, Hobart, Tasmania, Australia
Restricted access

Abstract

In the Indian Ocean subtropical gyre, historical temperature, salinity, and oxygen data with a median date of 1962 are compared with a hydrographic section taken at a mean latitude of 32°S in October–November 1987. Significant basinwide changes in all three hydrographic fields are observed below the mixed layer. On isobaric surfaces the main changes are (i) a warming of the upper 900 dbar of the water column with a maximum change in the sectional mean of 0.5°C, (ii) a freshening between 500 and 1500 dbar with a maximum freshening of 0.05 psu, and (iii) a pronounced decrease in oxygen concentration between 300 and 1000 dbar.

Examination of water mass properties shows that very significant water mass changes have occurred. On isopycnals subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW) have freshened and cooled. Both of these water masses are on average deeper in 1987. Using the analysis of , the changes of temperature at constant depth and at constant density are used to show that the water mass changes can most simply be explained by a surface warming in the source region of SAMW and by increased precipitation in the source region of AAIW.

The decrease in oxygen concentration can be explained simply by a slight slowing of the subtropical gyre allowing more time for biological consumption to decrease the oxygen concentration by water parcel translation from the formation area to the observation point. Estimates show that over the last 25 years there is an apparent decrease of the gyre spin rate of about 20% at the depth levels of SAMW; the estimated spin rate change decreases almost linearly with greater depth to zero at the oxygen minimum in Indian Deep Water (IDW). Below IDW the observed changes in oxygen concentration (and also the changes of temperature and salinity) are associated with the upward movement of isopycnals with no significant water mass change. The differences in temperature and salinity in the SAMW and AAIW are consistent with the relatively young age of these water masses inferred from CFC data.

Corresponding author address: Dr. Nathan Bindoff, Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, Tasmania 7001, Australia.

Email: n.bindoff@utas.edu.au

Abstract

In the Indian Ocean subtropical gyre, historical temperature, salinity, and oxygen data with a median date of 1962 are compared with a hydrographic section taken at a mean latitude of 32°S in October–November 1987. Significant basinwide changes in all three hydrographic fields are observed below the mixed layer. On isobaric surfaces the main changes are (i) a warming of the upper 900 dbar of the water column with a maximum change in the sectional mean of 0.5°C, (ii) a freshening between 500 and 1500 dbar with a maximum freshening of 0.05 psu, and (iii) a pronounced decrease in oxygen concentration between 300 and 1000 dbar.

Examination of water mass properties shows that very significant water mass changes have occurred. On isopycnals subantarctic mode water (SAMW) and Antarctic Intermediate Water (AAIW) have freshened and cooled. Both of these water masses are on average deeper in 1987. Using the analysis of , the changes of temperature at constant depth and at constant density are used to show that the water mass changes can most simply be explained by a surface warming in the source region of SAMW and by increased precipitation in the source region of AAIW.

The decrease in oxygen concentration can be explained simply by a slight slowing of the subtropical gyre allowing more time for biological consumption to decrease the oxygen concentration by water parcel translation from the formation area to the observation point. Estimates show that over the last 25 years there is an apparent decrease of the gyre spin rate of about 20% at the depth levels of SAMW; the estimated spin rate change decreases almost linearly with greater depth to zero at the oxygen minimum in Indian Deep Water (IDW). Below IDW the observed changes in oxygen concentration (and also the changes of temperature and salinity) are associated with the upward movement of isopycnals with no significant water mass change. The differences in temperature and salinity in the SAMW and AAIW are consistent with the relatively young age of these water masses inferred from CFC data.

Corresponding author address: Dr. Nathan Bindoff, Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart, Tasmania 7001, Australia.

Email: n.bindoff@utas.edu.au

Save