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Decadal Changes along an Indian Ocean Section at 328S and Their Interpretation


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Bindoff, NL and McDougall, TJ 2000 , 'Decadal Changes along an Indian Ocean Section at 328S and Their Interpretation' , Journal of Physical Oceanography, vol. 30, no. 6 , pp. 1207-1222 , doi: 10.1175/1520-0485.

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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 328S 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 0.58C, (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 Bindoff and McDougall
(1994), 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.

Item Type: Article
Authors/Creators:Bindoff, NL and McDougall, TJ
Journal or Publication Title: Journal of Physical Oceanography
ISSN: 1520-0485
DOI / ID Number: 10.1175/1520-0485
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