Long-term variations of surface and intermediate waters in the southern Indian Ocean along 32°S
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Variations of water properties in surface and intermediate layers along 32°S in the southern Indian Ocean were examined using a 50-year (1960–2010) time series reproduced from historical hydrographic and Argo data by using optimum interpolation. Salinity in the 26.7–27.3σθ density layer decreased significantly over the whole section, at a maximum rate of 0.02 decade−1 at 26.8–26.9σθ, for the 50-year average. Three deoxygenating cores were identified east of 75°E, and the increasing rate of apparent oxygen utilization in the most prominent core (26.9–27.0σθ) exceeded 0.05 ml l−1 decade−1. The pycnostad core of Subantarctic Mode Water (SAMW) and the salinity minimum of Antarctic Intermediate Water shifted slightly toward the lighter layers. Comparisons with trans-Indian Ocean survey data from 1936 suggest that the tendencies found in the time series began before 1960. Interestingly, cores of many prominent trends were located just offshore of Australia at 26.7–27.0σθ, which is in the SAMW density range. Spectrum analysis revealed that two oscillation components with time scales of about 40 and 10 years were dominant in the subsurface layers. Our results are fairly consistent with, and thus support, the oceanic responses in the southern Indian Ocean to anthropogenic climate change predicted by model studies.
KeywordsLong-term trend Oscillation Climate change Southern Indian Ocean Subantarctic Mode Water Antarctic Intermediate Water
We thank all members of the Ocean Climate Change Research Program, Japan Agency for Marine-Earth Science and Technology, for their comments, especially S. Hosoda, T. Hasegawa, and T. Ohira for their kind advice on our analysis. Two reviewers gave us valuable comments that have improved the manuscript. This study used data from the international Argo Program (http://www.argo.ucsd.edu). Argo is part of the Global Ocean/Global Climate Observing System. Data from Argo profiling floats are freely available.
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