We have improved a basic description (water characteristics and spatial distribution) of the Indian Ocean Subtropical Mode Water (IOSTMW) using an isopycnally averaged three-dimensional hydrographic dataset. Two mode waters and corresponding wintertime mixed layer depth maxima were observed north of the subtropical front (STF) in the South Indian Ocean: IOSTMW (within 25.8-26.2 σθ) in the region of 28-45° E and another subtropical mode water in the subtropical gyre (within 26.4-26.7 σθ) in the 60-80° E longitudinal band. Through careful examination of the spatial distribution and water characteristics of a core in the layer of minimum vertical temperature gradient (LMVTG), we identified that a mass of LMVTG corresponds to IOSTMW. The average water characteristics of the IOSTMW during approximately 1960-2004 were 16.54±0.49 °C, 35.51±0.04 psu and 26.0±0.1 σθ. The IOSTMW distribution area was estimated to be 25-50° E, 27-38° S. The formation region and approximate water characteristics of the second subtropical mode water were also estimated. Its probable formation region was 37-42° S, 60-80° E and north of the STF, with approximate water characteristics of 12.84±0.57 °C, 35.17±0.11 psu and 26.57±0.04 σθ.
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