Abstract
Abstract. 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.
Highlights
Mode waters, which are characterised by thermostads or pycnostads, are remnants of the deepest winter mixed layers and are widely distributed in the world’s interior ocean (Hanawa and Talley, 2001)
It is formed in the subantarctic zone (SAZ) which is bounded by the subtropical front (STF) and the subantarctic front (SAF)
We found that we could not detect the whole Indian Ocean Subtropical Mode Water (IOSTMW) as an layer of minimum vertical temperature gradient (LMVTG) using any single dT/dz threshold because temperature stratification below the IOSTMW was weak
Summary
Two mode waters have been widely recognized in the South Indian Ocean: Sub-Antarctic Mode Water (SAMW; McCartney, 1977, 1982) and Indian Ocean Subtropical Mode Water (IOSTMW; Olson et al, 1992). Since SAMW is the dominant mode water in the South Indian Ocean, its distribution area and water characteristics have been widely described (Fine, 1993; Toole and Warren, 1993; Wong, 2005). Winter mixed layer in the SAZ develops deeper than 300 m in 80–120◦ E, where the SAZ widens east of the Kerguelen Plateau (Wong, 2005; Sallee et al, 2006; Aoki et al, 2007) This increase of MLD leads to the injection of a thick layer of SAMW of densities greater than 26.7 σθ into the eastern South Indian Ocean.
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