General Distribution of Water Column Properties

Within the main thermocline, relatively low-NOs waters of the southern subtropical gyre are separated from those rich in NOs in the monsoon gyre to the north and the sub-Antarctic zone to the south by the Hydrochemical Front and the Subtropical Convergence (STC), respectively (Fig. 14.IB). The maximum in NOs generally occurs in the 1-2 km depth range north of the STC, and at 

At the sea surface, NOs concentrations are below the limit of detection ( 0.1 pM) in most areas including the equatorial Indian Ocean, which does not experience 

Together these processes maintain high rates of PP over much of the Arabian Sea during a large part of the year (Barber et ah, 2001). 

Zonal sections through the southern subtropical gyre at about Lat. 20° S show a trend of eastward intensification of the O2 minimum and its associated NOs maximum (Fig. 14.2). However, the east-west gradients in both properties are much weaker than the north-south gradients across the Hydrochemical Front for example, the lowest O2 concentration along 103 is 90 pM as compared to 5 pM along 107 (not shown but see Fig. 14.6). Even the modest intensification of the O2 minimum in the eastern tropical South Indian Ocean is most likely related to relatively poor ventilation rather than an enhanced O2 demand, a major factor for the acute O2 depletion in the North Indian Ocean (Sen Gupta and Naqvi, 1984). 

Farther north, moderate upweUing has been known to occur seasonally (during the southeast monsoon) along the southern coasts of Sumatra and Java (Susanto etal, 2001 Wyrtki, 1962), but stiU on a far smaller scale than in the Arabian Sea. Unfortunately, Htde is known about the biogeochemical processes along this coastal segment. 

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