Seasonal changes in the denitrification regime of the Arabian Sea

Abstract

Extensive observations made in the northeastern Arabian Sea during successive southwest (SW) and northeast (NE) monsoon seasons reveal significant changes in chemical composition of the oxygen-deficient waters. Distributions of nitrite and nitrate deficits, the latter computed from linear relationships established between the nitrate tracer 'NO' and potential temperature, and evaluated using a two end-member mixing model, are studied at four isopycnal surfaces to deduce some major features of the mid-depth circulation. The results suggest a complete reversal of the subsurface coastal circulation associated with the monsoons. During the SW monsoon, a northward undercurrent appears to carry low nitrite, low deficit and relatively well-oxygenated waters off the Indian continental margin, suppressing denitrification; this feature is absent during the NE monsoon. Vigorous horizontal mixing resulting from a seasonal reversal of currents at a depth of a few hundred metres, together with the lack of large east-west differences in the downward particulate flux, as revealed by some recent sediment trap experiments, could explain why the oxygen-deficient zone in the Arabian Sea is not confined to the upwelling centres along the boundaries. The results reinforce the assertion that mixed layer-induced nutrient pumping could account for a large fraction of new production in this region. Substantially higher deficits in inorganic combined nitrogen seem to occur during the NE monsoon suggesting that the renewal processes are most vigorous during the SW monsoon. It is suggested that the dominance of the NE monsoon circulation could lead to an intensification in denitrification during the glacial times.