On the renewal of the denitrifying layer in the Arabian Sea

Abstract

A one-dimensional (vertical) advection-diffusion model has been applied to the deep layer characterized by a linear potential temperature(theta)-salinity relationship in the Arabian Sea to estimate the velocity of ascending motion. The results indicate that the upward velocity increases southward. Consequently, the upward flux of oxygen per unit area associated with upwelling of deep water immediately south of the denitrification zone (between latitudes 5 and 10 degrees N) is almost twice the corresponding value for the zone of intense denitrification (between latitudes 16 and 21 degreess N). Coupled with a southward decrease in the supply of organic carbon from the surface layer, this seems to determine the southern and lower boundaries of the d. zone. When normalized to phosphate, there appears to be a deficiency in nitrate, estimated as lambda TgNa-1, in the water upwelling at ~3 km within the d. Zone, due to the downward diffusion of nitrate deficits from the d. Layer. Assuming that denitrification in the Arabian Sea conforms to global trends, the Liu-Kaplan model (1984) is applied to determine the renewal time of the denitrifying layer (ca.150-600m) as 1.6.-3.4a. This is consistent with some recent results supporting the view that Arabian Sea's contribution to global denitrification has been up to now severely underestimated