Evolution of cyclonic eddies and biogenic fluxes in the northern Bay of Bengal

Abstract. The Bay of Bengal has been traditionally known for its low primary productivity and varied reasons were attributed to it. The data analysis from the sediment traps deployed in the northern Bay of Bengal during the 5 yr from 1994 show episodic events of enhanced downward biogenic flux every year which was not related to monsoon-driven seasonal cycle. Satellite-derived sea level anomaly suggests that the episodic increase in the biogenic flux was associated with the presence of cyclonic eddies in the sediment trap location. Cyclonic eddy-induced down ward biogenic flux in the sediment trap location was larger than the amplitude, ∼40 mg m⁻² d⁻¹, of the seasonal cycle. The magnitude of the peak episodic fluxes were one-and-half to two-and-half times the annual mean flux, while the anomaly of peak episodic fluxes was at least equal to or greater than the magnitude of the seasonal flux value. Cyclonic eddies responsible for high biogenic flux during 1994 and 1996 were formed in the northern Bay of Bengal during February–March of respective years due to the interaction of northward flowing western boundary current and coastally trapped Kelvin wave. In contrast, cyclonic eddies during 1997 and 1998 were formed from the breaking of westward propagating Rossby waves. The sediment trap data provided the observational evidence that eddy-induced biological productivity is an important mechanism in the Bay of Bengal that contributes significantly to the mid-depth biogenic flux.