Weekly observations on dispersal and sink pathways of the terrigenous flux of the Ganga–Brahmaputra in the Bay of Bengal during NE monsoon

Abstract

The analyses of 64 sequential satellite images (October 1999–March 2001) of Indian Remote Sensing Satellite IRS—P4 ocean color monitor (OCM) (bands around 490, 555, and 670 nm) for total suspended matter (TSM), synchronous sea truth data acquisition, and salinity variations have been used to construct dispersal pathways of the surficial fluvial flux into the northern Bay of Bengal during the NE monsoon. From the spatial extent of the plumes of TSM (160–120 km) during October of 1999 and 2000, off the mouth of the Himalayan rivers the Ganga and the Brahmaputra (G–B), it is deduced that fluvial flux does not diminish concurrently with the termination of the southwest (SW) monsoon, as suggested by time-series trap experiments in the northern bay. During the NE monsoon, the G–B plumes move north to south initially off the mouth, and thereafter advects SW alongshore in the form of coastal sediment plumes, reducing the salinity of the coastal waters along the entire northern bay during October–December. We have observed a strong relation between enhanced episodic discharges of the Ganga–Brahmaputra (G–B) and augmented coastal turbidity during weekly events. It is also observed that even during such short (weekly) events of very high pulse of TSM discharge by the G–B system, the fluvial fluxes do not advect offshore into the deeper regions of the north-central bay, but are transported alongshore and distributed along the shelf. Our results, therefore, suggest that the reduced recovery of the fluxes in the sediments traps subsequent to the SW monsoon are not linearly related to the magnitude of fluvial flux of the G–B, but stems from the prevalent dispersal patterns.