The Amazon River Plume during AMASSEDS: Spatial characteristics and salinity variability

Abstract

The Amazon River discharge forms a plume of low‐salinity water that extends offshore and northwestward over the north Brazilian shelf. Observations acquired as part of A Multidisciplinary Amazon Shelf SEDiment Study (AMASSEDS) are used to characterize the spatial structure and temporal variability of the Amazon Plume. Four shipboard conductivity‐temperature‐depth (CTD) surveys spanning the shelf from 1°S to 5°N during rising (March 1990), maximum (May 1990), falling (August 1989), and minimum (November 1991) discharge show the Amazon Plume is typically 3 to 10 m thick and 80 to over 200 km wide. Northwest of the river mouth, the plume is often characterized by a wedge of low‐salinity water adjacent to the coast and a separate tongue of low‐salinity water extending offshore over the middle to outer shelf. A bottom front separating the low‐salinity plume water from oceanic water is consistently located between the 10‐ and 20‐m isobaths. A moored array deployed about 300 km northwest of the river mouth from February to June 1990 included inner and midshelf moorings in 18 and 65 m of water on which temperature‐conductivity measurements were made. The moored observations reveal salinity variations within the Amazon Plume of over 10 psu on timescales of days to weeks. This variability includes intermittent events in which plume water pools up in the vicinity of the river mouth and the plume width can exceed 200 km. These accumulation events are apparently due to wind events with a southeastward component which impede or block the normally northwestward freshwater transport. The resulting bulges in the plume are then released northwestward when the wind reverses. Volume budgets indicate the Amazon Plume entrains roughly twice the river discharge between the river mouth at the equator and 3°N. Estimates of gradient Richardson numbers from the moorings suggest entrainment, due to the strong semidiurnal tidal currents, occurs where the plume intersects the bottom and over the outer portion of the plume, where salinities approach oceanic values.