Modulation of Amazon River Plume: numerical studies

The Amazon River Plume (ARP) is a dynamic feature of the Amazon Shelf (AS), shaped by a combination of natural forces: density-driven currents, tides, and wind variability. To better understand its movement and modulation, we combined the robust Eulerian modelling (ROMS) and the effective Lagrangean framework (Opendrift) in a series of numerical experiments to examine how main forces determine the plume's structure and transport.
The ARP originates from the Amazon River’s immense discharge, forming a buoyant plume that spreads towards the open ocean. Without other influences, the ARP drifts northeast, following the river mouth's orientation. When tidal forces are introduced, the oscillatory motions enhance cross-shelf variability, pushing the plume further into the ocean. Tidal mixing, linked to the resonance of the M2 tidal harmonic, spreads the plume northwards, reaching nearly 2ºN. However, the freshest waters remain confined to the shallow AS, held in place by tidal effects.
Wind shear, though expected to drive more effective advection, mainly shifts parts of the plume northwest along the mid-shelf, adding coastal variability. Yet, in shallower areas, tidal forces dominate, maintaining their role as the primary control. While wind shear influences the plume’s shape, it is not the key driver of its northward movement.
The primary force modulating the ARP appears to be density-driven currents, generated by salinity and temperature gradients. When acting alone, these currents push the plume decisively into the Northern Hemisphere, reaching beyond 3ºN on the mid-shelf. At the shelf break, where the ocean deepens and the coastline's influence diminishes, the plume’s freshwater can spread further offshore.
When all forces act together—winds, tides, and currents—a complex balance emerges. Tides anchor the ARP to the shelf, preventing the freshest waters (salinity < 30) from escaping the AS. At the same time, currents and wind shear drive the plume northwest, extending its reach to around 4ºN.
This balance of forces highlights the dynamic nature of the Amazon River Plume. In an equatorial region where geostrophic balance is minimal, the vast discharge of the Amazon is shaped by regional forces, defining the behavior of one of the world’s largest river-ocean systems.