Wind-driven variability of the Amazon River plume on the continental shelf during the peak outflow season

Abstract

The hypothesis that variations of the Amazon plume are forced primarily by wind is further explored through a series of simplified numerical model simulations. The wind's role in the change in plume structure and the nature of this change are investigated for two events: a shift in wind direction from westward to southeastward and a reduction in magnitude of the westward wind speed. Under winds with a southeastward component, the plume is confined to below 5°N; this simulation represents a rare but illustrative event showing how the balance of forces is quickly adjusted under changing winds. The freshest portions of the plume expand eastward, but are confined near the river mouth, as observed. The cross-shelf and alongshelf dynamic balances are similar in magnitude to those with westward wind stress, but the balance between the equatorial jet and buoyancy-driven cross-shelf flow is altered, controlling a new along-shelf position of the front. During wind-relaxation events, the plume widens near the mouth as a result of strong, eastward cross-shelf velocities associated with an equatorial Kelvin wave.