Benguela Niños: Observations and Modelling

Abstract

A numerical ocean model for the South Atlantic was used to study the 1995 Benguela Niño in order to learn about the underlying forcing mechanisms of the phenomenon. By comparing observations and model output, it was found that the model WANE was unable to simulate the thermohaline signal of the 1995 event. It proved more successful at reproducing the current systems of the area. Another model simulation (OPA) was able to recreate both the surface and subsurface temperature signal of the 1984 and 1995 Benguela Niño. The main reason for the poor water mass representation during a warm event in WANE is probably the relaxation of the surface fluxes towards climatology. An investigation of the movements of a drifter buoy and winds during the 1995 Benguela Niño revealed a strong poleward current that moved against the southerly winds, indicating that local winds are not the main driving force of the phenomenon. It is believed that the origin of Benguela Niños is found in the western part of the equatorial Atlantic; as the trade winds off the coast of Brazil weaken, this relaxation leads to the generation of equatorial Kelvin waves which propagates across the equator and poleward along the coast of Africa. The east-west component of the wind in the western equatorial Atlantic was examined, and there seems to be a relationship between the winds off Brazil and the Benguela Niño. Both numerical model simulations and the wind data support the governing theory of the generation and forcing mechanisms of Benguela Niños.