The response of sound scatterers to El Ninõ- and La Niña-like oceanographic regimes in the southeastern Atlantic

Abstract

Oceanographic conditions off Angola alternate seasonally between upwelling in the austral winter and El Niño-like intrusions and downwelling in summer. During winter in regions deeper than 30 m, the water column consists of a top layer of warm, tropical water overlying cold, nutrient-rich, and hypoxic South Atlantic Central Water (SACW). Closer inshore the water becomes well mixed. In the stratified region, acoustic backscatter at 38 kHz matches the oceanographic structure. It is strong in the top layer, but declines sharply in the SACW. During summer, the water column is continuously stratified, and the SACW is absent from the shelf. The backscatter reveals multiple thin layers extending across much of the shelf. The scattering layers are often perturbed by internal waves. The combined evidence from multiple acoustic surveys and the existing synthetic-aperture radar imagery suggests that tidal internal waves are a pervasive phenomenon in Angolan waters. © United States Government, NOAA/NMFS/AFSC 2009.