Oceanographic and faunistic structures across an Angola Current intrusion into northern Namibia waters.

Abstract

It is thought that the penetration of Angolan waters through the Angola–Benguela Frontal Zone (ABFZ) into the much cooler Benguela regime may come about by a poleward slope undercurrent as well as by cross-frontal filaments. To test this hypothesis, two zonal transects off the northern Namibian coast were surveyed by CTD casts, current measurements and ichthyoplankton samples during April 1999. Simultaneous sea-surface temperature (SST), chlorophyll-a concentration, and wind data were obtained from satellite. The multidisciplinary results are described.An intense intrusion of the Angola Current into northern Namibian waters occurred with distinct signals of high temperatures and salinities, low chlorophyll-a concentration, and Angolan fish larvae of both the neritic and oceanic communities. This intrusion was temporarily displaced offshore by strong southeasterly winds, which also caused coastal upwelling and enhanced productivity. The expected slope undercurrent was not found. The Benguela Upwelling Front coincided with a sharp boundary between equatorward flow inshore, and generally southward flow offshore, whilst the offshore component of the Angola– Benguela Frontal Zone was located much farther south than anticipated, and showed only weak temperature gradients. Inshore of the Benguela Upwelling Front temperate Benguela fauna had characteristics of anomalously warm conditions. Tropical fish larvae offshore were clearly related to advection in Angola Current water, but not to recent spawning of their parents in it. Inconsistencies were observed in some cross slope boundaries between oceanic versus neritic fish larvae that can only partly be explained by Ekman drift of the surface layer, indicating that both the hydrographic and faunistic structures resulted from opposing meridional flows over time scales as different as 6 days to 4 weeks, intense mixing in the friction zone between them, and the westward displacement of a mesoscale gyre contributing its own anticyclonic flowfield. A conceptual transport model is presented.