Multidecadal Changes of the South Atlantic Ocean Angola–Benguela Front

The Angola-Benguela front (ABF) is a near-surface oceanic frontal zone where the southward-flowing Angola Current meets the northward flowing Benguela Current. This frontal zone is known to vary in position and intensity across different timescales, and these fluctuations can have a profound impact on the regional South Atlantic and southern African environment. In this study high-resolution simulations with a regional atmospheric model coupled to an intermediate-level mixed layer ocean model along with multiple atmospheric and oceanic reanalyses are analyzed to understand how and why the ABF has changed since 1980.

Model and observational results indicate a poleward shift of -0.05 to -0.55° latitude decade^-1 in the annual mean ABF position accompanied by an intensification of +0.05 to +0.13 K/100-km decade^-1 has occurred as ocean mixed layer temperatures have warmed (cooled) equatorward (poleward) of the front over the 1980 – 2014 period. The oceanic warming north of the ABF is associated with a weakening of vertical entrainment, reduced cooling associated with vertical diffusion, and a deepening of the mixed layer along the Angola coast. These changes coincide with a steady weakening of the onshore atmospheric flow as the zonal pressure gradient between the eastern equatorial Atlantic and the Congo Basin weakens. Oceanic cooling poleward of the ABF is primarily due to enhanced advection of cooler water from the south and east, increased cooling by vertical diffusion, and shoaling of the mixed layer depth. In the atmosphere, these changes are related to an intensification and poleward shift of the South Atlantic sub-tropical anticyclone as surface winds, hence the easterly mixed layer ocean currents, intensify in the Benguela upwelling region along the Namibian coast. These findings demonstrate that air/sea interactions play a prominent role in influencing the observed multi-decadal variability of the ABF over the southeastern Atlantic.