On the Angola Low interannual variability and its role for modulating ENSO effects in southern Africa

Southern African (SA) summer rainfall experiences large inter-annual variability, which is often associated with severe drought conditions. These have dramatic impacts on local communities (which depend on rain-fed subsistence agriculture), natural ecosystems, and wildlife. A key driver of SA summer rainfall inter-annual variability is the Angola Low, a summertime low-pressure system that affects the influx and convergence of low-level moisture fluxes into SA. Inter-annual variations of the Angola Low impact SA summer rainfall, reducing the seasonal and sub-seasonal prediction skills that arise from coupled atmosphere-ocean variability (e.g., ENSO). During the 1997/98 El Niño, for example, expected drought conditions failed to materialize because of a stronger and southward displaced Angola Low.

In spite of its importance, the inter-annual and decadal dynamics of the Angola Low, and its relationship with El Niño and other coupled atmosphere-ocean modes of variability, is still poorly understood, mostly because of the scarcity of atmospheric data in the region and short-term duration of atmospheric reanalyses. To bypass this issue, we use a long-term (3500 years) run from a 50-km-resolution coupled global circulation model capable to reliably represent the summertime SA large-scale circulation and teleconnections. We find that the Angola Low position, and to a lesser extent its strength, has additive effects to El Niño on summertime southern African rainfall. We further show that the meridional displacement and strength of the Angola Low are only weakly modulated by, and in large part independent of, El Niño and other coupled atmosphere-ocean modes like the Subtropical Indian Ocean Dipole. Comparison of the coupled run with a 1000-year run driven by climatological SSTs demonstrates that the inter-annual excursions of the Angola Low are associated with similar atmospheric anomalies over the southern Atlantic and Indian Ocean. As such, the Angola low is likely to be primarily controlled by internal extratropical atmospheric variability.