Circulation controls on southern African precipitation in coupled models: The role of the Angola Low

Abstract

In southern Africa, models from the latest Coupled Model Intercomparison Project produce a wide variety of rainfall climatologies. Differences between models in rainfall amount reach 70% in the rainy season (December–February; DJF), and the median model overestimates rainfall by between 15 and 40% throughout the annual cycle. This paper investigates the role of an understudied regional circulation feature, the Angola Low, in differentiating between model estimates of precipitation. In austral spring, the Angola Low is a heat low, driven by strong surface heating whereas in DJF it is more similar to a tropical low and is associated with moist instability. In the austral summer, we find that the simulated strength of the Angola Low is associated with between 40 and 60% of the intermodel variability in model mean rainfall across the subcontinent. The relationship is particularly strong along a northwest, southeast axis aligned from Angola down to the Mozambican Channel. Along this axis, models with stronger Angola Lows simulate enhanced, by up to 50 g kg−1 ms−1, northeasterly and northwesterly moisture transport. The enhanced southward moisture flux in models with relatively deep Angola Lows increases the rate of moisture convergence in central areas of the subcontinent and reduces moisture divergence across the Mozambican coast. The results highlight the need to better understand the links between the Angola Low and southern African rainfall and suggest that improving the simulation of the Angola Low can help to constrain model estimates of southern African rainfall.

Future Precipitation Projections over Central and Southern Africa and the Adjacent Indian Ocean: What Causes the Changes and the Uncertainty?

“…10). (iii) Finally, there is the intensity of the Angolan low and associated circulation changes, known to be an important control on present day precipitation in observations (e.g., Manhique et al 2011) and in models (Munday and Washington 2017). During wet (dry) years the strength of the Angola low tends to be stronger (weaker) with enhanced (suppressed) circulation bringing in moisture from the Atlantic and the Indian Ocean (Cook et al 2004;Munday and Washington 2017).…”

“…(iii) Finally, there is the intensity of the Angolan low and associated circulation changes, known to be an important control on present day precipitation in observations (e.g., Manhique et al 2011) and in models (Munday and Washington 2017). During wet (dry) years the strength of the Angola low tends to be stronger (weaker) with enhanced (suppressed) circulation bringing in moisture from the Atlantic and the Indian Ocean (Cook et al 2004;Munday and Washington 2017). Differential rates of future warming over land versus ocean may be expected to intensify and displace the Angolan low and we see evidence of differential response across models Of course, we would not expect the dynamical processes of intermodel spread in projected rainfall change to be consistent with all aspects of current interannual variability, given (i) the complex suite of interannual modes affecting the SA region and (ii) the highly variable and mixed ability of coupled models to represent the mean state and variability, both through local processes (Lazenby et al 2016) and remote teleconnections (e.g., Rowell 2013).…”

Later Wet Seasons with More Intense Rainfall over Africa under Future Climate Change

“…The SHL and AL are important drivers of rainfall seasonality and variability over West Africa and the wider Sahel (Lavaysse et al 2009) and southern Africa (Munday and Washington 2017) respectively. An index was required for quantifying the strength of the SHL and AL to establish whether changes in the strength of the SHL or AL will influence changing seasonality.…”

Variability in the Botswana High and its relationships with rainfall and temperature characteristics over southern Africa

“…This confluence exists between the meridional arm and the Angola Low. The latter is a shallow low that exists from the surface up to about 700 hPa during the summer half of the year (Mulenga et al, 2003;Reason et al, 2006;Munday and Washington, 2017) and is an important source region for the tropical temperate troughs (TTTs) or tropical -extratropical cloud bands that are the main synoptic rainfall producing weather system over subtropical southern Africa during summer (Harrison, 1984;Washington and Todd, 1999;Hart et al, 2010Hart et al, , 2013Manhique et al, 2011Manhique et al, , 2015. A typical TTT position is shown in the black dashed line in Figure 3 and extends in a NW-SE orientation from its tropical source (the Angola Low) to a midlatitude frontal system passing to the southeast of South Africa.…”