Abstract:
The association between bi-decadal rainfall variability over southern Africa and the rainfall
contributed by tropical cyclonic systems from the Southwest Indian Ocean (SWIO) provides a
potential means towards understanding decadal-scale variability over parts of the region. A
multi-decadal period is considered, focusing on the anomalous tropospheric patterns that
induced a particularly wet 8-year long sub-period over the Limpopo River Basin. The wet
sub-period was also characterized by a larger contribution to rainfall by tropical cyclones
and depressions. The findings suggest that a broadening of the Hadley circulation
underpinned by an anomalous anticyclonic pattern to the east of southern Africa altered
tropospheric steering flow, relative vorticity and moisture contents spatially during the subperiod
of 8 years. These circulation modulations induced enhanced potential for tropical
systems from the SWIO to cause precipitation over the Limpopo River Basin. The same
patterns are also conducive to increasing rainfall over the larger subcontinent, therefore
explaining the positive association in the bi-decadal rainfall cycle and rainfall contributed by
tropical cyclonic systems from the SWIO. The regional circulation anomalies are also
explained in hemispheric context, specifically in relation to the Southern Annular Mode,
towards understanding variation over other parts of the Southern Hemisphere at this time
scale.
