The study considers the influence of tropical systems (cyclones, storms, depressions) from
the Southwest Indian Ocean (SWIO) over the Limpopo River Basin and provides an outlook
towards projected decadal scale variability and change throughout and towards the end of the
21st century. These systems have been linked to widespread heavy rainfall and subsequent
flooding over the region. Due to the semi-arid nature of this area, variation and change in
time of significant rain contributing synoptic systems are very relevant to the agricultural
Combining several historical datasets, it is shown that tropical systems from the SWIO
contribute roughly 10% of the annual average rainfall as revealed by rainfall records of a
number of stations located over the eastern parts of the Limpopo River Basin. The
contribution to rainfall over the interior is further shown to be confined to the period January-
March (JFM). These systems are however shown to be responsible for a significant
proportion of widespread heavy rainfall events over parts of the Limpopo River Basin.
Furthermore, a pronounced cycle in the influence of these systems over the region is
identified and it is shown to vary with a cycle in the total seasonal rainfall known as the
Dyer-Tyson cycle over southern Africa at a bi-decadal (18-20year) scale. The association of this type of synoptic weather event (landfall of tropical system followed
by movement into the Limpopo River Basin) with the Dyer-Tyson cycle is used to identify
regional to Hemispheric circulation anomalies that explain the bi-decadal rainfall variation
and also to increase understanding of variation in the influence of tropical systems from the
SWIO over the region. At a hemispheric scale, the Southern Annular Mode (SAM) is shown
to be significantly correlated with regional anomalies during JFM that are associated with
tropical systems moving into the Limpopo River Basin and also above-normal seasonal
rainfall. As the SAM is known to be sensitive to external forcing and there exist regional
climatological associations over several parts of the Southern Hemisphere with it, it provides
a potential avenue towards exploring external drivers of decadal scale variability.
An external driver of the bi-decadal (18-20-year) cycle in climate records has been postulated
to be variation in lunar tidal forcing associated with the 18.6-year lunar nodal cycle.
Therefore, the association of the SAM with tidal forcing is investigated and proposed to be a
role player in the variation at the decadal time scale of tropical systems from the SWIO over
the Limpopo River Basin and also of the Dyer-Tyson rainfall cycle. Based on an observed
influence of tidal forcing on the SAM, an index is developed to estimate the seasonal JFM
SAM based on tidal forcing during Austral summer. The predictability of the seasonal JFM
SAM by the index is evaluated through multiple linear regression and a prediction is made
for the period ending in 2050, noting the association between the SAM and tropical systems
from the SWIO over the Limpopo River Basin.
Finally, towards understanding the projected changes by the end of the 21st century in the
climatology of tropical systems over the SWIO and in particular influences over the Limpopo
River Basin, simulations of an Atmospheric Global Circulation Model (AGCM) based on
SST simulations by several Assessment Report 4 (AR4) Global Coupled Models (GCMs) are
scrutinized. The findings are presented within the context of projected changes in regional
circulation anomalies and atmospheric temperature and humidity profiles relevant to the
tracks followed, development and intensification of tropical systems.