Tropical systems from the Southwest Indian Ocean into southern Africa: Impacts, variability and projected changes

Abstract

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 community. 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.