Early summer rainfall variability in the Congo Air Boundary Region

Maphugwi, Mulalo

Early summer rainfall variability in the Congo Air Boundary Region

Thesis / Dissertation

2024

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

Strong meridional rainfall gradients exist between the tropics and subtropics in southwestern Africa, bounded to the north by the moist Congo basin and the south by the Kalahari Desert. This region termed the tropical-subtropical divide (TSD) here, has recently faced one of the worst droughts in the last 40 years, contributing to local food insecurity. Compared to the rest of southern Africa, relatively little scientific attention has focused on the domain, partly due to long-term conflict preventing reliable observations. In this study, focus is placed on understanding rainfall characteristics and variability during the austral summer (October – April) across southwestern Africa using daily fifth generation of European Centre for Medium-Range Weather Forecast Reanalysis (ERA5) and Climate Hazard Group InfraRed Precipitation with stations (CHIRPS) data. Evaluation of gridded rainfall products in the region found that CHIRPS and ERA5 compare well with limited station data in the region. Using the ERA5 data, results reveal a significant decreasing trend of early summer (October-December) rainfall totals as well as rainy days since 1979 to present. The importance of the early summer rainfall is that it accounts for up to 60% of the total summer rainfall. There also appears a statistically significant long-term decreasing trend in rainfall onset (i.e., later onset), which typically occurs during the late October – early November months. A late onset could lead to dry early summers, a decrease in rainy season length, and severe droughts in the region in future. However, the late summer (January – April) increasing trend in rainfall totals might offset any large changes in summer rainfall. Correlation analysis reveals that although there is a significant relationship between early summer rainfall and rainy days and some of the main modes of climate variability and regional circulation systems, although it was highly variable across the domain. Thus, two sub-domains termed western and eastern region (“WR” and “ER”) were also investigated. A significant relationship between sea-surface temperature from the Niño 3.4 region and Indian Ocean Dipole (IOD) with rainfall and rainy days was only limited to the extreme southeast of Zambia. For late summer, when El Niño-Southern Oscillation (ENSO) has matured, there is a strong contrasting signal of significant positive (negative) relationship with rainfall totals and rainy days in the Angola Highlands (the rest of subtropical southern Africa). Angola Low index significantly and positively (negatively) correlates with early summer rainfall in the western (eastern) parts of the domain. The relationship between the Botswana High with early summer rainfall and rainy days is only limited to the eastern parts of the region. Lastly, a composite analysis of wet and dry years for the whole tropical-subtropical divide region as well as two sub-domains (western and eastern region) were investigated. Given the considerable variability within the region, anomalously dry and wet years are not consistent for the tropical-subtropical divide region, western and eastern region. Composites of wet years for the tropical-subtropical divide region and both sub-domains shows that the western Indian Ocean appears to act as the main source of moisture for the region. Typically, wet composites over the eastern region (western region) are characterized by stronger (weaker) Angola Low, while Botswana High influence over wet and dry years is only limited to the eastern region.