Browsing by Subject "rainfall"

Now showing 1 - 5 of 5
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    An analysis of heavy rainfall events over the Limpopo River Basin in southern Africa, their moisture sources and pathways
    (2020) Rapolaki, Ramontsheng Sakia; Reason, Christopher; Hermes, Juliet; Blamey, Ross
    Severe flooding events in subtropical southern Africa are not uncommon and can cause damage to infrastructure, lead to huge economic losses, and fatalities. Although extreme rainfall events can have far-reaching negative consequences, they can also provide large amounts of freshwater within a short time span, which supports the rain-fed farming upon which much of the population depends. However, the mechanisms through which extreme rainfall is produced in southern Africa are still not well understood. In particular, relatively little is known about where the moisture, a key ingredient in the rainfall, is sourced and how it is transported into the region. This thesis aims to address some of the gaps in this understanding by examining the moisture sources and subsequent moisture transport moisture into one of the key river basins in southern Africa, the Limpopo River Basin (LRB). The LRB, located in eastern southern Africa and spanning four countries, has experienced a number of extreme flooding events over the last three decades. Using CHIRPS satellite merged rainfall data for 1981-2016, the thesis identified the top 200 heavy extreme rainfall events in the LRB and the associated weather systems. It was found that tropicalextratropical cloud bands account for almost half of the events and tropical lows are responsible for just over a quarter. The remaining quarter of the events are associated with mesoscale convective systems and cut-off lows, the latter more important during transition seasons. Most of the events occur in the late summer when tropical lows and cloud bands are more common. Some relationships between the frequency of heavy rainfall events over the LRB and interannual climate modes of variability such as ENSO, SIOD, and SAM were found. Having examined the annual cycle of the top 200 heavy rainfall events, the analysis then applied the Lagrangian trajectory model HYSPLIT, with NCEP II reanalysis data as input, to backtrack air parcels from the LRB to their moisture source on seasonal scales and in terms of the types of weather systems involved. The resulting trajectories show that the seasonal transport of moisture over the LRB originates from seven moisture source regions; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical Northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. Important differences in moisture source regions and pathways exist between early (OctoberDecember) and late (January-April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. Generally, moisture v source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic systems involved. Thus, it was found that cloud band and tropical low events within the top 200 tend to have the Congo Basin as an important moisture source whereas this source is less evident for cut-off low events. To help assess the robustness of the composite synoptic approach, the final part of the thesis applied the Lagrangian analysis to the most severe case in the top 200 events over the LRB (11- 21 January 2013). It was found that this case was largely linked to three main moisture sources: (1) tropical northwest Indian Ocean, (2) the Agulhas Current / Mozambique Channel, subtropical Southwest Indian Ocean, and (3) continental sources over the Congo Basin and northern Tanzania. Generally, the moisture source regions and pathways for the January 2013 event agreed with the climatological moisture source regions over the LRB, apart from the obvious absence of the tropical southeast Atlantic source in this case. In general, the thesis has provided a better understanding of the characteristics of heavy rainfall events over the LRB in terms of their associated weather systems, seasonality, interannual variability, and moisture source regions and trajectories.
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    Early summer rainfall variability in the Congo Air Boundary Region
    (2024) Maphugwi, Mulalo; Blamey, Ross; Reason, Christopher
    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.
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    The impacts of indigenous herbivore grazing over five years (2004 - 2008) on vegetation dynamics in four distinct vegetation types of the winter-rainfall Little Karoo
    (2008) Madden, Christine; Hoffmann, Timm
    Vegetation in semi-arid regions is subject to change when heavily utilised by herbivores. Changes in species richness, species and growth form composition, total cover and plant palatability in response to rest (fenced) and grazing (open) treatments was investigated in Sanbona Wildlife Reserve over five years (2004-2008). This reserve is over 55 000 ha and has four dominant vegetation types: Little Karoo Quartz Vygieveld, Western Little Karoo, Montagu Shale Renosterveld and previously transformed Renosterveld classified here as Old Lands. There was no significant change in vegetation dynamics between the fenced or open plots. There were significant differences between years in some vegetation types. All vegetation types showed both treatments having similar shifts in floristic composition. Floristic composition deviated the greatest in 2008 in all vegetation types. This was attributed to an increase in summer and winter rainfall in 2008, as all plots were similarly affected. There was no observed impact of herbivores although disturbance from small rodents and baboons was observed in a few plots. The lack of detectable difference between the fenced and open sites was attributed to low stocking rates, good rainfall and insufficient time.
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    Influence of the Madden–Julian Oscillation on Southern African Summer Rainfall
    (2007) Pohl, B; Richard, Y; Fauchereau, N
    Composite maps of outgoing longwave radiation (OLR) anomalies over the Madden-Julian oscillation (MJO) cycle show marked intraseasonal fluctuations over southern Africa (south of 15°S). Large-scale convective clusters are seen to propagate eastward and then northward over the continent, mainly between 10° and 20°S. The corresponding response of the rainfall field presents the alternation, over the cycle, of dry and humid phases, which are both significant. Moisture flux anomalies indicate an intraseasonal modulation of the midtropospheric easterly flow over the Congo basin at 700 hPa; these fluctuations are coupled to meridional flux anomalies that extend from the tropical to the subtropical austral latitudes, and favor occurrences of wet or dry conditions over the domain. Though statistically significant, the influence of the MJO on southern Africa is however not homogeneous spatially, and only the tropical areas exhibit sharp periodicities in the 30-60-day period range. The OLR dipole observed in previous studies at the interannual and synoptic time scales between the hinterland parts of southern Africa and the southwestern Indian Ocean in the north of Madagascar is investigated next, as it also shows strong fluctuations at the intraseasonal time scale. The study points out that the dipole is partly influenced by the MJO, though the strongest periodicities are found for slightly longer periods (35-80 days) than those typically associated with the oscillation. The forcing of the MJO on the OLR dipole, though significant, remains thus partial.
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    Interannual memory effects for spring NDVI in semi-arid South Africa
    (2008) Richard, Yves; Martiny, Nadège; Fauchereau, Nicolas; Reason, Chris; Rouault, Mathieu; Vigaud, Nicolas; Tracol, Yann
    Almost 20 years of Normalized Difference Vegetative Index (NDVI) and precipitation (PPT) data are analysed to better understand the interannual memory effects on vegetation dynamics observed at regional scales in Southern Africa (SA). The study focuses on a semi-arid region (25°S–31°S; 21°E–26°E) during the austral early summer (September–December). The memory effects are examined using simple statistical approaches (linear correlations and regressions) which require the definition of an early summer vegetation predictand (December NDVI minus September NDVI) and a consistent set of potential predictors (rainfall amount, number of rainy days, rainfall intensity, NDVI and Rain-Use-Efficiency) considered with 4 to 15-month time-lag. An analysis over six SA sub-regions, corresponding to the six major land-cover types of the area reveals two distinct memory effects. A “negative” memory effect (with both rainfall and vegetation) is detected at 7 to 10-month time-lag while a “positive” memory effect (with vegetation only) is observed at 12 to 14-month time-lag. These results suggest that interannual memory effects in early summer vegetation dynamics of semi-arid South Africa may preferably be driven by biological rather than hydrological mechanisms.