Browsing by Author "Reason, Christopher"

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    A study of Mesoscale Eddies, the Agulhas current and the evolution of its meanders using satellite observations and numerical modelling experiments
    (2019) Braby, Laura; Backeberg Bjƶrn; Krug, Marjolaine; Reason, Christopher
    The Agulhas Current is the strongest western boundary current in the Southern Hemisphere and plays an important role in the exchange of heat and salt between the Indian and South Atlantic Ocean basins, thereby affecting global climate. The variability in the northern Agulhas Current is influenced by both cyclonic and anticyclonic mesoscale eddies, originating from the Mozambique Channel and south of Madagascar (known as source region eddies) and which propagate toward the offshore edge of the Agulhas Current. Using a combination of an eddy-tracking data set with in-situ surface drifter observations and altimetry-derived geostrophic currents, it is shown that source region eddies dissipate upon approaching the Agulhas Current. Their entrainment into the Agulhas Current affects its mean velocity and offshore position through a transfer of momentum, with anti-cyclonic eddies consistently increasing the Agulhas Currentā€™s velocity by 0.16 Ā± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 Ā± 0.16 m.s-1 and shifting the current up to 144 Ā± 85 km offshore. These velocity anomalies propagate downstream at rates of 44 km.d-1 (anti-cyclonic eddies) and 23 km.d-1 (cyclonic eddies). Whilst existing numerical models are successfully able to capture many aspects of the Agulhas Current, many models are unable to accurately represent the observed eddy dissipation and interaction processes, affecting our understanding of mesoscale variability within in the current. In this study, we compare two simulation experiments in a regional Hybrid Coordinate Ocean Model (HYCOM), where we change the wind forcing, and using an eddy tracking algorithm assess the local effect of the changed wind stress on source region eddies and their interaction with the northern Agulhas Current. There is an overall reduction in eddy kinetic energy (EKE) of 33% over the Agulhas Current domain. Changes in eddy pathways, properties and energy conversion terms, resulting from the change in forcing from absolute to relative winds (the wind speed relative to the current speed) have resulted in significantly different mesoscale eddies in the regional HYCOM. The effects of the change in wind forcing on the variability within the Agulhas Current were examined and the differences between the two simulations were found to be very small. Finally, the evolution of meanders in the Agulhas Current, including the properties and dissipation of smaller meanders as well as mesoscale Natal Pulses type meanders, were assessed using both HYCOM experiments and compared to satellite observations. The representation of smaller meanders (under 50km in size) improved with the changed in wind forcing. However, larger Agulhas Current meanders (greater than or equal to 50km) which previously occurred too frequently in the regional HYCOM, are now too infrequent in the regional HYCOM, with an average of 1.1 meanders occurring each year. A decrease in the frequency of larger meanders was observed from the location offshore of Port Edward (30.22Ā° E, 31.05Ā° S) to the region of the ACT array (27.48Ā° E, 33.35Ā° S), in the satellite data as well as both model experiments, indicating that some of the meanders have dissipated and that both regional HYCOM models are able to resolve this.
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    An analysis of anomalously wet summers in the South Western Cape of South Africa
    (2023) De Kock, Wade Matthew; Reason, Christopher; Blamey, Ross
    Unlike the rest of South Africa, the southwestern Cape (SWC) experiences most its rainfall in the austral winter (May-September). Due to interannual, intraseasonal and interdecadal variability, drought is a familiar occurrence. The SWC recently suffered from an extended dry period, known as the ā€˜Day Zero Drought' during 2015-2018, where greater Cape Town nearly ran out of piped water supply. Despite most rainfall in the SWC occurring from MaySeptember, considerable rainfall events have been known to occur during the summer (October-March). Such events could play a substantial role in mitigating winter droughts and multiyear droughts the region suffers from. Large Rainfall Events (LREs) during the summer of 2018/19 caused average dam levels in all major dams of the SWC to increase by more than 1%. The dam level increase is significant during the driest period of the year where dam levels decrease by several % per month. This study investigates all LREs during the summer (October-March) from 1979-2019 and their effects on major dam levels. Most summer LREs are found to be linked to atmospheric rivers (ARs) or cut-off lows (COLs), which together account for up to 88% of the top 75 LREs. Apart from one study characterising the considerable effect of ARs on winter rainfall, to date little research on ARs has been done for the region. Furthermore, COLs have been suggested to occur mostly during transition seasons. This thesis reveals that although ARs last shorter than COLs and lead to a smaller area receiving rainfall in the SWC, they both yield intense rainfall amounts with ARs concentrated around Greater Cape Town. After LREs have occurred, average dam volumes were shown to increase by up to 5% making LREs essential in drought recovery. Anomalously wet summers, which typically contain more LREs than average, are also mostly associated with cooler temperatures and less extreme hot days (90% decile). Rainfall totals are inversely correlated (r=-0.44) with extreme hot days. In addition, extreme hot days also show a significant increasing trend of 2.8 days/decade from 1979-2019. Along with increased cloud cover, weaker winds over dam catchment areas can be associated with 4 out the 5 wettest summer seasons. Of the 5 wettest summer seasons, only one (2013/14) occurred in the last two decades. Anomalously cool and wet summers, reduce the water consumption impact on dam volumes as well as help reduce the impacts of drier than normal winter seasons. Wet and cool summer seasons also reduce fire risk in the region which is important considering that the region is agriculturally productive and has experienced several devastating fires in recent decades, both in agricultural areas as well as in greater Cape Town. Although the extended summer contributes only about 30% of the year's annual rainfall, summer LREs occur during the most water demanding part of the year. Notably, increased summer LREs usually correspond with anomalously wet summers. This thesis finds that anomalously wet summers can be characterized by increased rainfall days which are linked to increased cyclonic anomalies over the region and westerly moisture fluxes shifted anomalously equatorward in the South Atlantic. These changes in circulation patterns are found to be linked to a negative Southern Annular Mode pattern and in the late summer, also linked to ENSO and the zonal wave number 3 pattern. Overall, trends suggest decreases in rainfall days in the Greater Cape Town region and in the nearby mountain areas where most major dams are located for the mid to late summer (December-March). These decreases in rainfall days can be related to poleward expansions of the South Atlantic High Pressure (SAHP) which then lead to decreases in storms impacting the SWC. With storm tracks occurring further poleward due to moisture corridor shifts and SAHP poleward expansions during recent years, there is a decrease in summer LREs in the SWC. Some of these poleward shifts are related to the tendency of the Southern Annular Mode to be in positive phase in recent decades. Since summer LREs are important in mitigating droughts in the region, future work needs to consider rainfall in all seasons rather than just the historical focus on winter rainfall which has been relatively well studied. This thesis shows the potential importance of anomalously wet summers as essential contributors to moisture in the region during the driest period of the year.
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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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    Assessing dry spell and wet day frequencies over southern Africa during the summer rainy season
    (2021) Thoithi, Wanjiru; Reason, Christopher
    Rainfall over southern Africa experiences substantial temporal and spatial variability which heavily impacts poor rural populations in the region that rely on rainfed agriculture for their livelihoods. Instead of totals, seasonal rainfall is better characterised by wet and dry events occurring within rainy seasons as knowledge of the frequency of such events is able to inform agricultural activity. Dry spells (pentads having <5 mm) and moderate wet days (10-30 mm) over southern Africa were assessed using high resolution (0.05ā—¦) Climate Hazards group Infrared Precipitation with Stations (CHIRPS) datasets over the period 1981/82-2018/19 during October-November (ON), December-February (DJF) and March-April (MA) using climatology, intensity-frequency and trend analysis. Correlations with SST over the tropical southeast Atlantic and climate modes namely, El Nino Southern Oscillation (ENSO), the Subtropical ĖœIndian Ocean Dipole (SIOD) and Southern Annular Mode (SAM) were computed. These, together with regressed atmospheric and SST fields were used to identify possible mechanisms for changes in dry spell and moderate wet day frequencies during austral summer. Two strong gradients in dry spell frequency were found to be present during DJF, one diagonal along the western margins of the Kalahari desert and the other meridional, lying across 20-24ā—¦S. Topographic influences on rainfall were observed near the Drakensberg and Chimanimani mountains, Mulanje massif and Madagascan highlands where dry spell frequency (DSF) (moderate wet day frequency (MWDF)) tended to be relatively lower (higher). A region which frequently experienced half of the season as dry was identified lying across 22-24ā—¦S (18-25ā—¦S) during DJF (MA), with a core in the central Limpopo River Valley where 85-100% (100%) of the seasons were dry for half the season. DSF and MWDF trends indicated that drying has occurred over central South Africa during ON whereas decreasing DSF and increasing MWDF trends pointed to a weakening diagonal and meridional gradient during DJF. Additionally, increasing MWDF trends over important agricultural areas have occurred during DJF. Trends over central South Africa, part of the diagonal gradient, were associated with changes in ENSO, SAM, the Botswana High and SST in the SE Atlantic whereas those in the western Botswana region, part of the meridional gradient, were associated with those in the SIOD, Mozambique Channel Trough and Mascarene High and SST in the eastern and western Pacific.
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    Climate variability and extremes in the Okavango River Basin, southern Africa
    (2023) Moses, Oliver; Reason, Christopher
    The Okavango River Basin (ORB) located in southern Africa is a region of highly sensitive and biodiverse ecosystems. It spans Angola, Namibia and Botswana, with the world-famous Okavango Delta located in the latter country. The ecosystems depend on the highly seasonal ORB streamflow, which is also the major source of freshwater for the rural population, most of whom depend on subsistence farming. Climate variability and extremes such as droughts, hot days and extreme rainfall events are not well understood over this region. Also, the relationship between climate and other aspects like vegetation and river discharge are not well understood. To contribute to a better understanding of this relationship, the thesis investigated relationships between rainfall, temperature, Normalized Difference Vegetation Index (NDVI) and river discharge, and their interannual variability and trends. It was found that at monthly and seasonal time scales, NDVI spatial patterns are closely related to those of rainfall than temperature. The NDVI-rainfall and NDVI-temperature relationships differ north of 18.9Ā°S where rainfall is higher than to its south. Correlations between NDVI and rainfall show lags of 1-2-months. Large areas across the region show significant warming trends in all seasons but mainly in October-December (OND), as well as wetting mainly in the north. The warming trend may imply more evaporation and desiccation which may exacerbate extreme event impacts such as severe droughts. Interannual variability of rainfall, NDVI and temperature is pronounced with significant correlations with El NiƱo-Southern Oscillation (ENSO), the subtropical Indian Ocean Dipole (SIOD) and the Botswana High for rainfall and temperature, and for NDVI with ENSO. The temperature (rainfall) correlations with ENSO and the Botswana were positive (negative), with the SIOD they were negative (positive), and the NDVI-ENSO correlations were negative. On longer time scales, the wet 2006-2013 period was analysed relative to much drier 1999-2005 epoch for OND. The 2006-2013 wetter conditions appear linked to La NiƱa Modoki conditions, regional circulation differences and warmer sea surface temperature near Angola. Extreme rainfall events over the ORB were analysed. The analysis was performed within a larger region in western central southern Africa (WCSA), given that many rainfall events extend beyond river basin boundaries. Focus was placed on extreme rainfall events accumulated over 1-day (DP1) and 3-days (DP3), during the main rainy season, January-April (JFMA). Due to data sparsity, the Climate Hazards Group Infrared Precipitation with Station data (CHIRPS) were used to identify these events. It was found that contributions of DP1 and DP3 events to JFMA rainfall totals are, on average, ~10% and ~17%, respectively, but in some years their contributions exceed 30%. Most of the events result from tropical-extratropical cloud bands, with tropical lows being also important. Interannual variability in extreme events is substantial and appears linked to ENSO and the Botswana High. Although ENSO influences the extreme events and rainfall totals more generally over southern Africa, by far the neutral JFMA 2017 season experienced the wettest conditions over the world-famous Okavango Delta region. Factors that contributed to these heavy rains included a deeper Angola Low, weaker mid-level Botswana High and anomalous westerly moisture fluxes from the tropical southeast Atlantic during January ā€“ early March. The second most intense rainfall event occurred on April 22nd, resulting from a cut-off low. DP1 frequencies show significant increasing trends, and similarly, rain-days and rain totals over many areas. These trends have important implications for agricultural and water management as well as wildlife conservation in the ORB. To contribute to a better understanding of drought over the ORB region, the thesis analysed various drought metrics. These include a Cumulative Drought Intensity (CDI) index, based on the product of maximum dry spell duration and maximum temperature anomaly, and the Standardised Precipitation-Evapotranspiration Index (SPEI). Strong horizontal gradients in frequencies of dry spells and hot days were found to shift south over the ORB from August to November as the tropical rain-belt shifts increasingly south of the equator, the Congo Air Boundary declines and the Botswana High strengthens and shifts south-westwards. By December, the tropical gradient in dry spell frequencies is unnoticeable while that across the Limpopo River and southern ORB region, where the Botswana High is centred, stands out. On seasonal time scales, October-November 2013-2021 is particularly hot and dry over the Okavango Delta region. The thesis provided evidence that this hot and dry epoch is related to a stronger and southward shifted Botswana High and reduced low-level moisture convergence. On interannual time scales, there were strong relationships with the Botswana High, and to lesser extent ENSO. A strong drying-warming trend was found in the early summer, linked to a significant strengthening of the Botswana High. These trends, in conjunction with the Coupled Model Intercomparison Project Phase 6 (CMIP6) projected early summer drying over southern Africa found in the literature, may impact severely on the sensitive ecosystems of the ORB, and on water availability as well as subsistence farming in the region.
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    Drivers of coastal sea level variability along the east and south of South Africa
    (2019) Nhantumbo, Bernardino JoĆ£o; Shillington, Frank; Backeberg, Bjƶrn; Nilsen, Jan Even; Reason, Christopher
    Sea level rise and variability is of great concern in the coastal areas where a significant part of the global population is settled. Therefore, understanding regional and local long-term sea level variability as well as its trend is critical. On the other hand, quantifying how the sea level has varied on different timescales and why, is critical for understanding sea level changes, and crucial for improving future global, regional, and local projections. In this study, monthly mean sea level records of seven individual tide gauges, from the east and south coast of South Africa were used to analyse the embedded timescales of variability. These timescales were separated through the Empirical Mode Decomposition (EMD) method. This is the first time that the EMD method has been applied to southern African tide gauge records. The sensitivity of the EMD method when dealing with data gaps was tested on artificially created gaps in monthly mean synthetic altimetry sea level records, representing the seven individual tide gauges under consideration. The missing values were filled by linear interpolation, average value and linear trend value. The results suggested that whichever gap filling method is applied, the separated EMD timescales will display a distorted temporal structure of the continuous time series. As a consequence, monthly mean tide gauge sea level records were optimised by filling the gaps as best as possible using satellite altimetry data and the adjacent tide gauge records where possible, and then the oscillatory timescales of variability were separated using the EMD method with the intent to determine their physical drivers. However, identifying a single driver for each separated timescale is challenging due to our limited knowledge of how sea level is linked to the various forcing mechanisms. Therefore, the timescales of sea level variability extracted using the EMD were grouped into sub-annual and interannual timescales, and their relationship to possible driving mechanisms was investigated. The sub-annual timescale indicates how sea level responds to the mesoscale and synoptic weather systems in the annual cycle, including seasonal and annual large-scale wind and atmospheric pressure pattern changes. The interannual timescale indicates an association with the climate indices including El NiƱo-Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode through large-scale sea surface temperature patterns and large-scale pressure and wind patterns. In addition, the results have suggested that the studied coastal sea level has an association with the Agulhas Current at both sub-annual and interannual timescale through absolute dynamic topography variations at the Agulhas Current core locations. However, due to limitations in Agulhas Current data, the study was limited to East London and Port Elizabeth and the results suggested that the Agulhas Current contribution is responsible for over 62% of the monthly sea level variability at East London. However, the results were not sufficiently consistent to suggest a firm conclusion at Port Elizabeth.
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    Drylines over the interior of subtropical southern Africa
    (2024) Van Schalkwyk, Lynette; Reason, Christopher; Blamey, Ross
    Drylines are surface airmass boundaries separating regions of very dry air from regions of moist air. Due to changes in airflow that occur along the interface of these boundaries, they are known to act as zones of surface convergence which can trigger convection. For this reason drylines have been studied globally. Drylines regularly form over the interior of subtropical southern Africa and have also been associated with thunderstorm development and often severe thunderstorm initiation, but their seasonal characteristics, formation mechanisms and the extent to which they influence convection, are largely unknown. By utilizing ERA5 reanalysis surface specific humidity and temperature data and an objective moisture and temperature gradient detection algorithm, a drylines dataset is compiled between 1979 and 2020 for the subtropical southern African interior plateau (SAP). Drylines occur most frequently during spring and summer and almost daily in December. In this region dryline locations are very sensitive to surface moisture variations and therefor a westward shift in peak dryline frequency occurs through summer as moisture increases from the east after the dry winter months. Drylines peak first over the eastern SAP during November, but over the central and western parts in December. During years with increased surface moisture weaker moisture gradients occur in the east during mid-summer, resulting in fewer drylines, but during years with decreased surface moisture, more drylines tend to occur. During mid-summer, dryline frequencies in the east are negatively correlated with dryline frequencies in the west so that during years with high dryline frequencies in the east, low dryline frequencies tend to occur in the west and vice versa. The dryline climatology also establishes that drylines occur most frequently over the climatologically drier western parts of the SAP (western plateau) during summer. Drylines on the western plateau are then further investigated and lightning data from 2010-2021 are used to distinguish between drylines which resulted in convective storm development (convective drylines) and those that did not (non-convective drylines). It is found that 76% of drylines which occur over the western plateau during the late afternoons, when convection is most likely in the west, are convective. Convective drylines peak during December months when 12 occur on average over the western plateau region. The formation mechanisms of convective vs. non-convective drylines are determined by constructing synoptic composites during early summer (October-December) and late summer (January-March). Low-level moisture flow composites for the two days leading up to a convective dryline show that the Limpopo River Valley and Zambezi River Valley are important moisture pathways through which moisture from the southwest Indian Ocean travel in the days leading up to a convective dryline. During early summer convective drylines tend to occur after sharp increases 7 in surface moisture ahead of the dryline, in the presence of a deep Kalahari Heat Low and an upper air trough to its west. During late summer the pressure gradients are not as intense, but the Kalahari Heat Low continues to be a key component of the synoptic circulation on convective dryline days. Compared to early summer, a higher number of days with moderate CAPE (>1000 J.kg-1 ) occur in the same location that has the highest frequency of drylines during late summer, and this results in a greater proportion of dryline-related thunderstorms in southwestern Botswana, northeastern Northern Cape and western Northwest. Dryline climatology results confirm that drylines frequently occur over the SAP and that they have a meaningful impact on thunderstorm development in the drier western interior. This drylines study is completed by an investigation of the month with the highest number of convective drylines since the convective dryline record began in 2010 and an analysis of its synoptic characteristics. During December 2021, 20 convective drylines occurred on the western plateau, 8 more than the average. Record rainfall occurred over the central interior of South Africa while the drier western interior also recorded above average rainfall. A detailed analysis is performed of a 6-day convective dryline event where thunderstorms developed along the dryline daily, before being steered towards the central interior by anomalously strong mid-level winds. It is shown that a significant positive correlation exists between dryline days on the western plateau and rainfall within the domain, but also further downstream. In this thesis, drylines are shown to be an important climatological feature of subtropical southern Africa throughout summer. Dryline locations are dependent on the seasonal influx of moisture from the southwest Indian Ocean via the Limpopo and Zambezi River valleys and are often located at the western boundary of the tropical rain band edge. Drylines frequently act as triggering mechanisms for convection and therefor also influence rainfall further downstream
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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 ā€“ 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 ā€“ 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 ā€“ 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 (ā€œWRā€ and ā€œERā€) were also investigated. A significant relationship between sea-surface temperature from the NiƱo 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 NiƱo-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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    Extreme rainfall events over the Pongola-Mtamvuna Water Management Area of South Africa
    (2022) Mpungose, Nomvula Bongekile; Reason, Christopher; Blamey, Ross
    Subtropical southern Africa experiences substantial rainfall variability both spatially and temporally, due to regional orography, geographic position, and local sea-surface temperatures. Extreme weather conditions such as droughts and floods are not uncommon and can result in both positive and negative socio-economic impacts. The Pongola-Mtamvuna Water Management Area (PM) located over north-eastern South Africa consists of communities that depend on rain-fed agriculture, as well as an inter-linked ecosystem and fresh water bodies that are dependent on rainfall. Extreme rainfall events and the systems that produce them are still not well understood, therefore, a detailed analysis of such events can contribute to an improved understanding and management of their associated risks. Here, the main focus is on the summer rainy season (October ā€“ March), rainfall variability is examined using CHIRPS daily rainfall data covering a period of thirty-seven years from 1981 ā€“ 2018. Extreme rainfall events are identified and classified for the PM area. The analysis points to the highest rainfall amounts typically occurring over low-lying coastal areas and near mountainous regions. About 60% of the extreme rainfall events were associated with tropical lows (40%) and MCS (20%). Cut-off lows (18%), cloud bands (16%), and tropical cyclones (6%) contributed to the remaining proportion. The highest frequency of events occurred during late summer months (January ā€“ March) when tropical lows and occasionally, tropical cyclones are more common. Rainfall over the PM has a statistically significant relationship with ENSO, most of the seasons with below-average rainfall and extreme events coincided with El NiƱo conditions. Odd cases where this was the opposite were more influenced by regional circulation anomalies which acted to enhance or reduce moisture over the land-mass thereby increasing conditions favourable/unfavourable for rainfall over the region.
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    The fundamental role of spectral scattering in the ocean colour Phytoplankton Functional Type signal
    (2018) Robertson Lain, Elisabeth; Reason, Christopher
    There is increasing interdisciplinary interest in phytoplankton community dynamics as the growing environmental problems of water quality (particularly eutrophication) and climate change demand attention. This has led to a pressing need for improved biophysical and causal understanding of Phytoplankton Functional Type (PFT) optical signals, in order that satellite radiometry may be used to detect ecologically relevant phytoplankton assemblage changes. This understanding can best be achieved with biophysically and biogeochemically consistent phytoplankton Inherent Optical Property (IOP) models, as it is only via modelling that phytoplankton assemblage characteristics can be examined systematically in relation to the bulk optical water-leaving signal. Harmful Algal Bloom (HAB) conditions in the Southern Benguela and various inland waters of Southern Africa require continuous observation by satellite due to the potential for significant negative environmental impacts. Current oceanic bio-optical models do not perform well in elevated Chlorophyll a conditions, but the high biomass conditions of Southern African inland and coastal waters lend themselves extremely well to the development of phytoplankton IOP models as the water-leaving signal is overwhelmingly phytoplankton-dominated. An initial validation of a new model of Equivalent Algal Populations (EAP) is presented here, and comparison is made with two prominent phytoplankton IOP models. The EAP model places emphasis on explicit biophysical modelling of the phytoplankton population as a holistic determinant of IOPs. By necessity due to its origins in highly scattering waters, a distinctive attribute of the EAP model is its comprehensive handling of the spectral and angular character of phytoplankton scattering. This emphasis is shown to have an impact on the ability to retrieve the detailed phytoplankton spectral scattering information necessary for PFT applications and to successfully simulate waterleaving reflectance across wide ranges of physical environments, biomass, and assemblage characteristics. The accurate description of a water body's Volume Scattering Function (VSF), and hence its phase functions, is critical to the determination of the constituent IOPs, the associated spectral water-leaving reflectance, and consequently the retrieval of PFT information. The EAP model offers the ability to provide phytoplankton population-specific phase functions, unveiling an opportunity to gain further insight into the causality of the PFT signal. This is a new modelling capability, and its application in case studies and sensitivity analyses has resulted in improved understanding of the PFT/assemblage-related signal, in particular the discovery that phytoplankton spectral scattering is the primary driver of the PFT-related signal. The required thresholds of PFT detection with respect to biomass, IOP budget and assemblage effective diameter are quantified. Key findings are that the backscattering-driven signal in the 520 to 600 nm region is the critical PFT identifier at marginal biomass, and that while PFT information does appear at blue and red wavelengths, it is compromised by biomass/gelbstoff ambiguity in the blue and low signal in the red, due primarily to absorption by water. The key findings and recommendations are hoped to provide considerable insight into PFT approaches with regard to in situ observation, sensor development and algorithm optimisation for the next generation of PFT investigations.
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    Open Access
    Long-lived mesoscale convective systems over Eastern South Africa
    (2022) Morake, Dedricks Monyai; Reason, Christopher; Blamey, Ross
    Previous studies on severe weather in South Africa have often focused on synoptic-scale systems such as cut off lows, tropical extratropical cloud bands, and tropical cyclones, with little attention placed on the smaller mesoscale convective systems (MCSs). On a global scale, MCSs are not only important as key rainfall producers, but are often responsible for flooding, severe winds, hail and sometimes tornadoes. MCSs in South Africa remain poorly understood with there being little evidence of any long-term climatology studies of these systems over the region. A climatology of large, long-lived MCSs over eastern South Africa for the extended austral summer (September-April) from 1985-2008 is presented. On average, 63 MCSs occur here in summer, but with considerable interannual variability in frequency. The systems mainly occur between November and March, with a December peak. This seasonal cycle in MCS activity is shown to coincide with favorable convective available potential energy (CAPE) and vertical shear profiles across the domain. Most systems tend to occur along the eastern escarpment with its sharp topographic gradients close to the warm waters of the northern Agulhas Current. The eastern escarpment can act as a convective trigger by mechanically uplifting sufficiently moist and unstable air masses. In addition, strong latent heat fluxes from the northern Agulhas Current help to create high-CAPE conditions. Typically, initiation begins in the early afternoon, MCS status is reached mid-afternoon, maximum extent early in the night and termination around midnight or shortly thereafter. The analysis also considered MCSs that developed over land versus those over the adjacent ocean. It is found that most MCSs initiate over land, but systems that initiate over the ocean tend to last longer than those that develop over land. The results also show that there are differences in the seasonal cycle between continental and oceanic MCSs, with oceanic systems containing two intraseasonal peaks (December and April). There is a relatively strong positive relationship between the southern annular mode (SAM) and early summer MCS frequency. For the late summer, the frequency of MCSs appears related to the strength of the Mascarene high and Mozambique Channel trough, which modulate the inflow of moisture into eastern South Africa and the stability of the lower atmosphere over the region. The 6 results indicated that there is considerable variability in the long-lived MCS frequency on interannual time scales and such variability can have considerable impacts on regional rainfall totals during the summer months over eastern South Africa. MCSs are known to produce heavy rainfall that is often associated with floods, which can be devastating to livestock, crops, infrastructure, and humankind. However, these systems also provide important rainfall within a short time span that is significant for rain-fed agriculture for a semi-arid country as South Africa. Using Climate Hazards Infrared Precipitation with Stations (CHIRPS) satellite rainfall data for 1985-2008, this study identified 38 daily extreme rainfall events which account for 40% of the top 50 extreme rainfall events over the two sub-domains of the eastern parts of South Africa that are linked to long-lived MCSs. Of the 38 events, systems duration ranged between 6-51 hours with 23 systems lasting for longer than 12 hours. Individual MCS-associated extreme daily rainfall events contribute between 21% - 54% to the total seasonal amount of rainfall over eastern South Africa. There is also noticeable interannual variability of seasonal rainfall over the northern and the southern domain, and considerable spatial variability in seasonal rainfall of MCS-related extreme rainfall events over the two sub-domains. In general, the thesis has contributed to a better understanding of the wider group of large, longlived MCS characteristics over eastern South Africa in terms of their distribution, frequency, life cycle, seasonal cycle and large-scale environmental conditions and the relationship between MCS frequency and interannual climate mode of variability such as El NiƱo-Southern Oscillation (ENSO), the subtropical south Indian Ocean dipole (SIOD), and SAM. The study also contributed to a better understand the role MCSs play in eastern South Africa summer rainfall and particularly extreme rainfall in the region.
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    Modelling dispersal and connectivity of broadcast spawning corals in the Western Indian Ocean
    (2018) Gamoyo, Majambo J; Reason, Christopher
    Coral reef degradation is happening at an alarming rate all over the world due to multiple stressors with elevated sea surface temperature being the root cause. Using the Regional Ocean Modelling System and an individual-based model for the western Indian Ocean, this thesis explored the general circulation patterns (both large and mesoscale) important to dispersal and connectivity of broadcast corals while identifying regions that act as a source of larvae and those that receive larvae. Because habitat destruction and fragmentation through severe bleaching and mortality threaten coral reef health, projected thermal stress from Global Climate Models was explored to quantify future bleaching scenarios that might impact the reproductive timing and larval dispersal. Evaluation of the ROMS configuration for the western Indian Ocean shows that the basin-scale circulation patterns of the region are appropriately captured with the mean volume transports consistent with those derived from observation. Using the eddy detection algorithm, a description of the Southern Gyre as a key aspect of the Somali Current system was identified. The Southern Gyre is associated with barotropic instabilities associated with the northward flowing Somali Current. Rossby waves arriving at the East African coast and the strength of the monsoon winds are also responsible for the evolution and intensification of the gyre. The aggregated trajectories from the Lagrangian model highlight the dominant dispersal pathways and barriers to dispersal following release. The general circulation plays an important role in the dispersal of reef larvae over the study region. At a short pelagic larval duration, most of the released larvae settle back to or near natal reefs, but as the pelagic duration increases, the number of isolated reefs and islands decreases. Even with increased pelagic duration, some reefs (e.g., Agalega and Tromelin) are completely isolated. The mean dispersal distance from release to settlement varied across the region with larvae released along the East African coast dispersed an average of 405 km before settling while those in the Seychelles archipelago dispersed about 101 km. Different blocks of clusters were observed with 16 clusters observed when the pelagic duration is shorter (5 days), compared to seven clusters when the pelagic duration is longer (60 days). The warming trends and bleaching thermal stress shows that among the 636 reef pixels in the study region, about 56% showed positive sea surface temperature trends during the study period (1985- 2016). The frequency of bleaching level thermal stress has also increased over the same period, a tendency that climate models project to continue. Even under optimistic scenarios (such as the Representative Concentration Pathway RCP 4.5), most coral reefs are projected to experience severe bleaching and possible mortality by the 2050s. Low to moderate thermal stress are projected over reefs along the East African coast and near the northwest tip of Madagascar and thus these regions may act as potential climate refugia while increasing the potential of reefs to cope with climate change.
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    Open Access
    Revisiting the links between the Southern Annular Mode and rainfall over the Western Cape region of South Africa
    (2018) Mahlalela, Precious; Reason, Christopher; Blamey; Ross
    The winter rainfall region of South Africa displays considerable interannual variability and prevalence to prolonged dry periods. Although not completely understood, a wide range of factors have been highlighted to contribute to this interannual variability. The relatively poor understanding of rainfall variability in this region is of concern considering the low rainfall received in 2015-2017, resulting in the City of Cape Town enforcing severe water restrictions due to dam levels falling dangerously low. The focus of this thesis is on the influence of the Southern Annular Mode (SAM) on rainfall over the region, the possible influence of El NiƱo Southern Oscillation (ENSO) is also considered. To achieve this, a correlation analysis was conducted using the Marshal (2003) SAM index and station rainfall anomalies over the region for the period 1979 to 2016. The results show that five (three) of the six driest (wettest) years were associated with a positive (negative) SAM phase. However, the relationship is found to be statistically insignificant at a 95% significance level. The relationship is also found to show spatial variability, with strong negative correlations over the West Coast, while a weak positive correlation is observed over the South Coast. Furthermore, a decadal analysis in the relationship found it to be statistically insignificant (at the 95 th significance level) for most of the study period, with an exception of the early winter over the West Coast which shows a strong negative correlation after 2015. A composite analysis showed that dry (wet) winters tend to be associated with a positive (negative) SAM pattern superimposed with a wave number 3 anomaly. In addition, there are La NiƱa (El ivNiƱo) ā€“ like SST anomalies in the tropical Pacific. These circulation and SST patterns are more or less observed during the generally dry 2015-2017 winters except that winter 2015 shows an El NiƱo SST anomaly.
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    Open Access
    Sensitivity of tropical cyclones in the Southwest Indian Ocean to the topography of Madagascar
    (2023) Christensen, Zachary; Reason, Christopher
    In this study, simulations from the Weather Research and Forecasting (WRF) model are used to investigate the sensitivity of tropical cyclone (TC) characteristics in the Southwest Indian Ocean (SWIO) to the presence of Madagascar topography. This is achieved through running two sensitivity experiments where the topography is i) flattened to a uniform plateau of 300 m height (FLAT run) or ii) ocean grid points completely replace Madagascar with sea surface temperatures (SST) interpolated from the neighboring South Indian Ocean (SEA). These are then compared to the control run (CNTRL), where the topography is unchanged. Each run receives the same boundary conditions at the surface and lateral boundaries from the Climate Forecast System run compares reasonably well with the modern European Centre for Medium-Range Weather Forecasts reanalysis data (ERA5). The model outputs are then run through the TRACK-1.5.2 program to identify and track TCs. It is found that the TC characteristics in CNTRL are reasonable when compared to those found in observations, which are provided by the International Best Track Archive for Climate Stewardship. Based on WRF being able to adequately represent the track and regional atmospheric circulation, the analysis of TC genesis and tracks in the three WRF runs indicates that these weather systems are sensitive to the Madagascan mountains. Each sensitivity run showed cyclonic high-pressure anomalies across southern Madagascar and enhanced low-level easterly winds, possibly from an enhanced South Indian Ocean high pressure system, causing TCs to track further westward into Mozambique when the mountains are either flattened or removed completely. Additionally, the SEA shows considerably more TCs generated in the far west of the basin, which are stronger than CNTRL and tend to follow a more zonal track towards and onto the mainland. These differences relate to Madagascar being replaced by warm SST with increased latent heat release. The results suggest that the island of Madagascar plays an important role in the tracks, intensity, and numbers of TCs found in the real world in the Mozambique Channel (MC) region.
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    Open Access
    The role of the large-scale modes of climate variability in the Southern African wave climate
    (2022) Oliver, Benjamin; Reason, Christopher; Veitch, Jennifer
    The wave energy flux along the southern African coastline regularly reaches extreme levels, seriously impacting coastal communities, infrastructure, as well as near-coast and offshore marine operations. Understanding the drivers behind past high wave energy events and their frequency is key to forecasting future events. Using both the in-situ wave buoy data recorded by the Council for Scientific and Industrial Research (CSIR) and satellite altimeter data, 2 global wave hindcast products are evaluated and the best-performing is chosen to assess long-term variability and trends around the coastline between 1979 and 2020. Seasonal trend analysis revealed significant increasing trends in offshore flux for all seasons, with spring having the strongest coastal trends. The role of the Southern Annular Mode (SAM), El Nino Southern Oscillation (ENSO), and the semi-annual oscillation (SAO) on the interannual monthly wave energy flux and direction variations were assessed. Individually each mode showed significant anomalies for at least one season, however often there are multiple active modes making things more complex. SAM has the strongest control on the flux anomalies with the negative (positive) SAM associated with above (below) average anomalies in both flux and westerly (easterly) direction anomalies. ENSO directly impacts the summer wave climate, and the SAO indirectly impacts the wave energy flux over spring and winter. All the in-situ wave height correlations showed changes when compared to partial correlations controlling for the other 2 modes. The SAO relationships showed the largest differences when accounting for the SAM and ENSO phases, generally reducing in strength. Constructive modal interference has led to both strong positive and negative anomalies in the past and will continue to do so in the future. The largest near-coast positive anomalies occurred under concurrent negative SAM and negative SAO, with more intense offshore anomalies under El NiƱo whereas the strongest negative anomalies occurred under a combination of La NiƱa with positive SAM and SAO phases.
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    Open Access
    The variability of Lagrangian transport in the southern Benguela Current upwelling system
    (2020) Ragoasha, Moagabo Natalie; Reason, Christopher; Herbette, Steven; Veitch, Jenny; Cambon, Gildas; Roy, Claude
    This study analyses the physical mechanisms that impact Lagrangian pathways and transport in the southern Benguela upwelling system (SBUS),an environment in which currents are key components of many important ecological processes, including the dispersal of marine larvae. Physical advection by currents is an important mechanism for egg and larvae transport success in the SBUS since the spawning areas and recruitment areas are separated by a long distance. High-resolution numerical model simulations of the SBUS coupled with particle tracking experiments are used to investigate Lagrangian pathways between the Cape Peninsula (34ā—¦S) and St Helena Bay(32ā—¦S) and how they are linked to the oceanic circulation. Transport success, given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released, is used quantify the alongshore connectivity between the two regions. We have identified and quantified the following physical drivers: (i) Benguela Jet, (ii) offshore Ekman transport,(iii) inner shelf poleward current, (iv) mesoscale eddies to be responsible for the spatial and temporal variability of the alongshore connectivity. The Benguela Jet was found to be the dominant driver of the connectivity at both seasonal and interannual timescales. Moreover, the presence of anti-cyclonic eddies near the shelf-edge negatively impact transport success by advecting particles into the open ocean. The opposite occurs with shelf-edge eddies as they transport particles onshore onto the shelf and the Benguela Jet contributing to positive transport success anomalies. These findings will provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs.
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    Upper ocean variability and tropical cyclones in the South West Indian Ocean
    (2018) Mawren, Daneeja; Reason, Christopher
    Tropical cyclones (TCs) are the most devastating weather phenomenon in nature with the powerful storm surge events occurring when severe and large TCs make landfall along coastlines. Although there have been significant strides in the TC track forecasts over the last 30 years, skills in TC intensity prediction still lag behind. Intensity may be impacted by the mixing length temperature (known as Tdy) and barrier layer thickness (BLT). Similar to cyclones in other tropical ocean basins, tropical cyclones in the South West Indian Ocean also cause significant social and economic damage in southeastern Africa and Madagascar. To forecast TC intensity more accurately, monitoring upper ocean conditions in the South Indian Ocean is of top priority. Two areas in the relatively poorly studied South Indian Ocean where such upper ocean characteristics of relevance to tropical cyclones need to be better understood are the Seychelles Chagos Thermocline Ridge (SCTR) and the Mozambique Channel. In the first part of the study, the variability of Tdy and BLT in the South West Indian Ocean, focused on the SCTR region and their relationships with tropical cyclones are investigated. It is shown that rapid cyclone intensification is influenced by large Tdy values, thick barrier layers and the presence of anticyclonic eddies. Both BLT and Tdy fields are modulated by the westward propagation of Rossby waves, which are often associated with ENSO. For example, the 1997-1998 El Nino shows a strong signal in Tdy, SST and BLT over the South West Indian Ocean. After this event, an increasing trend in Tdy occurred over most of the basin which may be associated with changes in atmospheric circulation. A rise in SST, Power Dissipation Index and frequency of Category-5 tropical cyclones also occurred post-1998. A case study of TC Bansi in the South West Indian Ocean and its relation to upper ocean heat content was presented. This tropical cyclone is of interest due to its unusual track and also because of all the damage it caused. Anomalously deep thermocline and high Tdy values were observed around December 2014-January 2015 in the South West Indian Ocean and analysis of the upper ocean structure during Bansi showed that its rapid intensification to Category 4 was related to its passage over a high Tdy (warm core) eddy region and a deep barrier layer. The second area focussed on, the Mozambique Channel, is not only a region of relatively high TC activity with highly vulnerable coastal populations, but also very energetic in terms of mesoscale ocean eddies and tidal forcing. Changes in upper ocean characteristics in the Mozambique Channel due to tidal forcing are examined as they may have significant impacts on the upper ocean structure and thus influence tropical cyclones which often occur in this region. Two experiments were conducted using the Regional Ocean Modelling System (ROMS); one forced with tides (Tide) and the other experiment without tidal forcing (NoTide). On seasonal time scales, the tidal forcing simulation shows warmer temperatures in the upper layer particularly near strong ocean currents (North East Madagascar Current and South East Madagascar Current). In Tide, warming near these currents is intensified during winter due to the southeast trade winds, while in summer, poleward advection of warmer waters south of 16-17 oS seemed more prominent. On weather time scales, these changes in the upper ocean structure, especially near the coast or in shallow regions can alter the intensity of passing tropical cyclones. When a storm encounters a warm anticyclonic eddy, as the case of TC Japhet studied in the thesis, the SST cooling by the cyclone is substantially reduced, the mixing length temperature is increased and the mixed layer is deepened. These changes can be important for TC evolution. SST variability over the South West Indian Ocean influences southern African summer rainfall and the regional atmospheric circulation either through regional modes as well as influences the landfall frequency of tropical cyclones on Mozambique (Vitart et al., 2003). Besides SST, a link has recently been found between the regional precipitation over southern Africa and tropical cyclone heat potential (a measure of upper ocean heat content) in the South West Indian Ocean (Malan et al., 2013). In this study, the relationships between an index of southern African summer rainfall (SARI) and Tdy in the South Indian Ocean at zero (January-March) and one season (October-December) lag were analyzed. A region in the southern Mozambique Channel, termed as Tdysmc, showed the strongest positive correlation with SARI at zero lag. Another strong but negative correlation with SARI at one season lag is found in the core of the Seychelles Chagos Thermocline Ridge region, termed as Tdycsctr. Composite analysis (neutral with respect to ENSO) indicated that when Tdysmc is enhanced over the South Mozambique Channel during JFM, positive rainfall anomalies prevail over large parts of subtropical southern Africa and the Congo Basin with reduced rainfall occuring over most of Madagascar and northern Mozambique. The rainfall differences are associated with enhanced easterly flow towards Madagascar transporting more moisture towards Mozambique and Tanzania, consistent with the increased rainfall. During positive Tdysmc JFM seasons, more tropical cyclones (TCs) were formed in the SWIO and more of them crossed the Mozambique Channel compared to negative Tdysmc seasons. Furthermore, during positive Tdysmc seasons, the landfalling TC was generated in the Mozambique Channel while during the negative Tdysmc ones, it was formed in the central South Indian Ocean. Positive Tdysmc seasons also have increased number of Category5 TCs in the Mozambique Channel. These results suggest that changes in the mixing length temperature, Tdysmc index which can be estimated from satellite data can be useful to monitor and potentially predict regional precipitation as well as the frequency and intensity of tropical cyclones that impact the south-eastern coast of Africa.
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    Variability and trends in rainy season characteristics of the Eastern Cape
    (2023) Mahlalela, Precious; Reason, Christopher; Blamey, Ross
    Forming part of south-eastern South Africa, the Eastern Cape province has been prone to extreme weather events such as floods and droughts. The region also displays considerable interannual rainfall variability with a tendency towards prolonged dry periods in recent decades. There is generally a poor understanding of the factors contributing to this rainfall variability. This is concerning considering the recent prolonged (2015 - 2020) drought, that has had major socio-economic effects particularly on the large impoverished rural population as well as on some urban areas where supplied water services have broken down in several cases. Even with some rainfall relief in the province during 2022, water shortages persist, particularly in the largest metropolitan area of Nelson Mandela Bay. The region is influenced by both midlatitude and tropical systems leading to a complex regional meteorology that hitherto has not been much studied compared to other parts of South Africa. Here, variability and trends in rainfall characteristics for the Eastern Cape are examined. Focus is placed on the spring (September-November) and summer (December- February) as these seasons contribute the largest proportion to annual totals. The spring season contributes between about 25-35% of the annual rainfall total, while the summer season contributes about 40-45%. Due to limited available station data, the Climate Hazards Infrared Precipitation with Stations (CHIRPS) data set is used. Comparisons with the available station data, provides confidence in the CHIRPS-derived results. On interannual time scales, the results indicate that dry (wet) springs over the Eastern Cape are associated with a cyclonic (anticyclonic) anomaly southeast of South Africa as part of a shift in the zonal wavenumber 3 pattern in the midlatitudes. Over the landmass, a stronger (weaker) Botswana High is also apparent with increased (decreased) subsidence over and near the Eastern Cape which is less (more) favourable for cloud band development and hence reduced (enhanced) rainfall during dry (wet) springs. The summer season shows significant El NiƱo Southern Oscillation (ENSO) and Southern Annular Mode (SAM) influences as well as from the Botswana High. Composites show that dry (wet) summers tend to be associated with a negative (positive) SAM pattern superimposed with a wave number 4 anomaly. According to CHIRPS data, the spring season has shown a significant decreasing trend in total rainfall as well as the number of light, moderate, and heavy rainfall days over most of the province since 1981. The summer signal is less consistent, with a significant increase in rainfall in some inland areas but a decrease in rainfall near the coast which is not found to be statistically significant. The observed summer trends are attributed to an increase in light and moderate rainfall days inland and a decrease in heavy rainfall days near the coast. An examination of the regional rainfall features suggests that the observed decrease in spring rainfall may be related to an observed decrease in the number of cloud bands during the spring while, the observed increase in rainfall inland during the summer might be associated with increased ridging along the south coast. Analysis of mid-century (2040-2060) CMIP5 rainfall projections suggests that there may be a flattening of the annual cycle over the Eastern Cape with the winter becoming wetter and parts of the summer drier. There is a large spread in CMIP5 model projections over the region with the multi-model mean projecting a very slight drying in both seasons. It is suggested that existing climate models may find representing the Eastern Cape region particularly challenging given its sharp gradients in land surface and ocean conditions and its complex meteorology