Browsing by Author "Reason, Chris"

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    An analysis of recent severe climate events over Tanzania
    (2009) Kijazi, Agnes Lawrence; Reason, Chris
    This thesis focuses on the analysis of the recent severe climate events over Tanzania. The circulation patterns over the Atlantic and Indian oceans responsible for inter-annual and intra-seasonal rainfall variability in Tanzania were analysed and the skill of Global Circulation Model (GCM) in capturing the inter-annual variability assessed. The results were tailored to the application for agriculture over North-eastern Tanzania to aid decision makers in the formulation of agriculture related policies for sustainable development. The thesis first touches on an analysis of the mean characteristics of selected meteorological fields comprising both MAM and OND rainfall seasons. Such processes were examined as necessary background information to determine the causes of rainfall variability. Two regions of deep convection were observed: one maritime (over the Indian Ocean), and one continental (over the Congo forests). High values of latent heat flux occurred in the Congo forests during the OND season, suggesting this region to be among the sources of moisture for said season. Generally, the easterly moisture flux dominates the region for both the OND and MAM seasons and this suggests the Indian Ocean to be the main source of moisture for the country's rainfall. The descending branch of the Walker type circulation was located over the Indian Ocean at about 55 °E and longitudinal shifts of this branch may cause rainfall anomalies. Similarly, the departure from any observed mean characteristics may cause climate anomalies over the region. Inter-annual time scale analysis indicates that the region experienced more dry than wet years during the period of study. However, dry anomalies were much weaker than wet anomalies, and the influence of El Niño on wet anomalies was suggested as one of the causes of this variation. Ascent (descent) over the study region coupled with a strong moisture flux convergence (divergence) was observed to be the dominant feature for wet (dry) years. On the intra-seasonal time scale that is so important for agricultural planning, wet years were characterised by longer than normal rainfall seasons, while for dry years the seasons were shorter than average. For the dry OND season, an out-of-season peak was evident that could be used to minimize the effect of drought. The circulation features show that the wet years were characterised by a cyclonic moisture flux and wind over the tropical South Indian Ocean east of Madagascar coupled with strong westerly moisture flux emanating from the equatorial Atlantic Ocean and south-easterly moisture flux from the Western Indian Ocean that enhance moisture flux convergence over the country. In contrast, the dry years show more or less the reverse circulation patterns over the Indian Ocean. An anticyclonic circulation occurs east of Madagascar with circulation patterns that divert moisture away from Tanzania. Comparison of ECHAM 4.5 model with NCEP/NCAR reanalysis over the study region indicated that the model was able to represent the circulation features associated with rainfall during the wet season while for the dry season the model failed to represent the circulation features. This suggests that local features (that are not well captured by the model) contribute significantly to the rainfall variability over the region, especially during dry seasons. The results obtained from this thesis have contributed to a better understanding of the mechanisms associated with rainfall variability over north eastern Tanzania. The research has addressed the gap that exists in the literature concerning what factors and mechanisms might drive rainfall variability at the sub-regional scale over East Africa. Furthermore, this study is the one of the first to assess the ability of an atmospheric model to replicate the observed circulation and rainfall patterns at this sub-regional scale for Tanzania.
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    An analysis of rainy season characteristics over the Limpopo region
    (2005) Phaladi, Raesetje Florina; Reason, Chris
    This thesis investigates characteristics of the summer rainy season over the Limpopo region that are important to the agricultural sector and other user groups. The Limpopo region supports a large rural population dependent on rain-fed agriculture as well as significant biodiversity, particularly in the Kruger National/Limpopo Transfrontier Park and is vulnerable to severe flood and drought events. REcently, the region has been impacted by severe drought (2002-2004) and flooding in late summer 2000. The rainy season characterisitcs investigated are the frequency of wet and dry spells during the rainy season and the onset date of the season.
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    An analysis of variability in the South Atlantic
    (2006) Colberg, Frank; Reason, Chris
    In this thesis the variability of the South Atlantic climate is investigated. In order to account for both large and regional scales, two ocean models are applied. An ocean general circulation model, forced with the National Centre for Environmental Prediction / National Centre for Atmospheric Research reanalysis is used to investigate the South Atlantic leading modes and also investigate the South Atlantic response to El Niño-Southern Oscillation induced climate variability. A regional ocean model is used to investigate variability in the South East Atlantic., especially in the Angola Benguela Frontal Zone.
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    Characteristics of wind fields and air-sea interactions over the upwelling region of the Somali coast
    (2008) Mafimbo, Ali J; Reason, Chris
    The mesoscale structure of the low-level wind field associated with a strong upwelling event was investigated. During July 2005 when a strong upwelling event occurred, the Somali jet was found to have oscillated at lower frequency of 3-7 weeks than the normal bi-weekly mode observed in several studies and the mesoscale winds exhibited high covariability with the prevailing SSTs. Strong values of alongshore winds were deduced from late June to mid-July. These winds weakened significantly in the third and fourth week of July. A large off-shore pressure gradient due to differential thermal properties of land and sea was also observed.
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    A climatology of potential severe convective environments across South Africa
    (Springer Verlag, 2016-11-15) Blamey, Ross; Middleton, C; Lennard, C; Reason, Chris
    Severe thunderstorms pose a considerable risk to society and the economy of South Africa during the austral summer months (October–March). Yet, the frequency and distribution of such severe storms is poorly understood, which partly stems out of an inadequate observation network. Given the lack of observations, alternative methods have focused on the relationship between severe storms and their associated environments. One such approach is to use a combination of covariant discriminants, derived from gridded datasets, as a probabilistic proxy for the development of severe storms. These covariates describe some key ingredient for severe convective storm development, such as the presence of instability. Using a combination of convective available potential energy and deep-layer vertical shear from Climate Forecast System Reanalysis, this study establishes a climatology of potential severe convective environments across South Africa for the period 1979–2010. Results indicate that early austral summer months are most likely associated with conditions that are conducive to the development of severe storms over the interior of South Africa. The east coast of the country is a hotspot for potential severe convective environments throughout the summer months. This is likely due to the close proximity of the Agulhas Current, which produces high latent heat fluxes and acts as a key moisture source. No obvious relationship is established between the frequency of potential severe convective environments and the main large-scale modes of variability in the Southern Hemisphere, such as ENSO. This implies that several factors, possibly more localised, may modulate the spatial and temporal frequency of severe thunderstorms across the region.
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    Cross-shore exchange in the northern Benguela and the related role of upwelling filaments Annethea Auguste Muller.
    (2013) Muller, Annethea Auguste; Reason, Chris; Mohrholz, Volker; Schmidt, Martin
    The purpose of this research is to quantify where, how much and by what mechanisms water leaves or comes on to the Namibian shelf and secondly to gain a better understanding into the circulation associated with upwelling filaments in the northern Benguela region and quantify their contribution to the exchange across the shelf edge. For the analyses results from a MOM-4 regional ecosystem model are combined with remote sensing and cruise data.
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    Cut-off low pressure systems and extreme rainfall over South Africa
    (2005) Singleton, Andrew Thomas; Reason, Chris
    This thesis is an investigation of cut-off low pressure systems over South Africa. These weather systems have been responsible for many of the flooding disasters that have affected South Africa, particularly the coastal regions, over recent decades. The thesis has two main objectives, namely, to construct a 30-year climatology of cut-off lows over South Africa, and to further understanding of the evolution of the low-level flow that leads to these systems producing extreme quantities of rainfall.
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    The dynamics and physical processes of the Comoros Basin
    (2013) Collins, Charine; Reason, Chris; Hermes, Juliet C
    The main objective of this thesis was to investigate the circulation in the ComorosBasin using observed and model datasets. These data were used to establish whether or not a Comoros Gyre exists and to investigate the nature of the eddy variability in the basin.The water masses in the Comoros Basin emulate those found further south in the Mozambique Channel. The presence of AAIW north of Madagascar confirmed that this water mass enters the Comoros Basin from the east while the presence of North Atlantic Deep Water showed that this water mass is capable of spreading northward over the Davie Ridge. The main currents in the Comoros Basin, the westward flowing NEMC and a poleward current along the western boundary, are under the influence of the monsoon winds. The NEMC intensifies during the Southwest monsoon in response to a strong wind jet which develops off the northern tip of Madagascar, whereas the poleward current weakens due to the opposing force imposed by the southwesterlies. Additionally, the circulation in the basin consist of meso-scale eddies of both polarities. Anti-cyclonic eddies, with lifespans of
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    Evaluation of an eddy resolving HYCOM simulation in the South Atlantic Ocean
    (2014) Giddy, Isabelle Sindiswa; Backeberg, Bjorn Christoph; Ansorge, Isabelle Jane; Reason, Chris; Campos, Edmo
    The South Atlantic Ocean acts as a key region of ocean teleconnections and water mass transformation, directly transporting waters from the Southern, Pacific and Indian Oceans towards the North Atlantic and feeding the upper branch of the Atlantic Meridional Overturning Circulation. To date, this region remains under sampled over long time periods. Models form useful platforms for the wholistic study of such regions where long term datasets are not available. However, it is important to first gauge to what extent the model deviates from available observations. In a detailed analysis, this study evaluates an eddy resolving 1/12° HYbrid Coordinate Ocean Model simulation in the South Atlantic and Agulhas regions with reference to available observations and literature. The outcomes of this research contribute to improving the configurations of future ocean models by identifying the limitations of the model analysed. In addition, analysis of the 50 year (1960-2009) simulation reveals that, in this model simulation, mesoscale activity and sea surface temperature trends in the Cape Basin are not related to Agulhas Current transport changes. These findings highlights the need for long term observational databases and further research into climate related trends and interannual variability of the ocean, in particular around Southern Africa.
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    Historical climate variability reconstructed from massive coral records in the western Indian Ocean
    (2012) Nicolas, Jean Vincent Arnaud; Reason, Chris; Zinke, Jens
    Coral δ¹⁸O and Sr/Ca records from massive corals in the western Indian Ocean (WIO) are used to establish the heterogeneous distribution of warming rates across the tropical and subtropical regions and to investigate if it corresponds with that from instrumental sea surface temperatures (SSTs). The coral records correlate with instrumental data better on monthly time scales compared to annual time scales. Coral thermal stress was assessed by the Degree Heating Months (DHM) technique and even though the coral DHM aligns quite well with instrumental DHM, the values from coral data are generally 2-3 times greater in magnitude than the DHM values from instrumental data. It was found that the accumulated thermal stress, calculated from coral and instrumental data, for the majority of the tropical and subtropical WIO sites has been increasing since the 1970’s. From 1870 to 1995, both the tropics and the subtropics have been warming in general, although with different and varying rates as recorded by the coral and the instrumental SSTs. It was further revealed that both the tropical and the subtropical WIO warmed during the summer and winter periods during 1870-1995. On longer time scales, the relationship between the coral records in the WIO and climate indices showed a significant interannual variability approximately centered at periods 3-6 years, indicating a probable link with ENSO and IOD. The extent to which coral reefs from different sites in the WIO are prepared to survive climate change based on historical SST variability and intensity of warming rates are described. It could therefore be suggested that some corals may be more favoured to survive warming climate compared to others because corals in the WIO are located in different oceanographic conditions and experience different climatic variations.
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    The influence of the Indian and Atlantic Oceans on wet and dry spells over southern Tanzania
    (2006) Likunama, Michael John; Reason, Chris
    Rainfall over Tanzania is highly variable. In recent decades the country has been devastated by floods and droughts. The Tanzanian population relies heavily on seasonal rainfall. Over the northern part of the country, the rainy season occurs in two phases, the short rains (October-December) and the long rains (March - May). Over the southern and the western areas, the rainy season occurs during November - May. This study aims at identifying factors that may playa role in the rainfall variability over the southern region of Tanzania. This study indicates that over the Indian Ocean, the sea surface temperatures (SSTs) over the northeast of Madagascar have a strong relationship with the rainfall in southern Tanzania during the OND rainfall, while the SSTs over the southeast Atlantic have a strong relationship during the JFM rainfall. It has also been revealed that the El Niño/ Southern Oscillation (ENSO) have influence on rainfall variability over the region. The warm/cold ENSO events would impact both a wet or dry year. Such contradiction has imposed a challenge to the forecasters of seasonal rainfall over southern Tanzania.
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    Influence of the Indian Ocean Subtropical Dipole on the Agulhas current
    (2014) Sovara, Mthetho Vuyo; Hermes, Juliet; Reason, Chris
    Modern studies have successfully linked Subtropical Dipole (SIOD) events to southern Africa’s austral summer precipitation patterns, however, none have investigated the SIOD’s influence on the Greater Agulhas Current System. Here, the SIOD climatology was developed using a Regional Ocean Modeling System (ROMS) configured with GFDL-CORE v.2b reanalysis winds and heat fluxes for the 1958-2007 period. This configuration allows for a relatively accurate spatial and temporal account of the Sea Surface Temperature (SST) and Sea Surface Height (SSH) variability in the Subtropical Indian Ocean (SIO). Simulation and evaluation of SIOD events was achieved through the application of the Empirical Orthogonal Function (EOF), Wavelet Analysis and Composite Map Analysis. The EOF applied to monthly SST anomalies for the months January to December during the years 1958-2007 in the SIO resulted in the SIOD phenomenon emerging as the second EOF mode and explaining 8.93 of the total variance of the SIO. Moreover, the EOF applied only to the austral summer (JFM) months emerges the SIOD as the first EOF mode and explaining 20.84 of the total variance in the SIO. ROMS model results and statistical correlation results suggest that SIOD SST variability is neither linked to the El Nino-Southern Oscillation (ENSO) nor the Tropical Indian Ocean Dipole (IOD) phenomena, notwithstanding that SIOD events have in the past, coincided with some El Nino and La Nina events. Composite map analysis results suggest no significant influence of SIOD events on anomalous Agulhas Current SST and SSH during positive and negative SIOD years. Examination of lagged statistical correlations also showed no significant relationship between the anomalous SIOD index and the satellite derived geostrophic velocity at the core of the Agulhas Current for the period 1993-2007.
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    The inter-annual variability in the onset of the enhanced chlorophyll-a east of Madagascar
    (2015) Oozeeraully, Yuneeda Bibi Naheed; Reason, Chris; Hermes, Juliet C; Collins, Charine
    Chlorophyll-a concentration is a measure of phytoplankton biomass and is therefore used as a proxy for primary production. The chlorophyll bloom occurring in the South western Indian Ocean is one of the major blooms in the open ocean and is subject to controversies about its formation, propagation and termination mechanisms. In this study, the region was divided into two sub-regions, the South East region (48°E-66°E, 24°S-30°S) and the East region (70°E-88°E, 24°S-30°S) and satellite datasets of chlorophyll-a, sea surface temperature, wind speed and direction and sea surface height were analysed. A bloom was characterized by twice the mean level of the sub-region (~0.08mg/m3) and three bloom years were identified: 2004, 2006 and 2008. The timing, propagation and termination of the bloom showed spatial and temporal variability. The wavelet analysis revealed a semi-annual signal for 2006 and 2008 and an annual one in 2004 due to the gradual decline in the concentration of chlorophyll-a. The transport of the bloom is affected by the passage of eddies where the concentration of chlorophyll-a is higher in the core and around the edges in cyclonic and anticyclonic eddies respectively. The bloom could be present at the sub-surface level throughout the years but is only apparent at the surface during specific years.
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    Interaction of the antarctic circumpolar current with topography: impacts on the Southern Ocean eddy dynamics
    (2013) Kobo, Nomkwezane Sanny; Reason, Chris; Monteiro, Pedro; Herbette, Steven
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    An investigation into dry and wet spell characteristics over Zambia and into the onset of the rainy season
    (2005) Hachigonta, Sepo Promise; Reason, Chris
    Recurring dry/wet spells and decreased agricultural productivity during the last decade in the southern and northern parts of Zambia point to the need for a clearer understanding of these events, their frequencies and their possible connection to large-scale climate modes. Apart from having a high number of dry spells during the December-February (DJF) core rainy season, the southern part of Zambia is frequently subjected to late onset dates and short durations of the rainy season whereas the northern and northwestern region frequently have high numbers of wet spells which in most cases lead to flooding. Using CMAP and station data, rainfall variability in terms of dry spells, wet spells, onset and cessation dates of the Zambian rainy season and the associated circulation anomalies are investigated. The study also looks at relationship between these extreme events with Nin03.4 SST anomalies. Low level easterly anomalies over Zambia are enhanced during seasons with high dry spells. As a result, there is reduced moisture penetration into Zambia from the Angola low and reduced low level moisture convergence over the country and hence increased dry spells. During the seasons with high numbers of wet spells, there are low level westerly anomalies over southern Angola and western Zambia implying a strong moisture influx from the tropical South East Atlantic and increased convergence over Zambia. It was shown that El Nino (La Nina) events typically result in above (below) average DJF dry spell frequency in Zambia. Early onset dates over the northern parts of Zambia were observed to occur during strong El Nino seasons and the strength of the observed relationship with Nin03.4 SSTs seems to be stronger for years during the 1980s than those during the 1990s. Early onset dates tend to occur for years with higher than average dry spell frequency in the following DJF season. This suggests that early onset may be a disadvantage in that it often leads to more dry spells during the subsequent peak growing period of the season.
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    An investigation into source and distribution of bromoform in the Southern African and Southern Ocean Marine boundry layer
    (2014) Kuyper, Brett; Reason, Chris; Waldron, Waldron, Howard; Palmer, Carl J; Labuschagne, Casper
    Bromoform is a climatically important atmospheric trace gas. It is released by macro- and microalgae into the ocean, and rapidly transferred to the atmosphere, where bromoform undergoes rapid photolysis yielding bromine radicals. These bromine radicals are known to participate in catalytic destruction of ozone at all levels throughout the atmosphere. This is especially important in the lower stratosphere. This destruction of ozone results in changes in the oxidative potential and decreases the greenhouse effect of the troposphere. A task-specific gas chromatograph with electron capture detector system, developed in-house, was used for the separation and quantitative detection of bromoform mixing ratios from environmental air samples. A custom thermal desorption unit was designed and built for use in this system along with a graphical user interface for the real-time collection and display of data. A limit of detection of 0.79 ± 0.09 ppt, with an overall precision of 12.7 % was achieved with this GC system and method. The quantitative detection of bromoform mixing ratios was made at the Cape Point, Global Atmospheric Watch station over a one month period in early spring 2011. Bromoform mixing ratios detected ranged between 2.29 and 84.7 ppt with a mean of 24.7 ppt. These mixing ratios appear to be generally elevated compared to previous studies, however, were still within the maximum values published. Local kelp beds around Cape Point and possibly anthropogenic inputs from Cape Town are likely to have been the dominant source of bromoform measured there, caused by changes in wind speed. A series of experiments were performed to explore the role of the bromoperoxidase enzyme in providing antioxidant protection in two diatom species (Phaeodactylum tricornutum and Chaetoceros neogracile_cf ) under different oxidative stresses. Carbon dioxide and nitrate limitation were induced as oxidative stresses. The mean per cell bromoform concentrations during the growth phase of the carbon limitation were 2.73 x 10¯¹⁷ and 8.68 x 10¯¹⁸ mol cell¯¹ for C. neogracile_cf and P. tricornutum, respectively. This decreased to 2.94 x 10¯¹⁸ and 3.87 x 10¯¹⁸ mol cell¯¹ during the limited phase. Bromoform production decreased to zero for P. tricornutum during the nitrate limitation. These experiments suggest that in these species bromoperoxidase is not utilised as an antioxidant pathway when under these nutrient limiting conditions.
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    An investigation into the impacts of the Benguela Niño on rainfall over southern Africa
    (2008) Hansingo, Kabumbwe; Reason, Chris
    The impacts of the Benguela Niño on southern African rainfall and circulation are investigated using an atmospheric general circulation model. The model used is the United Kingdom Met Office Hadley Centre Atmospheric General Circulation Model version 3 and experiments using idealizations of observed regional and remote SST anomalies during various Benguela Niño events were performed. It is found that SST forcing in tropical South East Atlantic induces a regional baroclinic response and that a Benguela Niño is capable of forcing anomalous wet conditions over western Angola on its own, via changes to uplift and evaporation over the SST forcing. It is also capable of forcing anomalous rainfall much further inland when the intensity is increased. An experiment with the tropical South East Atlantic SST anomaly shifted slightly further north produced a larger circulation and rainfall response in the model. Additional experiments with various SST anomalies in the South West Indian Ocean/central equatorial Pacific combined with those in the South East Atlantic were performed. These experiments are motivated by the fact that equatorial Pacific/South Indian Ocean SST anomalies of varying signs often occur at the same time as the Benguela Niño Events. The results suggest that depending on its sign, magnitude and location, SST forcing from the South West Indian Ocean may augment or oppose the southern African rainfall anomalies occurring during a Benguela Niño event to varying degree.
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    An investigation into the relative contributions of ENSO, Benguela Niño and the sub-tropical Indian Ocean dipole on summer rainfall over southern Africa
    (2014) Mpheshea, Lerato Elizabeth; Reason, Chris
    The region below the 5ᵒS latitude is defined here as southern Africa. This region is subject to high inter-annual rainfall variability attributed to the local and remote SST fluctuations. The main purpose of this study is to investigate the unique impact that SST conditions related to ENSO, Benguela Niño and subtropical Indian Ocean Dipole (SIOD) has on summer rainfall over southern Africa. To achieve this, a partial correlation method was employed to analyse the possible relationship between rainfall variability and interannual SST variability in the Niño3.4 index, Angola-Benguela front and SIOD index for the climatological period between 1950 and 2010. The results revealed ENSO as a prime mode of rainfall variability in southern Africa. The ENSO-rainfall relationship in the central region of southern Africa tends to be modulated by the Benguela Niño and SIOD signal. In the west coast region of Namibia and Angola, the Benguela Niño was found to be strongly linked to rainfall variability. However, this Benguela Niño-rainfall relationship tends to weaken when the effect of ENSO and SIOD is removed. The SIOD impact on the rainfall was found to be dependent on ENSO.
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    Links between the Seychelles-Chagos thermocline ridge and large scale climate modes and primary productivity; and the annual cycle of chlorophyll-a
    (2014) Dilmahamod, Ahmad Fehmi; Reason, Chris; Hermes, Juliet C
    The Seychelles-Chagos Thermocline Ridge (SCTR) is a region of upwelling present at 55°E- 90°E and 5°S-12°S in the southwest tropical Indian Ocean. It is a region of strong ocean-atmosphere interactions due to the high variability of the thermocline depth caused by the local Ekman pumping. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has shown high variability of surface chlorophyll-a (SChl-a) in the SCTR region. The Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) have also driven significant interannual variation of the depth of 20°C isotherm (D20) and SChl-a in the southern tropical Indian Ocean. A 50-years hindcast (RUN58-07) from a coupled bio-physical model was used to study the SChl-a concentration on an annual time scale and the interannual variability of D20 and SChla in the SCTR in response to IOD and ENSO events. Initial analysis revealed a high overestimation of SChl-a in the 50-year run. Therefore, a 44-years hindcast (RUN58-01) of the same coupled model was taken into consideration. Comparisons with observations show that the RUN58-07 reproduces the D20 and SSH better than the RUN58-01 but the RUN58-01 shows better agreement with SeaWiFS. Results reveal that the SCTR exhibits an annual cycle of SChl-a concentration, with a peak in austral winter (June-August) due to the strong southeasterlies, increasing wind stirring and induced upwelling. Vertical sections of the SCTR also indicate that an increase in surface concentration in austral winter is compensated by a decrease in subsurface phytoplankton blooms. Composite figures show that IOD events exhibit a greater influence on the subsurface and surface variability in the SCTR region. The IOD deepens and shoals the D20 in the SCTR and eastern Indian Ocean respectively whereas ENSO displays a weaker and less-extensive influence on the D20. The spatial distribution of SChl-a in the Indian Ocean is completely disrupted by IOD during which the SCTR becomes oligotrophic whereas the eastern Indian Ocean becomes highly productive. ENSO, however, does not display any significant biogeochemical signature in the SCTR. This study should improve our understanding of the interannual variability of the thermocline depth and chlorophyll-a in the SCTR region; and for the optimization of the management of fishery resources and marine ecosystems.
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    Mesoscale convective complexes over southern Africa
    (2012) Blamey, Ross; Reason, Chris
    A combination of numerous factors, including geographic position, regional orography and local sea surface temperatures, ensures that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events.