Browsing by Department "Department of Oceanography"

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    A 1000-year carbon isotope rainfall proxy record from South African baobab trees (Adansonia digitata L.)
    (Public Library of Science, 2015) Woodborne, Stephan; Hall, Grant; Robertson, Iain; Patrut, Adrian; Rouault, Mathieu; Loader, Neil J; Hofmeyr, Michele
    A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab ( Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.
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    A Comparison of Gas Exchange Models in the Estimation of CO2 Fluxes in the South Atlantic South of Africa for the Summer Season of 2008/2009
    (2014) Rainier, stephanie Megan; Monteiro, Pedro; Waldron, Howard
    There is a problem in the determination of air-sea CO2 fluxes because of the number of different relationships used in calculating gas transfer velocities. There is also a problem with the CO2 sink in the Southern Ocean being greatly underestimated. Data were collected underway using an autonomous pCO2 system during three separate relief cruises over the course of austral spring 2008 to austral autumn 2009 onboard the RV SA Agulhas in the South Atlantic Ocean. The wind speed product was extracted from QuikSCAT. Using the data we investigated the sensitivity of the five gas transfer velocity parameterisations to the uncertainty in the wind speed product of 2m.s-1. We found that the Stagnant Film Model was unresponsive. Liss and Merlivat's (1986) linear model for three wind regimes showed a gradual increase in sensitivity with wind speed. The quadratic relationship developed by Nightingale et al., (2000) also showed a steady increase in sensitivity with an increase in wind speed. Wanninkhof's (1992) quadratic relationship showed the greatest response at low wind speeds and then a continuing increase in response through medium to high wind regimes. The cubic relationship from Wanninkhof and McGillis (1999) showed the smallest response at low wind speeds but had the greatest response to the uncertainty in the wind speed product in medium and high wind regimes. We also calculated regional and seasonal averages of the CO2 flux with the five gas transfer velocities based on the different relationships between gas transfer velocity and wind speed. We found that there was a CO2 flux into the ocean ranging from 4mmol.m-2.day-1 to 12mmol.m-2.day-1 between 33.5 and 68°S, except during autumn between 45-50°S where there is a flux out of the ocean of 2mmol.m2 .day-1. Between 68-70°S the flux into the ocean strengthens to between 28mmol.m2 .day-1 and 52mmol.m-2.day-1. Gas transfer velocity is not dependant on wind speed alone, but currently it is the only variable that it measureable on a global scale. Further investigations are in place to measure gas transfer velocity in situ. The Southern Ocean is greatly undersampled spatially and temporally which leads to a lack of understanding 13 about possibly one of the most important CO2 sinks. New methods are being taken on to increase the sampling resolution.
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    A geographical and seasonal comparison of nitrogen uptake by phytoplankton in the Southern Ocean
    (2015) Philibert, R; Waldron, H; Clark, D
    The impact of light and nutrients (such as silicate and iron) availability on nitrogen uptake and primary production vary seasonally and regionally in the Southern Ocean. The seasonal cycle of nitrogen uptake by phytoplankton in the Southern Ocean is not fully resolved over an annual scale due to the lack of winter in situ measurements. In this study, nitrate and ammonium uptake rates were measured using 15N tracers during a winter cruise in July 2012 and a summer cruise in February–March 2013. The winter cruise consisted of two legs: leg 1 extended from Cape Town to the ice margin along the GoodHope line and leg 2 stretched from the ice margin to Marion Island. The summer cruise was mostly focused on the subantarctic zone of the Atlantic sector. In winter, nitrogen uptake rates were measured at 55 and 1% of the surface photosynthetically active radiation (sPAR). The summer uptake rates were measured at four light depths corresponding to 55, 30, 10 and 3% sPAR. The integrated nitrate uptake rates during the winter cruise ranged from 0.17 to 5.20 mmol N m−2 d−1 (average 1.14 mmol N m−2 d−1) while the ammonium uptake rates ranged from 0.60 to 32.86 mmol N m−2 d−1 (average 6.73 mmol N m−2 d−1). During the summer cruise, the mean-integrated nitrate uptake rate was 0.20 mmol N m−2 d−1 with a range between 0.10 and 0.38 mmol N m−2 d−1. The integrated ammonium uptake rate averaged 4.39 mmol N m−2 d−1 and ranged from 1.12 to 9.05 mmol N m−2 d−1.

    The factors controlling nitrogen uptake in winter and summer were investigated. During the winter cruise, it was found that the different nitrogen uptake regimes were not separated by the fronts of the Antarctic Circumpolar Current (ACC). Light (in terms of day length) and ammonium concentration had the most influence on the nitrogen uptake. In the summer, increases in the mixed layer depth (MLD) resulted in increased nitrogen uptake rates. This suggests that the increases in the MLD could be alleviating nutrient limitations experienced by the phytoplankton at the end of summer.
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    A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean
    (2012) Tagliabue, A; Mtshali, T; Aumont, O; Bowie, A R; Klunder, M B; Roychoudhury, A N; Swart, S
    Due to its importance as a limiting nutrient for phytoplankton growth in large regions of the world's oceans, ocean water column observations of concentration of the trace-metal iron (Fe) have increased markedly over recent decades. Here we compile >13 000 global measurements of dissolved Fe (dFe) and make this available to the community. We then conduct a synthesis study focussed on the Southern Ocean, where dFe plays a fundamental role in governing the carbon cycle, using four regions, six basins and five depth intervals as a framework. Our analysis highlights depth-dependent trends in the properties of dFe between different regions and basins. In general, surface dFe is highest in the Atlantic basin and the Antarctic region. While attributing drivers to these patterns is uncertain, inter-basin patterns in surface dFe might be linked to differing degrees of dFe inputs, while variability in biological consumption between regions covaries with the associated surface dFe differences. Opposite to the surface, dFe concentrations at depth are typically higher in the Indian basin and the Subantarctic region. The inter-region trends can be reconciled with similar ligand variability (although only from one cruise), and the inter-basin difference might be explained by differences in hydrothermal inputs suggested by modelling studies (Tagliabue et al., 2010) that await observational confirmation. We find that even in regions where many dFe measurements exist, the processes governing the seasonal evolution of dFe remain enigmatic, suggesting that, aside from broad Subantarctic - Antarctic trends, biological consumption might not be the major driver of dFe variability. This highlights the apparent importance of other processes such as exogenous inputs, physical transport/mixing or dFe recycling processes. Nevertheless, missing measurements during key seasonal transitions make it difficult to better quantify and understand surface water replenishment processes and the seasonal Fe cycle. Finally, we detail the degree of seasonal coverage by region, basin and depth. By synthesising prior measurements, we suggest a role for different processes and highlight key gaps in understanding, which we hope can help structure future research efforts in the Southern Ocean.
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    A novel approach to investigating chlorophyll-a fluorescence quantum yield variability in the Southern Ocean
    (2019) Bone, Emma Lewis; Vichi, Marcello; Thomalla, Sandy J; Bernard, Stewart; Smith, Marié E; Ryan-Keogh, Thomas. J
    The apparent fluorescence quantum yield of chlorophyll-a (ΦF ), i.e. the ratio of photons emitted as chlorophyll-a fluorescence to those absorbed by phytoplankton, serves as a first order measure of photosynthetic efficiency and a photophysiological indicator of the resident phytoplankton community. Drivers of ΦF variability, including taxonomy, nutrient availability, and light history, differ in magnitude of influence across various biogeographic provinces and seasons. A Multi-Exciter Fluorometer (MFL, JFE Advantech Co., Ltd.) was selected for use in in situ ΦF derivation and underwent an extensive radiometric calibration for this purpose. Wavelength-specific ΦF was determined for 66 in situ field stations, sampled in the Atlantic Southern Ocean during the austral winter of 2012 and summer of 2013/ 2014. Phytoplankton pigments, macronutrient concentrations, and light levels were simultaneously measured to investigate their influence on ΦF . While no relationship was observed between macronutrient levels and ΦF , an inverse relationship between light and ΦF was apparent. This was likely due to the influence of speciesspecific fluorescence quenching mechanisms employed by local populations. ΦF derived from ocean colour products (Φsat) from the Moderate Resolution Imaging Spectroradiometer (MODIS) were compared to in situ ΦF to assess the performance of three existing Φsat algorithms. Results indicate that accounting for chlorophyll-a fluorescence reabsorption, the inherent optical properties of the surrounding water column, and the sensor angle of observation, is crucial to reducing Φsat uncertainty. A hybrid combination of two of the algorithms performed best, and was used to derive Φsat for stations co-located to in situ iron measurements in the Atlantic Southern Ocean. A significant negative relationship was observed, indicative of the effects of iron availability on quantum yield and its potential as a proxy for iron limitation. However, separating the individual contributions of light, taxonomy, and iron limitation to Φsat variability remains a challenge. A time series analysis of Φsat was also undertaken, which revealed a prominent Φsat seasonal cycle. Ultimately, increased in situ sampling would expedite the development of improved Φsat algorithms; the routine retrieval of Φsat would offer insight into phytoplankton dynamics in undersampled regions such as the climate relevant Southern Ocean.
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    A preliminary investigation on the relationships between upwelling and commercial hake fishery in the Southern Benguela
    (2021) Memela, Nkuleleko; Vichi, Marcello; Hall, Russel
    The hake bottom trawl division is the largest component of the fishing industry in South Africa and it is one of the strong pillars of the food industry and the national economy. It is the main source of livelihood for many people in the West Coast and as such, finding ways of advancing it towards the direction of fourth industrial revolution is at the top of societal interests and a top priority for the major companies that are key players in the industry. Sea Harvest Group Limited is one of these key players and as such, it has undertaken to be a part of the study to improve predictability of fishing by collecting data which will contribute towards the scientific study of the patterns which determine the viability of some fishing locations over the others at different times under various conditions. The studied region is the West Coast grounds located in the southern Benguela at grid (32°S: 34°S, 16°E :19°E). The study is based on the hypothesis that the main driver of the availability of hake is the upwelling, separated into its coastal Ekman transport and curl-driven components. These two components of upwelling are driven by winds and they are known to stimulate primary production and support a larger marine food web. The correlations between these upwelling types and the mean monthly catch per unit effort (CPUE) of this region is assessed. The relationship between chlorophyll abundance and hake CPUE hypothesises lagged association of hake abundance to the underlying biological food chain driven by the upwelling events. A multiple regression model is then produced as a basic step towards quantification. The results suggest that Hake CPUE is lag correlated with upwelling and that some degree of predictability can be derived from the observation of combined upwelling patterns.
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    A reduction in marine primary productivity driven by rapid warming over the tropical Indian Ocean
    (2016) Roxy, Mathew Koll; Modi, Aditi; Murtugudde, Raghu; Valsala, Vinu; Panickal, Swapna; Prasanna Kumar, S; Ravichandran, M; Vichi, Marcello; Lévy, Marina
    Among the tropical oceans, the western Indian Ocean hosts one of the largest concentrations of marine phytoplankton blooms in summer. Interestingly, this is also the region with the largest warming trend in sea surface temperatures in the tropics during the past century—although the contribution of such a large warming to productivity changes has remained ambiguous. Earlier studies had described the western Indian Ocean as a region with the largest increase in phytoplankton during the recent decades. On the contrary, the current study points out an alarming decrease of up to 20% in phytoplankton in this region over the past six decades. We find that these trends in chlorophyll are driven by enhanced ocean stratification due to rapid warming in the Indian Ocean, which suppresses nutrient mixing from subsurface layers. Future climate projections suggest that the Indian Ocean will continue to warm, driving this productive region into an ecological desert.
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    A review of migratory behaviour of sea turtles off southerneastern Africa
    (2006) Luschi, P; Lutjeharms, J R E; Lambardi, P; Mencacci, R; Hughes, G R; Hays, G C
    The survival of sea turtles is threatened by modern fishing methods, exploitation of eggs and habitat destruction. Forming keystone species in the ocean, their extinction would disrupt the marine food chain in ways as yet unknown. The Indian Ocean has many breeding areas for sea turtles, the southernmost ones being on the Maputaland coast of KwaZulu-Natal, where loggerhead and leatherback turtles nest in large numbers thanks to long-lasting protection programmes. For the leatherback this is the only known nesting site in the entire western Indian Ocean. At the end of the reproductive season, both loggerheads and leatherbacks undertake migrations towards disparate feeding areas. To contribute to their conservation, the migratory behaviour of these animals needs to be understood. Here we review 10 years studying this behaviour using transmitters that telemeter data via satellite. It emerges that these species frequent widely dispersed areas ranging from the Atlantic Ocean to the Mozambique Channel. The migratory behaviour of leatherback and loggerhead turtles is, however, very different, probably due to their differing food requirements. While loggerhead postnesting movements have a truly migratory nature, the large-scale wanderings of leatherbacks are better described as prolonged sojourns in extended feeding areas.
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    A review of South African research in atmospheric science and physical oceanography during 2000-2005
    (2006) Reason, C J C; Engelbrecht, F; Landman, W; Lutjeharms, J R E; Piketh, S; Rautenbach, C J W; Hewitson, B C
    The purpose of this article is to review progress in the fields of atmospheric science and physical oceanography made by workers based at South African institutions over approximately the last 5 years. Research published by South African scientists working abroad is not included. Most published research in these fields falls within the broad areas of climate variability, climate change, aerosols and atmospheric pollution, seasonal forecasting, numerical modelling (both atmospheric and oceanic), and the physical oceanography of the Agulhas and Benguela current systems. Most but not all of the atmospheric science papers relate to South Africa or southern Africa; however, some work pertaining to the southern hemisphere as a whole or to other regions has been done. We note that funding and institutional support for atmospheric science and physical oceanography research in South Africa remains poor and this situation significantly hampers local efforts.
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    A study of Benguela Niños and Niñas from 1958 to 2015
    (2018) Imbol, Koungue Rodrigue Anicet; Rouault, Mathieu; Illig, Serena
    Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) records in combination with outputs from an Ocean Linear Model (OLM) and altimetric data are used to investigate the link between the equatorial Atlantic Ocean dynamics and the variability in the coastal region of Angola-Namibia at interannual timescales over 1998 to 2012. The PIRATA records help to define an index of equatorial Kelvin wave activities in the Equatorial Atlantic. There is a good agreement between PIRATA monthly dynamic height anomalies, altimetric monthly sea surface height anomalies (SSHA), and sea level anomalies calculated with an OLM at interannual time scales. This allows the interpretation of PIRATA records in terms of equatorial Kelvin wave propagations. Extreme warm or cold events in the Angola – Namibia area lag strong anomalous eastward equatorial propagations by 1–2 months. Remote equatorial forcing via equatorial Kelvin waves which propagate poleward along the west African coast as coastal trapped waves is at the origin of their developments. Results show a seasonal phasing, with significantly higher correlations between the equatorial index and coastal sea surface temperature anomalies (SSTA) off Angola-Namibia in October - April season. Then, a systematic study of all the Benguela Niño and Benguela Niña events before 1982 is done using an Ocean general circulation model in combination with the OLM outputs from 1958 to 2015. 26 anomalous strong coastal events (16 warm and 10 cold) are identified. The analysis of their evolution confirms the remote equatorial origin of most of these coastal anomalous strong events. Modelled meridional transport anomalies across the Angola Benguela Front (ABF) contribute to the development of these anomalous coastal warm events. Across the ABF, the results obtain with the net temperature transport are similar to the ones with net mass transport. Most anomalous events peak in October - April season. Lagged composites of surface temperature and wind stress anomalies in the equatorial and southeastern Atlantic reveal that both local and remote forcings develop simultaneously 1-2 months before the peak of Benguela Niño or Niña. At the monthly scale, local atmospheric forcing is more correlated with anomalous coastal events occurring in Southern Angola which is a non-wind-upwelling driven region. The results from this thesis open the possibility to predict Benguela Niño and Benguela Niña events using an index depicting the equatorial interannual variability associated with Interannual Equatorial Kelvin Wave propagation, especially from October to April when the coastal stratification is favourable to the imprint of coastal trapped waves in the surface layer.
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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 Björn; 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 Current’s velocity by 0.16 ± 0.17 m.s -1 . In contrast, entrainment of cyclonic eddies results in a decrease in velocity by 0.13 ± 0.16 m.s-1 and shifting the current up to 144 ± 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 (30.22° E, 31.05° S) to the region of the ACT array (27.48° E, 33.35° 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 ADCP study of subtidal scale density-driven exchange in Saldanha Bay, South Africa
    (2003) Stewart, Helen Frances; Monteiro, Pedro M S; Waldron, Howard; Brundrit, Geoff
    An ADCP and water-column temperature study was conducted to determine the circulation aspect of subtidal-scale, density-driven exchange in Saldanha Bay, South Africa. Density-driven exchange conditions develop in response to synoptic-scale wind events in the southern Benguela region, even under light (<5m s-') wind conditions. During a density-driven exchange event, directionally opposing bi-level flow, similar to an estuarine system, develops in response to remote upwelling-favourable winds. The bi-level flow component occurs in two distinct bands, bayward at 0-9m height from bottom and seaward 15-20m height off bottom, and is very sensitive to changes in wind forcing. Observations of current behaviour are added to the four-phase conceptual model of density-driven exchange developed by Monteiro and Largier (1999). In addition, estimates of bay flushing based on ADCP current velocities and the four-phase conceptual model are calculated and implications of shelf water influx into Saldanha Bay are discussed.
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    The Agulhas current system above the intermediate level
    (1977) Van Foreest, Dirk; Harris, T F W
    A first detailed picture of the Agulhas current is obtained from data of the Quasi synoptic Agulhas current cruise of March 1969. Reinterpretation of other hydrographic data was possible in the light of the 1969 cruise and the information obtained from satellite tracked buoys. For the 1969 cruise acceleration potential diagrams on various sigma-t levels are discussed and indicate that certain features in the current are of such a scale (length and time) that only closely spaced synoptic stations will show them. Of special interest are the following features: two main supplies to the current; a region of dynamical upwelling between East London and Port Elizabeth; the region where the current turns eastward (retroflection); the interaction with the South East Atlantic ocean; the planetary waves in the Agulhas return current over and east of the Agulhas plateau.
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    Agulhas current variability determined from space : a multi-sensor approach
    (2011) Rouault, Marjolaine; Shillington, Frank
    Satellite remote sensing datasets including more than 6 years of high frequency Sea Surface Temperature (SST) imagery as well as surface current observations derived from 18 years of merged-altimetry and over 2 years of Advanced Synthetic Aperture Radar (ASAR) observations are combined to study the variability of the Agulhas Current. The newly available rangedirected surface currents velocities from ASAR, which rely on the careful analysis of the measured Doppler shift, show strong promise for monitoring the meso to sub-mesoscale features of the surface circulation. While the accuracy of ASAR surface current velocities suffers from occasional bias due to our current inability to systematically account for the wind-induced contribution to the Doppler shift signal, the ASAR surface current velocities are able to consistently highlight regions of strong current and shear. The synaptic nature and relatively high resolution of ASAR acquisitions make the ASAR derived current velocities a good complement to altimetry for the study of sub-mesoscale processes and western boundary current dynamics. Time-averaged range-directed surface currents derived from ASAR provide an improved map of the mean Agulhas Current flow, clearly showing the location of the Agulhas Current core over the 1000 m isobath and identifying the region at the shelf edge of the north-eastern Agulhas Bank as one of the most variable within the Agulhas Current. To determine the variability of the Agulhas Current, an algorithm to track the position of the current is developed and applied to the longer merged-altimetry and SST records. Limitations associated with altimetry near the coast favour the use of the SST dataset to track the position of the Agulhas Current in its northern region. In the southern Agulhas, where the current lies further from the coast, altimetry is suited to monitoring the position of the Agulhas Current. The front detection analysis conducted on the SST dataset in the northern Agulhas reveals the complex nature of Natal Pulses. The downstream passage of the Natal Pulses is associated with the generation of secondary offshore meanders at the inshore edge of the current. Perturbations formed during the passage of Natal Pulses evolve rapidly to either dissipate, re-merge with the initial Natal Pulse or in some rare occasion, detach from the Agulhas Current.
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    Agulhas retroflection rings in the South Atlantic Ocean
    (1994) Duncombe Rae, Christopher Michael; Shillington, Frank A
    The western boundary current rings shed from the Agulhas retroflection may be responsible for a considerable transfer of heat, salt and energy from the South Indian into the South Atlantic Ocean. Few hydrographic measurements have been collected from Agulhas rings in the South Atlantic Ocean and their characteristics and influence on the waters of the Cape Basin through which they pass are thus little known. The temperature, salinity, and nutrient data presented in the thesis were collected from three Agulhas rings on a number of recent hydrographic cruises in the South Atlantic Ocean. Temperature profiles, conductivity-temperature-depth measurements, nutrient data, GEOSAT altimeter data, and NOAA-11 satellite imagery were used to investigate one of the rings in May 1989. It had previously been postulated that the rings could have an important effect on the Benguela upwelling system and this thesis demonstrates the interaction of the ring with a filament from the upwelling system. An adverse influence of this interaction on the anchovy larval population is postulated, and cited as a possible cause of the very poor anchovy yearclass of 1989. The other two rings were encountered during winter (August 1990 and June 1992), closer to the retroflection, and only hydrographic observations were possible. One of the rings showed a very deep isothermal surface layer and evidence of a deep pycnostad at its centre. The deep stad is shown to be likely due to vortex stretching and possible sources for the water in the stad are suggested. Comparative hydrographic characteristics, water mass structure, velocity fields, and the potential for contribution to interbasin transfer of the three rings are presented and discussed in the thesis.
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    An altimetry based examination of the path and variability of the Agulhas Return Current
    (2018) Fadida, Yotam; Hermes, Juliet; Malan, Neil
    As a result of climate change, partially driven by anthropogenic activity, strengthening wind stress over the worlds oceans is yielding a western boundary intensification and pole-ward shift of oceanic currents (Yang, 2016). Shifting wind regimes and perturbations in neighboring currents have been associated with variations in the flow path of western boundary currents (WBC) and their extensions (Talley, 2011; Combes and Matano, 2014; Nakamura and Kazmin 2003). WBCs, typically characterised by fast flowing and warm currents, are key regions of heat and salt transport as well as for oceanic carbon uptake, air-sea heat flux and nutrient transport, thus positional shifts may have far-reaching implications (Gray and Palter, 2017). While meridional trends (latitudinal migrations) have been observed in analogous WBCs and their extensions, no long-term investigation (decadal) into the flow path and position of the Agulhas Return Current (ARC) has been carried out. Now, with over 25 years of altimetric, satellite remote observation data available, a better understanding of the flow path, seasonal cycle, meridional trends and eddy kinetic energy (EKE) of the ARC is possible. Here we provide a detailed description of the flow path of the ARC together with several mechanisms that may be contributing to the current’s apparent stability. In addition we demonstrate that the documented western boundary intensification and pole-ward shift may be manifesting itself in two ways in the ARC, an increase in regional EKE and a southward trend found mainly in the flow-path of the eastern section of the ARC. Our results show a strong link between the EKE of the region and the Subtropical Indian Ocean Dipole, both of which are associated with the Southern Annular Mode (SAM), illustrating the effects that southward shifts in the subtropical high pressure system have on regional and mesoscale climate modes. The combination of a stable, topographically forced flow path with observed EKE increases, demonstrate the effects of wind stress intensification on a region highly influenced by its bathymetry. Further research into the effects of enhanced eddy activity is necessary, as it has been shown to affect regional primary production (Falkowski, 1991; Oschlies, 1998) and air-sea interactions, thus having potential ramifications for regional aquaculture, weather and fisheries, as well as calculations/models concerned with heat-flux and carbon exchange.
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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 ‘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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    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 assessment of four decades of wave power variability - a critical requirement for coastal resilience
    (2022) Hall, Candice; Ansorge, Isabel; Jensen, Robert E; Wang, David W
    Wave power estimates and trend assessments are crucial for coastal management and resilience, as increases in wave power introduces significant risks of flooding and shoreline erosion. This study evaluates wave power trends at 29 National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) moored buoy sites with associated U.S. Army Corps of Engineers (USACE) Wave Information Study model estimates within the North Pacific Ocean, Hawaiian Islands, Gulf of Mexico and North Atlantic Ocean. This work is the first conclusive study to show spatially and temporally comparative observational and model wave power results, providing new information on the accuracy of model estimates using wave power as a proxy. Wave power data were interpolated to augment missing values and detrended for seasonality to facilitate testing of interannual and interdecadal trends in wave power. Results show that the majority of the eastern Pacific Ocean and Hawaii wave power trends are downward, with mixed slope wave power trends apparent within the Atlantic Ocean and Gulf of Mexico. Observational and model results show that wave power peaks in long term interannual trends are similar with respect to timing, but not magnitude. Variability in the wave power trend direction within each region suggests that site specific wave power trends should not be generalised to represent a large region, with regionally grouped annual maximum 90th percentiles obscuring the variability of individual site results. Prior to the calculation of these wave power estimates, a thorough interrogation of the quality of the observational wave data was conducted. Three tasks achieved confidence in these observational datasets: a) an evaluation of the effects of changing NDBC instrumentation technologies on data quality; b) the development of an independent, self describing, archive that mitigates for historical data storage issues; and c) the subsequent removal of identified discontinuities within the time series datasets. Instrumented buoy intercomparisons within the Pacific Ocean and U.S. Great Lakes prove that the recently deployed NDBC 2.1-m hulls show an increased wave data accuracy when compared to the legacy NDBC 3-m hulls for significant wave height, average wave period, and spectral signal-to-noise ratio, which allows for an increase in swell energy retention in the lower frequency spectral range. With confidence in the newly deployed NDBC platforms, this work then addressed NDBC data accessibility issues, as data are stored in multiple archives with unique storage, metadata, and quality control protocols. Known storage and quality control inconsistencies were removed and the validated data for all NDBC moored buoy stations are stored within a USACE Quality Controlled, Consistent (QCC) Measurement Archive, which is now a public database of best available historical NDBC data with verified metadata. Spectral wave data from this USACE QCC Archive were interpolated for frequency equivalency and used to recalculate the required wave power input parameters to ensure consistency through the historical datasets, successfully removing a number of previously identified time series discontinuations. With these data discontinuity corrections, uncertainties and inaccuracies are removed from the estimated wave power trends. Overall, this study highlights the undeniable need for accurate and consistent observational data that are essential for a realistic estimation of local wave climate studies, a vital requirement for all coastal risk management considerations. Although these observational and model wave power trends are U.S. specific, the methodologies developed within this work are applicable to datasets in any region.