Browsing by Author "Hermes, Juliet"

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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 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 techniques to successfully monitor the oceanography of the complex uThukela Marine Protected Area
    (2022) Ramsarup, Neha; Morris, Tamaryn; Hermes, Juliet
    Marine Protected Areas (MPAs) are increasingly being established to restore and protect coastal and marine environments. The newly established uThukela Marine Protected Area (MPA), located on the central KwaZulu-Natal Bight along the northeast coast of South Africa, has been identified as a key ecological region. The region provides recruitment and is a general nursery area for marine life on the Bight. Knowledge of the oceanographic dynamics in the region is essential for understanding the functioning of the ecosystem and the effectiveness of the MPA. This study analysed changes in ocean temperatures at 20 sites along the Bight over a 41-year period using in situ beach and UTR data, and satellite data. Significant warming of 0.03°C/year occurred at beach sites within and around the MPA. Beach temperatures increased at an average rate of 0.02°C/year across the Bight, during both summer and winter, whereas UTR temperatures warmed by 0.14°C/year during summer and 0.08°C/year during winter. At the event scale, a case study of a persistent decrease in in situ temperatures along the entire Bight during summer 2017/2018 was investigated. The decrease in temperatures was hypothesised to be as a result of the combined effects of a La Niña event and a Natal Pulse, which could have significant consequences for temperature-sensitive species. Temperature trends observed in beach temperatures over the 41 years were not reflected in beach temperatures over the past decade, highlighting the importance of large datasets when investigating climate change. Warm biases of up to 2°C observed in satellite-derived temperature measurements, and its failure to replicate trends seen in the beach data, suggests that in situ temperature measurements in MPAs are better suited for long-term monitoring efforts. These findings can help assess the success of MPAs and to guide monitoring and research activities within the region.
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    Climate and environmental change along the East Coast of South Africa: perspectives from a local marine resource- dependent community and scientific researchers
    (2019) Duba, Tania; Hermes, Juliet; Blamey, Ross; Raemaekers, Serge
    Coastal areas are very susceptible to environmental problems such as sea-level rise, coastal flooding, increased frequency and intensity of extreme events, and changes in marine ecosystems that are arising from global climate change and variability. In the South African context, the Agulhas Current is important for its crucial role in regional climate and weather as well as the fishing livelihood of the coastal communities along the east coast of South Africa. Despite the efforts made to understand the Agulhas Current and the impacts of climate and environmental change, the shelf region remains poorly understood mostly due to the difficulties associated with observing and modelling such strong currents. The marine resource users in the fishing communities along the east coast of South Africa show long term dependence on the neighbouring ocean going back at least three generations. These communities provide long term, rich, detailed, and contextualized environmental knowledge from their daily interactions with the sea. This study seeks to investigate the local climate and environmental change knowledge of the fishers based on their own observations, perceptions, and experiences. The convergence/divergence of the marine resource user’s knowledge with the traditional scientific findings is explored using a broad, participatory methodology including desktop literature analysis, interviews and an adopted version of the Rapid Vulnerability Assessment (RVA). Results show that fishers in Tshani-Mankosi have observed changes in the rainfall, sea surface temperature and wind patterns in their community. According to the fishers, sea surface temperature and annual rainfall seem to have decreased while winds and rainfall related extreme events have increased. Similar observations were noticed in the scientific research at a larger spatial and temporal scale. Key differences and similarities between the two types of knowledge come from factors such as knowledge construction processes, scales, type of data output and parameters of interest. Finally, the study reveals opportunities and challenges of research collaboration between the community and scientific researchers.
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    Deriving a policy document towards an early warning system for estuaries in South Africa: case study Great Brak estuary, Eden District, Southern Cape
    (2020) Stander, Johan; Ansorge, Isabel; Hermes, Juliet
    South Africa's estuaries and their surrounding communities are becoming increasingly vulnerable to storm surges and accompanied estuary flooding. These events are largely due to increasing severity of storm surges combined with growing housing and commercial developments. A particularly severe weather event in 2007/2008 highlighted the pressing need to understand the processes involved and the urgency to develop proactive response and management actions to mitigate the effects of future storm events on these coastal areas. Scientific research on estuarine flooding is limited not only for South Africa but within the international community as well and only recently has received committed attention from policy makers. It is clear that our current knowledge of South African estuary flooding events remains rudimentary; while necessary action to mitigate such events are poorly understood and planned. The aim of this PhD thesis is to devise and implement an Estuary Early Warning – Emergency Preparedness and Response Guide for stakeholders and government policymakers. This guide will target South Africa's coastal region by analysing past information on storm surges and estuary flooding, particularly in the low-lying southern coast region of the Western Cape, South Africa. The key objective of this thesis is to assess the best processesfor the issuing of estuary alerts and to better standardise them so that the response remains in line with multi-hazard early warning standard procedures and practices within South Africa. A further aim is to provide a comprehensive national guideline on how best to effectively disseminate and communicate such information and to establish an Estuary Early Warning (EEW) – Emergency Preparedness and Response Guide (EPRG), which forms part of the South African Multi-Hazard Early Warning System (MHEWS). It is critical that this EEW meets general principles accepted internationally for an effective Early Warning System. This thesis addresses the following key elements namely: (1) Risk identification, (2) Key drivers and contributions to estuary flooding, (3) Monitoring and alert early warning system, (4) Alert dissemination and (5) Response actions. Such pioneering work is an essential tool to translate science into policy, a crossover field, which remains poorly implemented.
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    Downstream evolution of ocean properties and associated fluxes in the Greater Agulhas Current System: Ad hoc Argo experiments and modeling
    (2020) Morris, Tamaryn; Ansorge, Isabel; Hermes, Juliet; González, Borja Aguiar; Lamont, Tarron
    The evolution of cyclonic eddies across the Southern Mozambique Chanel and the downstream evolution of the Agulhas Current was investigated using Argo floats, in combination with output from ocean general circulation reanalysis models. Two dedicated experiments were undertaken in April and July 2013, whereby eight floats were deployed within two separate cyclonic eddies. Floats were set to either daily and five-daily profiling from 1000 db to the surface, with park depths ranging from 300 db to 1000 db. The two cyclonic eddies propagated southwestward across the Mozambique Channel from southwest Madagascar to the KwaZulu-Natal Bight, a distance of approximately 1300 km, in approximately 130 days at a mean speed of 0.13 m s−1 . Estimates indicate the April (July) eddy showed mean trapped depths of 595 ± 294 m (914 ± 107 m), volume transport of 13.4 ± 5.2 Sv (21.2 ± 9.1 Sv), heat flux of -0.07 ± 0.06 PW (-0.2 ± 0.09 PW) and freshwater flux of 0.04 ± 0.04 Sv (0.09 ± 0.05 Sv). These results highlight the role of Madagascar cyclonic eddies as transporters of cooled and freshened source waters into the Agulhas Current. During a third experiment, six floats were deployed in the Agulhas Current, and exited the current within 9 - 12 days at mean speeds of 0.51 – 0.76 m s−1 . An evolution of properties was shown from north to south for both Argo data and model output; for volume transport (16.76 – 38.18 Sv; 17.70 – 32.51 Sv), heat fluxes (0.85 – 1.79 PW; 0.99 – 1.91 PW) and salt fluxes (0.60 – 1.37 x 1012 kg s−1 ; 0.63 – 1.17 x 1012 kg s−1 ). This study illustrates the first near-real time survey of the Agulhas Current, and a potential method of quasi-synoptic surveys using Argo float technology. These experiments highlight alternative methods of studying regions of turbulence by altering the mission parameters of Argo floats. Increased observations of eddies and Western Boundary Currents are critical to our understanding of the global oceans and impacts on the earths climate. Even more so for the understudied Indian Ocean.
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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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    SEAmester – South Africa’s first class afloat
    (2016) Dorrington, Rosemary A; Fawcett, Sarah; Gammon, David W; Henry, Tahlia; Hermes, Juliet; Hölscher, Beate; d’Hotman, Jethan; Meiklejohn, Ian; Morris, Tammy; Pinto, Izidine; du Plessis, Marcel; Roman, Raymond; Saunders, Clinton; Shabangu, Fannie W; de Vos, Marc; Walker, David R; Louw, Gavin
    The International Society for Burns Injuries (ISBI) has published guidelines for the management of multiple or mass burns casualties, and recommends that 'each country has or should have a disaster planning system that addresses its own particular needs.' The need for a national burns disaster plan integrated with national and provincial disaster planning was discussed at the South African Burns Society Congress in 2009, but there was no real involvement in the disaster planning prior to the 2010 World Cup; the country would have been poorly prepared had there been a burns disaster during the event. This article identifies some of the lessons learnt and strategies derived from major burns disasters and burns disaster planning from other regions. Members of the South African Burns Society are undertaking an audit of burns care in South Africa to investigate the feasibility of a national burns disaster plan. This audit (which is still under way) also aims to identify weaknesses of burns care in South Africa and implement improvements where necessary.
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    Seasonality of the Agulhas Current with respect to near- and far-field winds
    (2018) Hutchinson, Katherine; Penven, Pierrick; Beal, Lisa; Ansorge, Isabel; Hermes, Juliet
    The Agulhas Current plays a critical role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain poorly understood. Model studies predict an austral winter-spring maximum in poleward volume transport, whilst observations reveal an austral summertime (February-March) maximum. Here, the role of winds on Agulhas Current seasonality is investigated using shallow water models, satellite measurements, and a 23-year transport proxy based on observations. A one-and-a-half layer reduced gravity model is shown to successfully reproduce the seasonal phasing of the current. This seasonality is found to be highly sensitive to the propagation speed of Rossby waves, which determines the arrival time of the wind stress signal at the western boundary. By matching Rossby wave speeds to those observed using altimetry, an Agulhas Current with a maximum flow in February and a minimum flow in July is simulated, agreeing well with observations. Near-field winds, to the west of 35◦E, dominate this seasonality, as signals from more remote wind forcing dissipate due to destructive interference while crossing the basin. Local winds driving coastal upwelling/downwelling directly over the Agulhas cannot, alone, account for the observed seasonal phasing, as they force a NovemberDecember maximum and June minimum in flow. The seasonal response to Indian Ocean winds is also investigated using a barotropic (single layer) model with realistic topography. A barotropic adjustment cannot explain the observed Agulhas Current seasonality, predicting a wintertime maximum in transport. The results from the barotropic simulation are similar to previous model studies, where seasonality is dominated by a southward propagation of signals via the Mozambique Channel, suggesting that these models are too barotopic in their response to the winds. Findings from this study elucidate the role of near-field winds and baroclinic processes in determining the seasonality of the Agulhas Current.
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    The South African tide gauge network and its contribution to maritime safety
    (2021) Farre, Ruth Elizabeth; Hermes, Juliet; Brundrit, Geoff; Ansorge Isabel
    The use of tidal information and its application to, and value within, the study of climate change, sea-level rise and the tides effects on marine life is well known. However, little work has been done on the more technical aspects of tidal measurements and the use within maritime safety in accordance with international standards. This thesis will address the key technical elements in the design and realisation of the national tide gauge network around the South African coastline and how the data collected can be used to increase maritime safety around the South African coast, as well as the additional benefits the data may have both nationally and internationally. The South African Navy's national tide database, both actual records and predictions, as well as current and historical records, was used in order to establish the requirements for a sustainable tide gauge network. In addition, various case studies and data analysis were carried out, as were personal communications with various naval personnel. Establishing and maintaining a sustainable tide gauge network requires research and testing of various tide gauges. Identification of the correct locations to install the tide gauges is key to its success. Although the current South African Tide gauge network is not complete it does provide adequate coverage and is vitally important in the production of navigational products. The manner in which the raw collected tide data is processed and used for creating predictions meets international requirements and standards. Tidal predictions are a necessity for shallow water maritime operations and shallow water navigation, under keel and masthead clearances. The tide gauge network provides data to various organisations for storm surge and tsunami warnings while contributing to the “blue economy” through projects like Operation Phakisa. This thesis provides a comprehensive overview of South Africa's tide gauge network, investigates and justifies the reasons behind the locations and gives in depth examples of the essential use of tidal information for maritime safety.
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    Variability in frontal zones in the Southern Ocean along the Greenwich Meridian
    (2009) Billany, William T B; Reason, Chris; Hermes, Juliet
    The study found that the meridional positions and gradients of MADT exhibit interannual variability as well as considerable trends over the 15-year time-series. The meridional zones along the Greenwich Meridian also displayed clear trends over the period from June 2002 - November 2007 in integrated SST. The trends suggest a warming and shift in the climate state driving some of the variability in the ACC, which is consistent with other scientific studies.
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    Wind-driven upwelling on the Western Agulhas Bank: a study using ERA5 and WASA3 winds
    (2023) Birkett, Giovanna; Hermes, Juliet
    The Southern Benguela Upwelling System (SBUS), located along the west coast of South Africa, is unique in that it is the only eastern boundary upwelling system bounded at both ends by warm water masses, and with no continental boundary between it and the adjacent western boundary current, the Agulhas Current. The Western Agulhas Bank (WAB), defined as the region over the Agulhas Bank from Cape Point to Cape Agulhas, is the southernmost region of the SBUS however it has a markedly different coastline orientation, and upwelling dynamics in this region are understudied. This study compares two different modelled wind products, ERA5 (31 km resolution) and WASA3 (3.33 km resolution), assessing coastal winddriven upwelling in the WAB. Wind-driven upwelling, derived from the higher resolution WASA3 dataset is used to identify and investigate the seasonality of three discrete regions of upwelling within the WAB. WASA3 is shown to capture stronger orography-induced wind stress curl which drives the locations of upwelling cells and contributes 67.23% to the total upwelling velocity observed at Cape Hangklip in January, just more than double the contribution of alongshore wind stress driven Ekman pumping. Theoretical wind-driven upwelling in Cape Hangklip is shown to be stronger than in the Cape Peninsula upwelling cell in January, despite previous literature describing the Cape Peninsula as a stronger upwelling cell. This indicates that other factors not accounted for in this study likely suppress upwelling at Cape Hangklip, such as shallow bathymetry and stratification. The study highlights the importance of using a high-resolution wind product to resolve small scale features, emphasising that numerical models using low resolution wind products such as ERA5 should be used with caution when interpreting simulated upwelling velocities. Given the socioeconomic importance of the WAB, and the effect of the upwelling on aquaculture, fishing and tourism in this region, it is suggested that WASA3 or a similar high resolution wind dataset is used to force numerical models with the aim of gaining a greater understanding of the upwelling dynamics in this region.