Browsing by Subject "Oceanography"

Now showing 1 - 20 of 203
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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 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 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 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.
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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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    Analysis of variable scatterometer wind fields in the Benguela upwelling region
    (2001) Johnson, Ashley Stratton; Shillington, Frank; Nelson, G; Roy, Claude
    The dissertation seeked to understand the effectiveness of satellite based data collection of wind fields along the west coast of southern Africa, pertaining to particular synoptic atmospheric systems. A comparison between data from two automatic coastal weather stations and measurements obtained by the NASA scatterometer (NSCAT) for the period 1 December 1996 to 31 May 1997 yielded a correlation coefficient of 70%.
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    Antipodal receptions in global acoustics
    (1997) Courtney, Jennifer Susan; Brundrit, Geoff
    Global Acoustic Propagation is a recently developed scientific discipline within the study of long range underwater acoustic propagation. Acoustic propagation over extremely long ranges involves a combination of effects from earth curvature and the global distribution of oceanographic and geophysical features. Antipodal receptions, that is to ranges of the order of 20Mm (1 Megameter = 1 000 km), require underwater acoustic propagation to very long ranges and thus effects due to the form of the earth and the range dependence of the sound speed field within the ocean can not be ignored. The purpose of this thesis is to investigate the nature of antipodal receptions with reference to the form of the earth and horizontal sound speed variations within the ocean and thereby contribute to the new and specialized field of study, Global Acoustic Propagation. Close to an acoustic source acoustic energy diverges so that local signal strengths decrease with distance from the source. However, as the antipodal region is approached acoustic energy that has not been blocked by bathymetric features will refocus, counteracting the distance loss rule. Thus at antipodal sites there is a good prospect of receiving a focused signal. Even so, the ellipsoidal form of the earth and the horizontal variability within the sound speed field of the ocean means that the re-focusing will result in a region of enhanced signal rather than an exact antipodal point. The precise detail of the advantage of placing receivers in the antipodal region will depend upon the paths taken by the acoustic energy and the characteristics of the acoustic medium encountered along each path. The proposal for this thesis is to develop an algorithm to determine the nature of the antipodal region under certain physical assumptions. The physical assumptions are referred to as the geometric assumption, which refers to the form of the earth, and the refractive assumption, which refers to the horizontal variability of sound speed within the world oceans.
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    Assessing dry spell and wet day frequencies over southern Africa during the summer rainy season
    (2021) Thoithi, Wanjiru; Reason, Christopher
    Rainfall over southern Africa experiences substantial temporal and spatial variability which heavily impacts poor rural populations in the region that rely on rainfed agriculture for their livelihoods. Instead of totals, seasonal rainfall is better characterised by wet and dry events occurring within rainy seasons as knowledge of the frequency of such events is able to inform agricultural activity. Dry spells (pentads having <5 mm) and moderate wet days (10-30 mm) over southern Africa were assessed using high resolution (0.05â—¦) Climate Hazards group Infrared Precipitation with Stations (CHIRPS) datasets over the period 1981/82-2018/19 during October-November (ON), December-February (DJF) and March-April (MA) using climatology, intensity-frequency and trend analysis. Correlations with SST over the tropical southeast Atlantic and climate modes namely, El Nino Southern Oscillation (ENSO), the Subtropical ËœIndian Ocean Dipole (SIOD) and Southern Annular Mode (SAM) were computed. These, together with regressed atmospheric and SST fields were used to identify possible mechanisms for changes in dry spell and moderate wet day frequencies during austral summer. Two strong gradients in dry spell frequency were found to be present during DJF, one diagonal along the western margins of the Kalahari desert and the other meridional, lying across 20-24â—¦S. Topographic influences on rainfall were observed near the Drakensberg and Chimanimani mountains, Mulanje massif and Madagascan highlands where dry spell frequency (DSF) (moderate wet day frequency (MWDF)) tended to be relatively lower (higher). A region which frequently experienced half of the season as dry was identified lying across 22-24â—¦S (18-25â—¦S) during DJF (MA), with a core in the central Limpopo River Valley where 85-100% (100%) of the seasons were dry for half the season. DSF and MWDF trends indicated that drying has occurred over central South Africa during ON whereas decreasing DSF and increasing MWDF trends pointed to a weakening diagonal and meridional gradient during DJF. Additionally, increasing MWDF trends over important agricultural areas have occurred during DJF. Trends over central South Africa, part of the diagonal gradient, were associated with changes in ENSO, SAM, the Botswana High and SST in the SE Atlantic whereas those in the western Botswana region, part of the meridional gradient, were associated with those in the SIOD, Mozambique Channel Trough and Mascarene High and SST in the eastern and western Pacific.
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    Assessing the impacts of assimilating satellite SST in addition to along-track SLA into a HYCOM of the Agulhas System
    (2017) Rapeti, Tharone; Backeberg, Bjorn Christoph; Rouault, Mathieu
    The greater Agulhas Current System, is considered to be the largest western boundary current in the Southern Hemisphere, with only the Gulf Stream, and possibly the Kuroshio, considered to be larger globally (Bryden et al., 2005). The Current System is a crucial factor for determining the mean state and variability of the regional marine environment, resources and ecosystems in the region, regional weather, as well as the global climate on a broad range of temporal and spatial scales. Due to an absence of a coherent in situ and satellite-based observing system in the area, modelling and data assimilation techniques are utilised. These both further the quantitative understanding of the ocean dynamics as well as providing better forecasts of this complicated western boundary current system. In this study, we compare two assimilation experiments using the Ensemble Optimal Interpolation (EnOI) data assimilation scheme in a regional implementation of the Hybrid Coordinate Ocean Model (HYCOM). In the first experiment, we assimilate along-track satellite sea level anomaly (SLA) data only, and in the second experiment we assimilate both along-track sea level anomaly (SLA) as well as satellite sea surface temperature (SST) data. The objectives of the study are to investigate the impacts of assimilating SST along with SLA into the regional HYCOM model, with the hopes of improving the model performance. The long term aim of this experiment is to develop a regional ocean prediction system. The additional assimilation of SST along with SLA into the HYCOM model, has improved upon the representation of the SST field across the region by reducing the error. However, with regards to velocity, surface eddy kinetic energies (EKE), as well as subsurface velocities, the updated SST model shows less improvement. A velocity bias can be seen as the reason for underperformance in these aspects. The model still struggles to recreate subsurface water masses, underestimating salinity in the upper 500 m; assimilating T/S profiles in the future could improve on this. The assimilation of SST has improved upon the SST-SSH correlation in the model, as well as the spatial distribution and accuracy. The assimilation of SST along with SLA has had many positive impacts, with unfortunately, a few negatives. The shortcomings of the numerical model will have to be improved upon and additional assimilation variables should be tested in further studies, to provide a solid forecasting system.
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    Assessment of early 20th Century climate model simulations of Antarctic Sea ice using historical commercial humpback whale catch data
    (2021) Mazomba, Thando; Vichi, Marcello
    The story of Southern Hemisphere humpback whales through time presents multiple narratives. This study integrated two of those narratives to better understand our climate over time - the ecological behaviour of this species as well as their exploitation in the last century. The changing of our climate is largely better understood from the introduction of satellites in the late 20th century when data could be collected at higher spatial and temporal levels. Prior to this, data were scarce, especially for remote areas such as the Southern Ocean. The trust on climate models to produce valuable projections rely on how skilled they are in reproducing the historical climate; therefore their results require assessments against as many observations as possible to further increase their reliability. The Southern Ocean being an integral component to climate regulation, it is important to try understand its oceanographic features. The seasonal sea ice cover represents one major feature of this system. This study proposes to use other sources of data for the early 20th century that will help closing the gap prior to satellite observations. Humpback whales migrate poleward during the austral summer to feed on Antarctic krill at a proximity to the ice edge. Humpback whale catch locations in the early 20th century corroborate with this foraging behaviour. Using humpback whale catch location data as a benchmark, the study aims to assess the skill of climate models in simulating sea ice edge location for the early 20th century. Sea ice edge is directly related to sea-ice extent, which is an important variable in the research of sea ice dynamics over time. This is especially true in the face of rapid climate change where accuracy of sea ice changes is very important. The study therefore also aims to assess climate model climatological seasonal cycle of sea ice extent results from climate models against literature and contemporary observations. Comparisons between each model's results are also carried out. The humpback whale catch effort, as per IWC data, mostly covered the Atlantic and the Indian sector of the Southern Ocean. For this reason, the study focused on these two sectors for the analyses. Decade 1930-1939 showed the highest catch numbers consistently throughout the months of the study. The simulated past century and recent climatological seasonal cycle sea ice extent show a wide variety of responses between the models, with the majority of them underestimating the seasonal cycle based on previous literature and contemporary observations. This indicates the need to improve the sea ice physical processes in models to better capture the specific Southern Ocean processes. The ensemble median of ice edge location from the models apparently follow the latitudinal pattern of the whale catch locations, which are assumed in this study to mark the topography of the sea ice edge. However, they simulate a sea ice edge equatorward of the edge derived from an ensemble analysis of humpback whale catch locations. The variance explained by the coefficient of determination between the models and the whale catch distribution is low, with the highest value of one month being low as well. This indicates that only a portion of the simulated edge follows the reconstructed sea ice features.
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    An assessment of the reliability of fossil pilchard and anchovy scales as fish population indicators off Namibia
    (1986) Shackleton, L Y; Johnson, R F
    Fossil fish scales hold potential for ellucidating past fish population fluctuations. A system of classification for scales from the pilchard, Sardinops ocellata, and the anchovy, Engraulis capensis, is presented. Both species show an unexpected range of scale types. The classifications reduce errors in distinguishing between the scales of the two species to ±2,5%. Scale loss from these fish is quantified under laboratory conditions. Pilchard lose 1,56 scales/fish/day due to death and 1,50 scales/fish/day due to deciduousness over their expected lifetimes. For anchovy the figures are 2,42 scales/fish/day (death) and 0,48 scales/fish/day (deciduousness). Application of these scale-loss studies to scales preserved in the anaerobic sediments off Walvis Bay, Namibia, shows that deciduousness is the dominant process contributing scales to the sediments. This basic information on pilchard and anchovy scale loss is used to interpret counts of scales in the laminated interval of a core taken from the diatomaceous muds off Walvis Bay. This pilot study shows that: i) the Namibian Fishery was dominated by pilchard in the past as it was prior to the collapse in the early 1970's and, because of this, is distinctly different from the anchovy/anchovetta-dominated east Pacific systems; and ii) major pre-fishery stock fluctuations do appear to be reflected in the sedimentary record. Further scale studies on larger sediment samples are recommended to ellucidate the Namibian fish stock fluctuations.
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    The atmospheric boundary layer above the Agulhas current
    (1996) Lee-Thorp, Andrew Michael; Lutjeharms, Johann R E; Rouault, Mathieu
    This thesis describes the atmospheric boundary layer above the Agulhas Current using shipboard meteorological measurements and rawinsonde ascents. The juxtaposition of the warm Agulhas Current and cool shelf waters is shown to have far-reaching effects on the overlying atmosphere. Air-sea fluxes of momentum, sensible and latent heat and resultant boundary layer characteristics demonstrate high horizontal inhomogeneity. The results suggest that this inhomogeneity is permanent. The spatial heat flux gradient is reflected in the overlying atmosphere by a transition in stability of the boundary layer and potential cumulus formation from the cool shelf to the warm current. For airflow perpendicular to the Agulhas Current an internal boundary layer was observed to develop at the inshore sea surface temperature front. Onshore-moving air accumulated a significant quantity of moisture during its trajectory over the current. When airflow is parallel to the current an atmospheric moisture front exists along the axis of the inshore sea surface temperature front. The mean thermodynamic structure of the atmosphere was investigated. An inversion capped the boundary layer whilst a second, higher-level subsidence inversion was found which acts to limit the vertical development of cumulus clouds and therefore the redistribution of heat and moisture above the boundary layer. The results presented in this thesis are useful in two ways. The Agulhas Current has frequently been linked to South African climate. This is the first dedicated study which quantifies and characterizes the atmospheric boundary layer in this region. Secondly, maritime airmasses are dramatically modified above the Agulhas Current. The resultant large horizontal inhomogeneity, its vertical extent and permanence suggest that its inclusion is vital to any successful climate model. Atmospheric general circulation models have been criticized for not taking into account regions of strong horizontal inhomogeneity. The results of this thesis support this argument and highlight the need for similar studies. Bibliography: pages 116-123.
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    Atmospheric circulation and sea surface temperature anomalies during the dry summers of 2001/02, 2002/03 and 2003/04 in Southern Africa
    (2005) Mehari, Michael Futwi; Rouault, Mathieu
    Drought is a phenomenon associated with the lack or shortage of water. It means less than normal or no water is available. It primarily originates from lack of precipitation. Lack of precipitation leads to depletion of storage of I. soil moisture that results in dry land crop failure and dying-off grazing and other vegetations, II. of ground water, which results in drying up of springs, streams and boreholes and Ill. of water in man-made reservoirs, which results in stress to households, industry, power stations and irrigation schemes (Davis, 1983). There are three types of drought {Thomas, 1965): meteorological, agricultural and hydrological droughts. Meteorological drought occurs when the rainfall is abnormally low. Agricultural drought exists when the soil is depleted to the extent that crop harvests are reduced significantly (Davis, 1983). Agricultural drought has a common time scale of a season (3 to 6 months) (Harsch E, 1992). Agricultural drought can also be caused by excessive rain or flood leading to a damage of crops. Hydrological drought is associated with scarcity of precipitation on a longer time scale (1- 2 years or more) and its effect is on ground water supply (Meigh et al, 1992). Meteorological drought can be seen as a subset of agricultural drought. If there is agricultural drought then there is also meteorological drought. On the other hand, agricultural and hydrological drought can be out of phase, each having different signatures (Rouault and Richard, 2003).