Browsing by Author "Brundrit, Geoff B"

Now showing 1 - 12 of 12
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    The characterisation of synoptic circulation patterns in Saldanha Bay
    (1996) Bilski, Sidney Warren; Brundrit, Geoff B
    Saldanha Bay, on the west coast of South Africa, is the only deep water port between Cape Town and Walvis Bay. It is separated into two smaller bays, Small Bay and Big Bay, by an iron-ore jetty built in 1975. With its sheltered environment it is an ideal site for the development of a mariculture industry, but a conflict of interest arises between the mussel farmers and the use of the iron-ore jetty and other sources of pollution. This thesis is a contribution to an effort to understand how the requirements of the mariculture industry in respect of food provision and clean water can be met. Seven field trips were made to Saldanha Bay with the aim of studying the circulation characteristics in the various regions of the bay. It was found that drogues were an effective method of measuring currents in Saldanha Bay, with the best method of drogue tracking being with the use of a Differential Global Positioning System (DGPS).
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    Empirical dynamics of a small scale coastal upwelling region
    (1991) Bain, Cairns Alexander Robertson; Brundrit, Geoff B
    The study investigates the dynamics of a small space scale (less than 10 km) coastal upwelling region at the temporal scales spanning hours to years. Three to four year time series data sets of, sea temperatures at different depths (2m, 5 m and 8,5 m) one kilometer offshore, of wind and of waves, obtained from Eskom for the Koeberg nuclear power station site study near Melkbosstrand (33° 41'S, 18° 26'E) were digitized on an hourly basis. An emphasis is placed on the study of the wind and sea temperature data, the latter being an unique data set in the South African context. The data were filtered into different frequency bands (<12,0 <0,5 <0,025 cpd). Simple statistics, linear correlation and spectral analysis were used to characterize these bands. Dominant temporal scales were identified as the seasonal, event (synoptic) and diurnal time scales. The characterization of the latter two time scales were supplemented with field work which inter alia measured: sea temperature profiles and transects; sea surface temperature distribution with the airborne radiation thermometry technique and Lagrangian currents.
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    Extreme sea levels around the coast of Southern Africa
    (1995) Searson, Sarah; Brundrit, Geoff B
    Tide gauge data from ten ports around the coast of Southern Africa are used to study the nature and behaviour of extreme high sea levels with a view towards predicting the likelihood of extremes occurring in the future. A recorded sea level height can be thought of as a combination of an astronomical tide and a weather determined component. In Southern Africa tides are typically 2 to 2.5 metres in range and the non-tidal residual accounts for up to 50 cm. Sea level is governed by local tides and local meteorology and there is great similarity in the magnitudes and timing at all ports. However tides are found to be the dominant contribution to extreme levels, hence the long term character of tidal variations is important in Southern African extremes. The fortnightly, equinoctial and 4.4 year tidal cycles determine the expected sea level extremes. A prediction technique developed here makes use of the tidal dominance by calculating the likelihood of exceedance for a specific month in a particular year. For any month the highest tide is known and an extreme will depend on the necessary surge occurring. Probability is derived from the surge distribution for that month, carried out for each month in a year, and the results presented as an exceedance chart.
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    A fine-scale volumetric census of the water masses of the Agulhas retroflection area
    (1990) Valentine, Henry Richard; Brundrit, Geoff B
    Two studies of the quantification of water masses around southern Africa are described in this thesis. The first, a pilot study, is concerned with the volumetric analysis of historic hydrographic data from the region off the South African west coast. It uses data that have been collected with various instruments over a considerable period of time and with relatively poor vertical and horizontal resolution. Nevertheless meaningful and significant results have been obtained: (i) The T/S characteristics of the upwelling domain and the open-ocean are significantly different and show no overlap. Upwelled water is derived from different parts of the South Atlantic Central Water mass. (ii) Upwelled water, irrespective of its particular origin the Central Water mass, eventually is heated to a limit of 23°C. (iii) The volume of water in each bivariate T/S class of Central Water, which is available for upwelling, are constant. This implies that the intensity of upwelling may be estimated from the salinity of the upwelled water only. The second or main study is concerned with the Agulhas Retroflection area. This is located in the broader South-East Atlantic/South West Indian ocean region, which is identified as a serious gap in a previously published fine-scale volumetric census of the world ocean. Recently collected, high quality hydrographic data make it possible to rectify this. This is the principle objective of this thesis. Some results include: I. The subtle differences between the Central water of South Atlantic Ocean origin and that of the South Indian ocean were addressed. These differences are real and distinguishable as is shown using both historic data and high quality hydrographic data. 2. For the first time, reliable quantitative estimates for the warm and thermocline water masses are produced. The warm, saline surface water of the Agulhas Retroflection area contributes very little to the overall volume of the upper 1500 m of the water column. 3. The low salinity water from south of the Subtropical Convergence is spread widely (in T/S space) on the less saline side of the three-dimensional volumetric diagrams and has very little volume (less than 1% of the total volume). 4. The more saline, higher volume section of the Central Water in this region corresponds to South West Indian Central Water. 5. Twenty five percent of the total volume of water in this region is contained in only 21 fine-scale bivariate classes. 6. Fifty percent of the water (contained in 71 classes) has a temperature below 3°C. If the lower volume. non-ranked classes were added then almost seventy percent (67,26%) of all the water in the Retroflection area would be colder than 3°C. The results of this census are compared with those of the world ocean census. Although the volume per bivariate class interval is not of the same order of magnitude, the results are remarkably similar. The water masses of the Retroflection area are less diverse than those of the world ocean.
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    Forced synoptic coastal-trapped waves along the southern African coastline
    (1995) Van Ballegooyen, Roy C; Brundrit, Geoff B
    The presence of subinertial (2 to 20 day) oscillations in sea levels and currents over the continental shelf surrounding southern Africa is well-documented, however presently available observational data alone are largely ~nsuitable or insufficient to adequately characterise the shelf circulation in terms of coastal-trapped wave (CTW) theory. In this dissertation, an analysis of atmospheric pressure and sea level data from 1980 to 1990, a scale analysis of the equations of motion and numerical modelling are used to characterise the subinertial coastal-trapped wave motions off the west coast and south coasts of southern Africa. Appropriate scaling of the equations of motion and numerical analyses of the subinertial CTW response at a number of locations along the west and south coasts of southern Africa indicate that the large scale response of shelf waters to synoptic scale or "weather band" atmospheric forcing is predominantly barotropic. This permits the use of the linear, vertically integrated, shallow water wave equations on a J-plane to model the response of the West Coast shelf waters to "weather band" atmospheric forcing. The CTW dispersion curves have been calculated for a number of shelf profiles along the West and South Coast and are used in the interpretation of the sea level analyses and the numerical modelling results, and further to investigate the effect of increasing water column stratification on the subinertial CTW response
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    Intra-seasonal convection dynamics over Southwest and Northeast Tanzania : an observational study
    (1997) Mpeta, Emmanuel Jonathan; Jury, Mark R; Brundrit, Geoff B
    Intraseasonal convection oscillation over the northeastern and southwestern Tanzania during MAM and DJF seasons respectively are examined using December, 1979 to May, 1994 pentad (5-day mean) Outgoing Longwave Radiation (OLR) as an indicator of convective cloud distribution. Area-averaged OLR indices are derived for the two areas. Time series of OLR indices for MAM and DJF indicate large quasi-periodic OLR fluctuations in some years and small fluctuations in other years. Periodogram analyses results reveal that dominant periodogram values for the oscillations were different in different years over both areas. Dominant periodogram peaks with periods more than 6 pentads (30 days) occurred 40% of the time on the average. Based on the pentad OLR time series plots deep convection and their precursors are composited. The time evolution of composite OLR maps reveal that patterns of low OLR values (indicating deep connection) shift north-eastwards coupled with low OLR values associated with mid-latitude troughs and linked to the ITCZ. Composite of kinematic and thermodynamic parameters associated with deep conJection and precursors are composited.
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    A low frequency underwater sound propagation model
    (1993) Stander, Marthinus Petrus; Brundrit, Geoff B
    All wave-theoretic underwater sound propagation models attempt to derive the acoustic field originating from a sound source in a specific environment by either solving the wave equation directly or by solving an approximation there-of. This dissertation describes a normal mode direct solution with emphasis on the application as well as the theoretic analysis capability of the specific model, called NORMAN.
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    Modeling studies of acoustic propagation through the Agulhas Current region
    (1994) Courtney, Jennifer Susan; Brundrit, Geoff B
    The motivation for this thesis arises from the apparent global warming problem. It is common knowledge that the ocean is an important heat sink and that to predict global warming trends it is important to measure changes in the ocean temperature. Measurements of ocean temperature are subject to large local variability associated with mesoscale features. In order to measure global warming trends a method is required to determine large scale averages, over large oceanic ranges, in order to remove the inherent variability associated with mesoscale features, such as currents and mesoscale eddies. This requirement can be met by acoustic thermometry.
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    The sea surface heat balance in the Benguela upwelling region
    (1987) Guastella, L A-M; Brundrit, Geoff B
    The surface heat balance of the Benguela upwelling area on the west coast of southern Africa is analysed. Measurements of the components of the heat balance were made in the St Helena Bay area from 14-21 October 1986. Additional long-term data was obtained from Alexander Bay and Cape Town. An average net heat gain of 227 W.m⁻² was received over the eight days of the field study. The presence of cold water determined that latent heat loss by the sea surface was small, while the sensible heat flux represented a small gain by the sea. These two turbulent heat fluxes are roughly equal and opposite and therefore approximately cancel each other. Use of a model, assuming idealised conditions, indicated that most turbulent heat exchange between the air and takes place in the nearshore region where air-sea contrasts are greatest. The net radiation was found to provide a good estimate of the total heat balance, thus the major contributing term to a high heat balance over the Benguela area is the input solar radiation. Minimal synoptic variation in the heat balance during the eight-day field programme was observed, but additional global radiation data analysed revealed that synoptic variations over the 3-6 day period are in fact more significant than the longer term seasonal variations. Both synoptic and seasonal variations in the heat balance are greater in the south than in the north. The high heat flux into the sea surface is capable of increasing the temperature of the upwelled water at a fairly rapid rate. During summer the heat exchange is capable of increasing the temperature of the upper 10 m mixed layer by as much as 0.65°C over one day. This input heat is used to realise the high biological potential of the upwelled waters.
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    Southern African climate anomalies, summer rainfall and the Angola low
    (1999) Mulenga, Henry Mubanga; Jury, Mark R; Brundrit, Geoff B
    Anomalous climatic conditions have contributed to poverty, wlnerability and unemployment, which are major concerns of many southern African governments. Western countries continue to give food aid during drought periods and are looking for new and effective ways of supporting national food security plans (Walker, 1989a). Food security is a very difficult problem in Africa. A number of agricultural national programs have been planned to alleviate the problem but crop yields and living standards continue to deteriorate in many African countries. Unfavorable macro-economic conditions, debt repayments, civil war, political instabilities and mismanagement of resources make the situation more complex. It may be considered that anomalous climatic events (droughts or floods) are important factors, which contribute to acute food shortage. Seasonal rainfall forecasts are an important management tool for donor countries as well as local farmers. The failure to utilize forecasts based on sound scientific knowledge would negate attempts at achieving food security (W orId Meteorological Organization, 1996). It is for this reason that rainfall is an important input parameter in attaining self-sufficiency in food. Rainfall varies in space and time over southern Africa (preston-Whyte and Tyson, 1988; Walker 1989b; Mason, 1992; Rocha, 1992; Makarau, 1995). Further investigations of year-to-year variability of southern African summer rainfall are required in order to understand mechanisms and make accurate seasonal forecasts. Therefore, research work in climate should have high priority in African countries. The need to understand and predict the interannual variations of the atmosphere and the oceans has resulted in formation of global programs like the Tropical Ocean Global Atmosphere (TOGA) and World Climate Research Program (Climate Variability and Predictability, CLIVAR, 1995 and 1998). 2 Determination of the dynamics of droughts and floods continues to be a major problem. Correct prediction of extreme events such as droughts, floods, cold and warm spells involves knowing the mechanisms as well as the local and remote forcings. Several mechanisms have been suggested but more empirical research is required to understand and predict climate variability of southern African on intra-seasonal and inter-annual time scales. This study focuses on inter-annual and intra-seasonal variability of southern African climate with the view of identifying climatic regional and local features that influence summer rainfall and its fluctuations over southern Africa as whole. In order to achieve this, determination and refining of rainfall-SST anomaly relationships is carried out. Teleconnection patterns and coherent structure of interannual variability are revealed. Atmospheric mean meteorological features over Southern Africa and adjacent oceans have direct impact on summer rainfall. Thus determination of mean features using up-graded new and longer data sets provides a basis for investigating inter-annual and intraseasonal variability. This study presents mean characteristics of peak summer (December, January and February) based on 14 years ECMWF gridded data set. This study is motivated by a desire to understand climatic controls of interannual summer rainfall variability over southern African. The intensity of summer rainfall is modulated on synoptic, intra-seasonal, interannual and decadal time scales.
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    Vortices of the Mozambique ridge current
    (1985) Gründlingh, Marten Luther; Brundrit, Geoff B
    During a cruise of the R.V. Meiring Naude in August 1975, anomalous values for temperature, salinity and nutrients were recorded over a deep-sea region of the Southwestern Indian Ocean. It was thought that this oceanographic anomaly may represent part of a cyclonic mesoscale vortex of unknown origin. The scant information available at the time on the circulation in this area precluded any of the known, steady currents from being possible generators. Only once before, in 1962, had a similar observation been made, and its significance had not been recognised. In the period 1976 to 1982, several hydrographic cruises were executed on the R.V. Meiring Naude in the region 27 - 33°S, 32 - 43°E, to locate similar features and to find answers to the following questions: Was the 1975 anomaly really a vortex (i.e. a rotating body of water)? What are the physical and dynamic characteristics (i.e. temperature, salinity, density, velocity, volume transport, energy) and distribution of such vortices? How and where are the vortices generated, and what are their lifetime and eventual fate? In all, more than 500 routine hydrographic stations were occupied to collect data on water properties. Most of these stations extended to a depth of 1 000 m, while about 20% went to at least 1 800 m. Initially, hydrosondes designed and built by the Council for Scientific and Industrial Research were employed, but a Neil Brown Instrument Systems' CTD (conductivity-temperature-depth) microprofiler was used from 1979 onwards. Satellite-tracked buoys and infrared imagery were used to derive information on the circulation patterns, thus extending the coverage of the small research vessel. The drift rate of the ship and current measurements from a drifting array of current meters augmented the calculations of geostrophic velocity, volume transport and energy, and provided insight into the flow dynamics of the water.
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    Wind shear and differential upwelling along the South Western tip of Africa
    (1984) Jury, Mark Robert; Brundrit, Geoff B; Keen, Cecil S
    Along the SW tip of Africa (30-35° S, 17-20° E) topographic irregularities shear the wind stress field giving rise to coastal upwelling with an alongshore variability. The relationship between wind shear and differential upwelling is established using a blend of oceanographic and meteorological investigations. Mesoscale aerial survey case studies form the observational basis from which the spatial variations in winds and upwelling are compared. Coastal winds, controlled by the pressure gradient between the South Atlantic Anticyclone and a summertime interior low, become modulated through interactions with the circumpolar jet stream. Low level winds accelerate over portions of the southern Benguela current region through deflections forced by three pronounced capes, two with mountain ridges exceeding 1000 m. Selected case studies are utilized to contrast the characteristics of deep and shallow wind flow, and the exposed and sheltered offshore regions. The spatial variability of the low level wind and sea surface temperature fields is correlated by means of aerial survey techniques applied at alongshore spacings of 10 to 50 km. Vertical transect and profile data collected at altitudes up to 1 km illustrate the variable depth of wind flow. Vertical wind shear controls the interaction of topography and winds. Field results show that vertical shears of -2 (10⁻²)s⁻¹ produce horizontal wind vorticities of -6 (10⁻⁴)s⁻¹ and alongshore sea surface temperature gradients of 1°C (10km)⁻¹ , a characteristic of summertime upwelling in the southern Benguela region.