Browsing by Author "Harris, T F W"
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- ItemOpen AccessThe Agulhas current system above the intermediate level(1977) Van Foreest, Dirk; Harris, T F WA 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.
- ItemOpen AccessCharacteristics of ocean gravity waves off the Cape South West coast(1974) Shillington, Frank Alan; Harris, T F W; Hennessy, Thomas RichardOcean gravity waves have been recorded near Melkbosstrand with a Wemelsfelder float type wave recorder on a sea tower situated in water 13m deep and one Kilometre offshore. The half hour records, taken twice daily between July 1972 - June 1973, have been analysed in a format uniform with that of Draper (1966). Measurements have been made from the wave records to check the value of the statistical ratios of Longuet- Higgins (1952) and Draper (1963). Ratios of maximum wave height to root mean square amplitude are lower than the theoretical values and reasons for this are discussed in the light of the records being taken in shallow water. Ratios of maximum wave height to the average upper one tenth wave height and average upper one third wave height agree closely with the theoretical values. Spectral analysis using the methods of Blackman and Tukey (1958), with 36 degrees of freedom and 100 lags have been applied to half hour records digitized once per second. Frequency, time diagrams contoured at equal energy values have been constructed for the period May and part of June 1973 on the basis of twice daily spectral measurements at 5 mHz intervals. The wave events, which include generation of swell from near South Georgia, (54°S, 37°W), have been compared with the South African Weather Bureau synoptic charts in order to identify generation centres. Repeated energy values near the 50 mHz band seem to be generated by winds of a lower velocity than required by Moskowitz (1964). Predominant siting of fetches at distances of the order of 1000 nautical miles accounts for the bulk of wave generation between latitudes 40° - 50°S. One storm appears to have originated near the Drake passage, but no events have been detected with waves passing through the passage. Analysis of wave spectra from several storms generating maximum wave heights over 5 metres, show that fully developed seas are seldom, if ever, present in Cape waters.
- ItemOpen AccessA descriptive physical analysis of water movement in the South West African Indian Ocean during the Northeast monsoon season(1971) Lutjeharms, Johann R E; Harris, T F WThe Agulhas Current is unique as a western boundary current in having as its source of surface water currents that are linked to the variable current regions of the North Indian Ocean. In which way the Agulhas Current derives its supply from these currents and how these currents interrelate, is very poorly understood. In an attempt to make a contribution to the understanding of this flow an isentropic analysis in depth on seven σt-surfaces for the whole South West Indian Ocean during the Northeast Monsoon season was carried out. This analysis was augmented by the calculation of the velocities according to the Witte-Margules equation on each σt-surface at ten selected vertical sections. In addition the detailed volume transport and the velocity structure for each vertical section were calculated. The results pointed to a significant variation in transport and velocity structure from year to year. The large scale circulation in depth of the ocean is described. It is found that the Agulhas Current derives its supply of water from different sources at different depths and that at depth a large measure of recirculation of Agulhas Current Water takes place. It is not thought that this has been reported before and these findings might be of importance to the understanding of the current system as a whole. A few innovations in the standard oceanographic methods are described and two computer programs to aid analyses are presented.
- ItemOpen AccessThe nearshore dynamics of Matroos Bay : field and theoretical investigations(1977) Gunn, Bruce William; Harris, T F WField and theoretical investigations were made of the nearshore circulation of Matroos Bay, a small embayment on the south-western coast of Africa. The field study revealed two dominant circulation types, denoted "anticlockwise" and "clockwise". The anticlockwise circulation type was characterized by longshore currents flowing from the back of the bay to the northern headland where a rip current was located. The clockwise circulation type was characterized by longshore currents flowing from the northern headland to the back of the bay where a rip current was situated. Anticlockwise circulation types were primarily found to occur during periods of low waves, the clockwise circulation during periods of high waves. Winds and coastal currents were found not to have a major influence on the circulation within the bay. The theoretical study was based on a model of longshore currents in a semi-circular bay (O'Rourke and Le Blond, 1972). This study showed that a change of the height of the wave could lead to a change in the direction of flow of the longshore current. In agreement with the field study, a low wave height was found to result in an anticlockwise circulation type, a high wave height resulting in a clockwise circulation type. The change from one circulation type to the other depended on the relative strength of two opposing mechanisms. One mechanism arose from the obliquity of the wave approach to the shore. This mechanism generated longshore currents towards the back of the bay and was dominant during high waves. The other mechanism resulted from the non-uniformity of the wave height around the bay and generated longshore currents flowing from the back of the bay. Coastal currents were measured as part of the field study. A fair correlation was-observed between the direction of the coastal current and that of the wind, particularly if the winds were moderate to strong. Temperature and salinity measurements were also made. These properties showed seasonal variations in accord with the principles of upwelling. Surface water was found to have a salinity similar to that found at a depth of 400 to 600 metres in the South Atlantic Ocean. The water was upwelled from a greater depth during the summer than during the winter. Heating of the water was found to be greater in summer than in winter whilst dilution of the water, due to rainfall, was found to be negligible, even during winter. The interchange of water between the bay and the coastal circulation was found to be dependent on the circulation type.
- ItemOpen AccessOne-dimensional wave spectrum analysis of wind waves off Cape Town(1970) Van Ieperen, Maarten Pieter; Harris, T F WWind waves in the vicinity of Cape Town, South Africa, were measured by means of an N.I.O. (England) ship-borne wave recorder, in water depths varying from 12 - 140 metres. One dimensional frequency spectra were computed from the records, by power spectrum analysis, based on the method of Blackman and Tukey, using an I.B.M. 1130 Computer. Assumptions of normality and stationarity were tested. The Gaussian assumption of the waves was found to be acceptable for the waters off Cape Town. At the 5% critical level, using the chi-square test, 3 out of 23 records tested were found to be non-Gaussian. No significant difference between the Gaussian properties of the deep and the shallow stations was found. Tests for stationarity applied to 3 selected records showed 1 record as clearly non-stationary. However, this recording was obtained under fluctuating wind conditions. Comparison of the total variances of the power spectra obtained in shoaling water showed a systematic decrease of variance with depth. Normalised spectra did not show a systematic selective attenuation of the variances with frequency. Factors which might have caused the systematic reduction of the total variances with depth have been examined. The bottom friction factor for this coast was estimated. The mean value of the bottom friction is 0.22. This is higher than found by other workers.
- ItemOpen AccessA two-mode numerical model with applications to coastal upwelling(1981) Van Foreest, Dirk; Harris, T F WA first 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 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. Volume flux calculations (reference 1100m.) show that the two supplies to the Agulhas current could be estimated at 15 sverdrups each. The main part of the current turns back into the South West Indian ocean. An amount of roughly 5 sverdrups was lost East Atlantic. Property distributions although difficult to interpret, were used to construct the acceleration potential diagrams as the contouring of these diagrams was not always obvious. For a more detailed discussion of the combined data the current system was divided into areas. These areas are: Source area (including the inflow south of Durban and the countercurrent at Durban; Dynamical upwelling region; Mixing and retroflection area; Planetary wave region.