Abstract
M. Sc.
The main aim of this study was to analyse ships' drift data for the South West Indian and South East Atlantic Oceans in order to find out more about the general characteristics and seasonal flow patterns of the surface ocean currents in these regions. A ship's drift observation consists of the difference between a course steered between two positions (estimated arrival time is based on the speed of the ship through the water) and the course made good during the same period (figure 1.1). The speed of the drift can be computed by dividing the vector difference by the time elapsed. Since the most important contributors to the drift of a ship are the surface ocean currents, ship's drift data can be analysed in order to provide information about the speeds and directions of flow of the surface ocean currents in a particular area. Data, showing the mean prevailing winds, were also obtained from oceanic climatic atlases in order to investigate the relationship between the winds and the surface ocean currents. The ships' drift data as well as the wind data were used to investigate the following characteristics of the ocean currents in the study area: Geographic location including their spatial and temporal variability Speeds and directions of flow including their spatial and temporal variability The relationship between the currents and the mean prevailing winds and The eddy kinetic energy (a measure of variability around the mean flow) associated with the currents. Additional features investigated included: The discontinuity of the Mozambique Current The source currents of the Agulhas Current, and The possible existence of an offshore branch of the Agulhas Current, known as the Mozambique Ridge Current. The geographical locations, as well as the speeds and directions of flow of the main surface ocean currents in the study area were successfully determined, with the exceptions of the Angola, South Atlantic, Agulhas Return, South Indian Ocean and Antarctic Circumpolar Currents. These exceptions can be traced to the lack of data in the regions in which these currents occur. A large seasonal difference in the speeds of flow of the South Equatorial (in the Indian Ocean), the East Madagascar and the northern Mozambique Currents was found. The southern Mozambique, the Agulhas and Benguela Current as well as the currents inshore of the Agulhas Current over the Agulhas Bank were shown to exhibit small seasonal differences in their speeds of flow. Owing to a lack of data, very little could be determined regarding the seasonal variability of the Angola, South Atlantic, Agulhas Return, South Indian Ocean and Antarctic Circumpolar Currents. A seasonal variability pertaining to the geographical location of the extreme northern Mozambique Current was determined, but no seasonal variability was found in relation to the geographical location of any of the other currents in the study area. In the northern parts of the South West Indian Ocean a strong relationship was found between the mean prevailing winds and the surface currents. The main driving force for the currents in this region would seem to be the winds. However, as these currents flow southwards and become stronger, as is the case for the Agulhas Current, the relationship between the winds and the currents appears to be somewhat weaker. This may be owing to the fact that the main driving force for these currents is not the winds but the inflow of water from the north. In regions where the currents are weak, such as the Benguela Current region and the Agulhas Bank area, the relationship between the winds and currents was also shown to be strong. The eddy kinetic energy gives a measure of the variability around the mean flow of a current. Often this variability is manifested by the presence of eddy features. Some regions exhibited high values of eddy kinetic energy. These areas included: Parts of the East-Madagascar Current Parts of the Mozambique Current The Agulhas Current, including the region of the Agulhas Retroflection, and Parts of the South Equatorial Current in the Indian Ocean. Two areas of special interest regarding the eddy kinetic energy are the regions of the Agulhas and Madagascar Currents Retroflections. In these areas values of eddy kinetic
energy greater then 750 cm 2/s2 occur (figure 4.34). This appears to reinforce the findings of previous researchers, that have postulated the existence of current retroflections in these areas and have indicated that these areas are regions for the formation of eddy features. Unfortunately the maps and charts generated from ships' drift data do not provide clear representations of the retroflections of the East Madagascar and Agulhas Currents. This is owing to the high degree of spatial and temporal variability of these current retroflections. Thus, despite the difficulty of providing conclusive evidence for the existence of these features from ships' drift data, this investigation into the eddy kinetic energy in this study may give additional evidence for the possible presence of the East Madagascar and Agulhas Current Retroflections. The Mozambique Current was found to be a discontinuous, ill-defined western. The Mozambique Current was found to be a discontinuous, ill-defined western boundary current. Both the Mozambique and the East Madagascar Currents were shown to be source currents for the Agulhas Current, but probably in a weak and intermittent manner. The source water from the East Madagascar Current may be in the form of eddies. The existence of an offshore branch of the Agulhas Current, could not be verified from ships' drift data. This study makes two significant contributions to the descriptive physical oceanography of the South West Indian and South East Atlantic Oceans. These are: A detailed analysis relating to the seasonal variability of the currents. In the case of the Benguela Current ; the- investigation into the seasonal variability was more detailed than any previous study. A comprehensive investigation pertaining to the eddy kinetic energy associated with the currents. This was done on a one degree grid scale. The only other study which used ships' drift data to examine the eddy kinetic energy for these regions used a five degree grid scale, which could lead to the smoothing out of high values associated with narrow western boundary currents.