A fine-scale volumetric census of the water masses of the Agulhas retroflection area

Valentine, Henry Richard

A fine-scale volumetric census of the water masses of the Agulhas retroflection area

Master Thesis

1990

Publisher

University of Cape Town

Department

Department of Oceanography

Faculty

Faculty of Science

Abstract

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.

Keywords

Oceanography

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