Browsing by Author "Chang, Nicolette"

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    Descriptive analysis of a virtual transect through the Benguela Upwelling System
    (2003) Chang, Nicolette; Shillington, Frank; Roy, Claude
    With the limitations of in situ observations of the ocean, numerical modeling of the ocean can easily provide clues to the structures and processes that lie therein. This project is an example of the examination of numerical model output. In this case, the area of interest is an area of ocean off Namaqualand which displays a reaction to local wind forcing. A transect was performed off the coast using output from ROMS (Regional Ocean Modeling System), forced under realistic winds obtained from weekly averaged ERS data. Temporal and spatial analysis yield information on the climatological characteristics of this region.
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    Investigation of Wind Variability in the South Atlantic Sector of the Southern Ocean and the Influence on the Upper Ocean in a Numerical Ocean Model
    (2019) Moalusi, Tumelo Comfort; Vichi, Marcello; Chang, Nicolette; Nicholson, Sarah
    Several papers have linked climate variability in the Southern Ocean (SO) with the Southern Annular Mode (SAM), which has seen an increase in the positive phase since the mid-1960s, due to the Antarctic ozone depletion and emissions of greenhouse gases. The SAM is recognized as the main mode of atmospheric variability in the SO. The SAM index allows an understanding of the latitudinal movement (south-north) of the westerly wind belt circling Antarctica and has significant impacts on Antarctic surface temperatures, ocean circulation, and many other aspects of Southern Hemisphere climate and thus the global ocean. During negative phases of the SAM Index, westerlies intensify and move north, bringing about more (or stronger) storms, and low pressure systems over southern Australia. The changes associated with SAM forcing may have impacts on carbon uptake and storage in the SO directly through upwelling and outgassing, and indirectly, by influencing nutrient cycles and phytoplankton activity. Understanding the variability of the wind field in the SO and how it affects ocean circulation, climatic and oceanic variables is important. Thus, this thesis presents the relationship of the SAM index and the upper ocean, specifically analysing sea surface salinity (SSS), sea surface temperature (SST) and the mixed layer depth (MLD), in the Southern Atlantic sector of the SO as presented in numerical ocean models. Two resolutions of NEMO ocean model are compared: a) eddy-permitting (SATLANTIC05), b) eddy-resolving (SATLANTIC12) models, with horizontal resolutions of ½ and 1/12 °, respectively. In situ data from 2013 World Ocean Atlas is used as a benchmark for the analysis. Our model‐based analysis confirms previous studies done on the influence of the SAM on the SO, that a strong relationship exists. The SAM index is positively correlated with wind speed in the Antarctic Zone (AZ) and negatively correlated in the Subantarctic Zone (SAZ). The impacts of this is clear in the upper ocean. These correlations between SAM index and the selected variables at these selected locations confirms that the SAM index corresponds with cool surface temperatures at higher latitudes and a weak cooling at midlatitudes during positive phase, which differs regionally.
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    Open Access
    Numerical ocean model study of the Agulhas Bank and the cool ridge
    (2008) Chang, Nicolette; Shillington, Frank; Penven, Pierrick
    The oceanic structure and circulation of the Agulhas Bank, the very wide continental shelf area off South Africa, has been explored in this thesis. The Agulhas Bank is a complex ocean region influenced by shelf processes as well as a nearby western boundary current, the Agulhas Current on its eastern margin and the Benguela Upwelling system on its western margin. In addition, a cold water feature, known as the cool ridge, has been observed on the Eastern Agulhas Bank. A consistent dynamical description is not available but it is commonly observed as a south-westerly flow of cold water in the upper water column, roughly following the 100m isobath and extending seawards off the coast. The formation of the cool ridge has also been investigated in this thesis by means of a numerical ocean model. Previous studies on the Agulhas Bank have been limited temporally and / or spatially. Thus, the Agulhas Bank as a whole has been inadequately sampled to provide a comprehensive representation. In order to remedy these deficiencies, the Regional Ocean Modeling System (ROMS) was used to study the Agulhas Bank dynamics. A large-scale model of the surrounding oceans, the SAfE (South African Experiment) configuration of the ROMS model, was used to force a one-way embedded finer-resolution model over the Agulhas Bank. This produced, 8 years of model data at an approximate horizontal resolution of 8km and 32 vertical terrain-following levels. Two main experiments were performed to understand the nature of the Agulhas Bank. Firstly, the "Reference Experiment" derived a seasonal ocean climatology of the Agulhas Bank. Secondly, the "No Agulhas Experiment" was carried out in an approach in which the Agulhas Current was removed from the shelf edge. A comparison of these two experiments yielded the influence of the Agulhas Current on the Agulhas Bank. The ROMS model was able to reproduce the main observed seasonal structure and circulation of the Agulhas Bank as well as the cool ridge. The Agulhas Bank showed marked seasonality, with its two-layer structure being significantly influenced by the Agulhas Current. The direct influence of the Agulhas Current on the Bank occurs on the Outer Agulhas Bank by the Agulhas Current itself or an Agulhas Current filament. Ekman veering by the interaction of the Agulhas Current with the bottom topography on the slope of the eastern Agulhas Bank advect cool water vertically onto the Bank. This strengthens the thermocline from below, in contrast to surface warming by solar insolation in summer and the Agulhas Current in winter. Cold waters, upwelled over the shelf edge, indirectly affect the greater Agulhas Bank by their advection by the predominantly westward mean currents. The most significant influence of the cold shelf-upwelled waters are in bringing cold waters to shallower depths over most of the Agulhas Bank. This may influence the waters that upwell at the coast, which on the eastern Agulhas Bank (without the Agulhas Current) are trapped under a thick warm surface layer.