Modelling the Agulhas Ocean Current: with a focus on the related shallow water hydrodynamics in and around the Durban Bay, South Africa.
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Date
2021
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Abstract
The Agulhas Ocean Current is a powerful and persistent western boundary current that flows along the continental shelf edge off the eastern coast of South Africa in a southerly direction. In addition to the tide- and wind-induced currents, the Agulhas Current influences the nearshore currents in Durban Bay, for example, the Durban Eddy. eThekwini Municipality and Deltares proposed that Durban be used as a pilot study to investigate the capabilities of developing a Delft3D Flexible Mesh (D-Flow FM) model that integrates the combined forcing of ocean currents, tide and wind in a coastal model domain, which once successfully achieved, is to be integrated within eThekwini’s Forecast Early Warning System.
The aim of this study is to use D-Flow FM to accurately model the Agulhas Current and analyse its effects on the nearshore waters of Durban. Output from global ocean models, such as the NEMO models operated by E.U. Copernicus Marine Service Information (CMEMS) in the Global Reanalysis Multi-Model Ensemble Product (GREP), include the Agulhas Current but are not suitable for coastal applications due to their relatively coarse resolution and absence of tidal forcing. For this reason, model output from the GREP was downscaled to a coastal scale using a D-Flow FM model of KZN with appropriate boundary conditions and evaluating the use of a new Delft3D nudging technique.
The final D-Flow FM model that was developed applied a relatively high-resolution grid on top of the continental shelf. A stable ocean current was simulated by forcing the model with a full set of 3D ocean boundary conditions, including currents, salinity, temperature, sea surface anomalies. In addition, the model was forced with tide and wind. Model tests showed that the nudging technique was not required when applying a suitable model extent. Realistic currents seem to develop along the edge of the shelf in the D-Flow FM model as observed from literature and measurements.
The final coastal model output was compared with Acoustic Doppler Current Profiler (ADCP) data from ACEP (African Coelacanth Ecosystem Program) and data from a SADCO (Southern African Data Centre for Oceanography) database off Durban. The results of the D-Flow FM coastal model correlated better with these measurements when statistically compared to data from the GREP.
The D-Flow FM coastal model was used to reproduce the Durban Eddy and analyse its modelled characteristics. The modelled duration of the eddy was between 8 to 10 days, which included the formation and dissipation of the eddy. A monthly average in agreement with previous observational studies of 1.66 eddies was seen from model outputs, with the reversal of currents along the Durban coast whenever an eddy was present.
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Masters Degrees. University of KwaZulu-Natal, Durban.