The relationship between coastal sea level variability in South Africa and the Agulhas Current

Abstract

Sea level observations suggest that western boundary currents, such as the Gulf Stream, contribute to coastal sea-level variability and rise. Since the influence of the Agulhas Current on coastal sea-level variability and rise has not yet been quantified, this study considers how variations in the transport and position of this current affect sea-level variability along the South African coast. This was achieved by analysing timescales of variability in the time series of an Agulhas Current transport proxy and absolute dynamic topography at the core position, at the ACT array, comparing these to monthly mean sea level records of East London (33°S, 28°E). The timescales embedded in each of the datasets were separated through the Empirical Mode Decomposition (EMD) method. Monthly gridded satellite altimetry sea level anomalies were also utilised to extend the analysis to other tide gauge locations on the east and south coast of South Africa. The results suggest that, on timescales ranging from a few months to decades, coastal sea-level is associated with the Agulhas Current through absolute dynamic topography variations in the Agulhas Current core. Absolute dynamic topography variations in the Agulhas Current core are found to be in response to displacements of the current both near and offshore from the coast during eddies and meandering events. The study suggests that the Agulhas Current variations should be taken into account for better coastal planning, management and engineering. • Sea level variability along the South African south and east coast is studied. • Coast to Agulhas Current gradient approach determines its path and core position. • Empirical Mode Decomposition is used to extract different timescales of variability. • Position of the Agulhas Current core and its transport affects coastal sea-level. • A relationship exists at sub-annual and interannual timescales.