The Natal Bight Coastal Counter-Current: A modeling study

Abstract

Output from a high-resolution ocean model, a wind reanalysis and a particle tracking tool are used to improve our understanding of the shelf circulation in an embayment off South Africa's east coast, known as the KwaZulu-Natal Bight. This region spans across roughly 140 km of coastline and is located between 29°S and 30°S. It is influenced by the strong, south-westward flowing Agulhas Current on its offshore edge, while its shelf is dominated by weak and variable currents. On the KwaZulu-Natal Bight's shelf, realistic high-resolution model simulations indicate the presence of a mean north-eastward flow: the Natal Bight Coastal Counter-Current. The mean surface circulation depicts a Natal Bight Coastal Counter Current stretching along the 50 m isobath from the southern to the northern section of the KwaZulu-Natal Bight while progressively becoming narrower and weaker northwards. The mean vertical structure of this counter current extends throughout the water column and at its origin, it almost connects with the Agulhas Undercurrent. In this region, the Natal Bight Coastal Counter-Current is about 20 km wide and has an average speed of 20 cm/s at its core, which may exceed 100 cm/s during individual events. The passage of southward propagating anticyclonic eddies offshore of the Agulhas Current are associated with a southward flow along the southern KwaZulu-Natal Bight region and the interruption of the otherwise north-eastward shelf currents. While the circulation in the KwaZulu-Natal Bight is primarily driven by perturbations at the Agulhas Current front, there is also some indication of a direct wind-driven influence in coastal waters, inshore of the 50 m isobath and north of 29.5°S. Virtual particle tracking experiments show that the Natal Bight Coastal Counter Current may increase connectivity between Marine Protected Areas within the KwaZulu-Natal Bight, where the current greatly increases the water retention. This may trap nutrients from coastal origins on the shelf, together with any suspended particles such as larvae. Therefore, the Natal Bight Coastal Counter-Current has the potential to increase the suitability of this habitat for larval settlement.