Wind- and eddy-driven upwelling over submarine canyons inshore of the northern Agulhas Current

Abstract

The iSimangaliso Wetland Park, located along the coastline of northern KwaZulu-Natal, hosts South Africa’s only subtropical coral reef system. Episodic upwelling events are observed inshore and at the heads of Diepgat, Leadsman and Leven canyons within the iSimangaliso Wetland Park and may assist in larval dispersal and act as a buffer against coral bleaching. Over a 12-month period (1 June 2018-31 May 2019), two cooling events were identified from surface and subsurface temperature measurements extracted from the Sea Surface Temperature (SST) product of the Operational Sea Surface Temperature and Ice Analysis (OSTIA) satellite and data collected from underwater temperature recorders, respectively. Lagged correlations between subsurface temperatures confirmed that the anomalous cooling events were upwelling events (of 2-7 hours). The approximate upwelling rate was greatest at Diepgat Canyon, which suggests that upwelling events are enhanced by the physical structure of the canyon head and its shallow termination point which may influence the affected ecosystems. Wind measurements from the Mbazwana weather station and satellite sea-level anomalies were used to investigate the respective roles of wind stress and mesoscale ocean variability as driving forces of the upwelling events. The results indicate that the less intense and shorter cooling event was driven by a cyclonic eddy event, but that the stronger cooling event was driven by a combination of both forcing mechanisms.