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.