Upwelling along the southeast African coastline and links to cold water temperature anomalies at Sodwana Bay, South Africa

Abstract

Coral bleaching is a major concern for the health and survival of coral reefs worldwide, as it can significantly reduce coral growth and increase coral susceptibility to disease. The coral reefs at Sodwana Bay, located on the northeastern coast of South Africa, have shown resilience to coral bleaching which has been attributed to cold water temperature anomalies observed at Sodwana. This study investigates the regional hydrodynamic processes associated with 63 temperature anomalies identified at Sodwana between 1994 and 2015. The study found that 65% of the anomalies are associated with remote upwelling of cold water near the Delagoa Peninsula, followed by advection from the Delagoa Bight towards the Sodwana region. The cold upwelled water advects directly from the peninsula along the shelf towards Sodwana or recirculates within the Delagoa Bight before advecting along the coastline to Sodwana. The remaining 35% of the anomalies were associated with local upwelling within the KwaZulu-Natal Bight south of Sodwana.

The study also found that the separation of the strong intermittent southward stream from the Delagoa Peninsula is strongly linked to the upwelling at the Delagoa Peninsula. The cold upwelled water is then advected to the Sodwana region by regional flow patterns, resulting in a temperature anomaly at Sodwana. The local upwelling is similarly linked to an increase in the Agulhas Current velocity magnitude around Sodwana and flow separation of the current from the coastline at the northern extent of the KwaZulu-Natal Bight.

The cross-correlations between the conditionally averaged flow fields associated with remote and local upwelling near the Delagoa Peninsula and the instantaneous flow fields at the time of the upwelling showed a positive correlation for all the anomalies over the 21 years. The anomalies associated with remote upwelling have an average correlation coefficient of 0.67 with a maximum correlation of 0.96. The anomalies associated with local upwelling have an average correlation coefficient of 0.74 with a maximum correlation of 0.98.

This study shows that the unique geometry of the southeast African coastline is a key factor associated with the cold water temperature anomalies at Sodwana. The interaction between the regional hydrodynamics and coastline features such as the Delagoa and Natal Bights, are crucial for the upwelling that eventually leads to the cold water temperature anomalies at Sodwana. Without these coastline features and the intermittent strengthening of the southward streams along the coastline due to the high eddy kinetic energy associated with the region, it is likely that these temperature anomalies would not occur at Sodwana. Without these temperature anomalies, the Sodwana coral reefs would become more vulnerable to climate change and coral bleaching.