Refining the role of bathymetry, hydrodynamics and upwelling at various scales along the coral reefs at Sodwana Bay, South Africa

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Hydrodynamics and physical processes that occur at various length and time scales strongly influence coral reefs. Therefore, understanding the interactions between reefs, hydrodynamics and other physical processes is crucial for the maintenance and survival of reef systems. Coral reefs around the world are under increasing threat to global climate change, and additionally coral bleaching is a major concern for the health and survival of these reefs. Some marginal coral reefs are situated in areas where the complex ocean flow patterns interact with topographical features, providing possible refuges to rising ocean temperatures and coral bleaching. A prominent example is the Sodwana Bay coral reef system which has shown resilience to coral bleaching. This resilience has been attributed to cold water temperature anomalies that cause short-term temperature fluctuations on the reefs. This study explores hydrodynamics at various scales around the Sodwana Bay coral reefs and associated short-term temperature anomalies using a flexible mesh hydrodynamic model of the southwest region of the Indian Ocean, nested within a global ocean model. The nested hydrodynamic model better replicates the observed temperature anomalies when compared to the the reanalysis NEMO global ocean model. The higher model resolution around Sodwana results in less numerical mixing and smoothing of the temperature fields in the nearshore region when compared to the reanalysed NEMO global ocean model leading to a better replication of the local hydrodynamics around the Sodwana region. The anomalies investigated were associated with remote upwelling of cold water near the Delagoa Peninsula, followed by advection from the Delagoa Bight towards the Sodwana region. The separation of the strong intermittent southward stream from the Delagoa Peninsula is strongly linked to the upwelling at the Delagoa Peninsula. An analysis of the hydrodynamic patterns during the anomaly periods reveal that when the strong southward stream reattaches to the coastline, it typically does so south of Sodwana. The reattachment of the stream has an inertial effect and pushes the flow of water against the coastline which deflects the flow northwards up past Sodwana resulting in a northward current reversal along the Sodwana coastline which agrees with observed current reversals during the anomaly periods by insitu measurements taken on the Sodwana reefs. The model also revealed that local upwelling occurs within the Sodwana canyons during this event, making the water in the canyons colder than the surrounding water. When the locally upwelled water spreads over the reef system, the anomaly amplitude is enhanced by approximately 20 %.