Numerical modelling of the upwelling and associated hydrodynamics at various scales along the coral reefs at Sodwana Bay, South Africa

Abstract Coral reefs are strongly influenced by hydrodynamics and physical processes on various length and time scales, and understanding coral reef systems are crucial for the maintenance and survival of the reefs. Coral reefs around the world are under increasing threat to global climate change, and coral bleaching is a major concern for the health and survival of these reefs. Certain coral reefs are situated in high latitude areas where the complex ocean flow patterns interact with topographical features, providing certain refuges to rising ocean temperatures. 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. A flexible mesh hydrodynamic model of the southwest region of the Indian Ocean was developed to investigate short-term temperature anomalies on the Sodwana coral reef system located along the northeastern coast of South Africa. The model successfully replicates the observed temperature anomalies over a chosen representative year. The modelled on-reef temperatures during the temperature anomaly events were also compared to temperatures near Sodwana from the reanalysis NEMO global ocean model. The higher model resolution around Sodwana results in less numerical effects and smoothing of the temperature fields in the nearshore region when compared to the reanalysed NEMO model. A representative anomaly in Febru-ary 2004 was also investigated using the nested model to delineate the impact of local hydrodynamics in and around submarine canyons on the representative temperature anomaly.