Cold water pathways and regional flow patterns in the southwest Indian ocean associated with temperature anomalies on the Sodwana reef system, South Africa

Abstract

Coral reefs around the world are under increasing threat to global climate change and coral bleaching events that can result in severe damage and mortality to the reef systems. Some coral reefs are situated in areas where the complex ocean flow patterns and their interaction with topographical features around the reefs provide certain refuges to the rising ocean temperatures and coral bleaching. The Sodwana reef system experiences cold water temperature anomalies that cause short-term temperature fluctuations on the reefs. It is suggested that these temperature anomalies provide relief from bleaching and are an integral part of the coral’s survival at Sodwana. Previous studies have shown that the temperature anomalies are linked to regional scale hydrodynamic features surrounding Sodwana. We investigated the cold water pathways and regional flow patterns associated with the temperature anomaly events between 1994 and 2015 using data from the global reanalysed NEMO ocean model output. A comparison between temperature measurements taken on the Sodwana reefs and temperature data extracted from the model near Sodwana shows a good agreement and that the model replicates the temperature anomalies. This indicates that the temperature anomalies are associated with the regional hydrodynamic flow structures replicated in the NEMO model. We used a Lagrangian particle backtracking model and the hydrodynamic data from the NEMO model to delineate the pathways associated with the 63 temperature anomalies observed between 1994 and 2015. The pathways delineated from the Lagrangian particle backtracking model originate to the northeast of Sodwana within the Delagoa Bight region, indicating that hydrodynamics around the Delagoa Bight is linked to the cold water temperature anomalies observed at Sodwana. A statistical analysis of the modelled regional Eulerian flow fields showed that two representative regional flow fields are associated with the anomalies and representative pathways. The first representative flow field features a high-speed concentrated southward stream that separates from the Delagoa Bight peninsula and flows along the seaward edge of the Delagoa shelf and a cyclonic eddy structure within the Delagoa Bight. The second representative flow field features the southward stream that separates from the Delagoa Bight peninsula and flows directly to the Sodwana region with a significantly smaller separation zone within the Delagoa Bight. The second representative flow field shows no observable cyclonic eddy structure. The southward stream observed in all representative flow fields reattaches to the South African coastline near the Sodwana region. We also show through the mean particle vertical displacement and temperature change along the anomaly pathways that the nearshore zone within 50 km of Sodwana is a region of enhanced upwelling and mixing during the anomaly periods.