Temperature variability of a western boundary current temperate embayment

Coastal sea temperature is closely linked to the earth’s climate system and to biogeochemical cycles operating at global, regional and local scales. Algoa Bay, situated along the warm-temperate south-east coast of South Africa, is a highly dynamic log-spiral embayment influenced by a western boundary current exhibiting pronounced seasonality and interannual variability. This study provides an overview of sea temperature in Algoa Bay, characterising temperature dynamics and describing both the mean state and variability. The thermal structure exhibited variability in stratification at daily, monthly and interannual timescales. Monthly temperatures displayed a well-defined summer thermocline, with evidence of cold-water intrusion at offshore stations and at depth. Interannual variability indicated warm conditions during 2012/13 and 2014, contrasted by cooler conditions in 2009-2011, 2015-2017, 2022 and 2023 with evidence of annual, biennial and quadrennial cycles. Five temperature regions were identified using hierarchical clustering. Our results confirm that sea temperatures in Algoa Bay oscillate between warm and cool phases over time, with clear station- and depth-dependent trends. Although these trends were not statistically significant, Surface and Subsurface (10 m) waters exhibited gradual warming over time, while temperatures below displayed a cooling tendency. Similarly, temperature variability also increased over time in all regions, except in Offshore deep waters where variability declined. In addition, 85 marine heatwaves and 87 marine cold-spells were identified, underscoring the need for further investigation into their drivers and impacts. Overall, these results provide a foundational overview of the complex sea temperature dynamics of Algoa Bay that can inform mitigation and management planning for aquaculture, climate change resilience, fisheries, energy infrastructure and biodiversity conservation.