Temporal changes in sea surface temperatures and extreme events along the southern Agulhas Current

Abstract

Over the past several decades, both mean sea surface temperatures and extreme thermal events in and around the Agulhas Current have undergone notable shifts. Bayesian ensemble modelling identified significant changepoints in the seasonality and trends of southern Agulhas Current temperatures. Quantile regression further demonstrated that at coastal sites within upwelling cells along the eastern and western margins of the Agulhas Bank, mean and lowerbound (10th percentile) temperatures have declined, in contrast to the general warming observed across the broader Agulhas Current. While marine heatwaves have increased in frequency and dominate extreme events throughout much of the Agulhas system, the 2007 changepoint revealed a reversal at four of the five coastal sites: marine cold spells became the prevailing extreme events, rising sharply in both frequency and intensity, in the same coastal regions where mass fish kills and shoreline wash-ups have been observed over the past five years. This study demonstrates that following a critical shift in the temperature regime, extreme thermal events have become both more frequent and more severe, with potentially profound consequences for marine ecology and ecosystems.