Analysis of estuarine marine heatwaves in an upwelling system: The Ría de Arousa as a case study

Marine heatwaves are prolonged periods of anomalously high water temperatures and have significant ecological and economic impacts. While these events are well-documented in open ocean systems, their characteristics and drivers in estuarine environments, particularly within upwelling systems, are less understood. This study analyzes estuarine marine heatwaves (EMHWs) in the Ría de Arousa, a productive estuary influenced by the upwelling dynamics of the NW Iberian Peninsula, using measured data from 2008 to 2023. Thirty-eight EMHW events were identified, with their occurrence strongly linked to oceanic thermal anomalies.

Among large-scale climate indexes, the Atlantic Multidecadal Oscillation (AMO) exhibited a significant positive correlation with EMHWs, explaining 13.8 % of their variability. In contrast, the East Atlantic (EA) and North Atlantic Oscillation (NAO) indexes showed no significant relationship. MHWs and extreme temperature events (METDs) at the continental shelf and open ocean were the strongest predictors of EMHWs. In contrast, atmospheric heatwaves (AHWs) have a minor influence.

The upwelling index exhibited a weak negative correlation, explaining only 0.17 % of EMHW variability, while wind intensity accounted for 0.32 %, suggesting a limited buffering effect of upwelling on EMHWs. A marked seasonal pattern was observed, with EMHWs being more intense in October than in summer, likely due to reduced upwelling activity in autumn. While upwelling may exert a mild regulatory effect, its capacity to mitigate extreme events appears limited. Understanding these interactions is crucial for improving predictive models and managing the impacts of EMHWs in ecologically and economically significant estuarine environments.