Marine heatwaves variability and trends in the Patagonian Shelf

Abstract. Marine heatwaves (MHWs), have doubled in frequency globally in recent decades and are becoming longer, more intense, and increasingly disruptive to marine ecosystems. However, despite their growing ecological and biogeochemical importance, major productive coastal systems remain understudied, particularly in the Southern Hemisphere. Here, we provide the first comprehensive characterization of MHWs across the Patagonian Shelf (PS), one of the most biologically productive marine regions on Earth, using 40 years of satellite-derived daily sea surface temperature (SST) data. We first assess how the choice of MHW detection method (fixed versus moving climatology) and SST-dataset selection affect MHW metrics. Then we quantify MHW frequency, intensity, duration, and long-term trends, revealing that the PS experiences on average 1.9 ± 2 MHWs yr−1 with a mean cumulative duration of 23–28 d yr−1 and an average intensity of 1.36 ± 0.3 °C. We show that MHW activity varies substantially across the region, with the northern sector and the outer shelf experiencing the most frequent and intense events (>2 events yr−1 and >2 °C). A notable increase in MHW days (+5–10 d per decade) is observed in the northern PS, whereas no significant trends are observed to the south (i.e., south of 48° S). These trends are consistent with background warming of the ocean in this region, suggesting a mechanistic link, whereby long-term warming enhances the likelihood of MHWs occurrence and duration. We further demonstrate that a component of MHW variability can be attributed to the El Niño Southern Oscillation, which exerts a stronger influence on the intensity of thermal anomalies than on the cumulative duration of the events. Together, these findings constitute the first comprehensive assessment of MHWs on the PS and provide essential insight for anticipating their ecological and climatic impacts in one of the Southern Hemisphere's key marine ecosystems.