Marine Heatwaves Variability and Trends in the Patagonian Shelf

Abstract. Marine heatwaves (MHWs), defined as periods of persistently anomalous warm ocean temperatures, have doubled in frequency worldwide in recent decades and are becoming longer, more intense, and increasingly disruptive to marine ecosystems. In this study, we use 42 years of satellite-derived daily sea surface temperature (SST) data to characterize the frequency, intensity, duration, and long-term trends of MHWs in the Patagonian Shelf (PS). On average, the PS experiences 2.5 events year^-1, with a cumulative duration of 20 to 30 days annually and intensities ranging from 0.5 °C to 2.5 °C. The northern PS shows clear evidence of an increase in MHW days (+5–10 days decade^-1), whereas no significant trends are observed in the southern region (i.e., south of 48° S). Across the PS, MHW intensity exhibits a modest downward trend of roughly −0.2 °C decade⁻¹. Part of MHW variability is attributable to the El Niño Southern Oscillation. In particular, the highest annual total of marine heat-wave days was observed during the strong La Niña event of 1998 and both MHW intensity and duration tend to increase during La Niña episodes, with MHW intensity showing a more consistent association with La Niña conditions. We also examine the influence of the MHW detection method, fixed versus moving climatology, on MHW statistics. We find that over the PS, the methodological impact on key MHW metrics is minimal, especially when compared to the deep ocean, where substantial background SST trends amplify methodological differences. These findings underscore the necessity of region-specific assessments of MHWs to elucidate their future evolution and pace of change within the broader context of climate change.