Identification of Marine Heatwaves and Their Characteristics in the Tidal Elbe River

Marine heatwaves (MHWs), prolonged periods of unusually high ocean temperatures, have been observed worldwide and are expected to increase in both intensity and frequency due to anthropogenic climate change. This rise in MHW frequency and intensity has led to significant biological and ecological shifts, including changes in species distributions, large-scale mortality, and the decline or extinction of local species. The tidal Elbe discharges into the German Bight. It plays a crucial role in the region's ecology, serving as a dynamic estuarine ecosystem where freshwater from the river meets saline water from the North Sea. Within the framework of the ElbeXtreme project, we aim to better understand extreme events like MHWs to enhance the resilience of coastal ecosystems and human communities.

To this end, we MHW events in the tidal Elbe region from the upper estuary at Bunthaus to the lower estuary at Cuxhaven to better understand their characteristics, trends, and impacts over the period from 1988 to 2023. The characteristics of MHWs, such as intensity, duration, and frequency, are compared across different measuring stations sustained by local authorities and institutions. Surface water temperature data show high confidence (p-value < 0.01) in the observed increasing trend (0.2°C/decade) in temperature anomalies across all stations. Moderate MHWs are frequently observed, with an annual mean of 2.3 events, at various stations from Bunthaus to Cuxhaven. Only a few strong MHW events (2-3× the local difference between the climatological mean and the climatological 90^th percentile) were identified in 2000, 2007, and 2018. The characteristics of MHWs show spatial variability along the estuary. The annual mean duration of events is approximately 33 days, decreasing from Cuxhaven to Bunthaus. In contrast, the annual mean intensity of events increases moving upstream. Meanwhile, the annual mean number of events along the estuary shows no significant change. Although the number and duration of MHWs do not show seasonal variation, summer and autumn exhibit stronger MHW intensity compared to spring and winter.