Quantifying the effects of observational time resolution on marine heatwave detection

Abstract. Marine heatwaves (MHWs) are increasingly recognized for their significant impacts on coastal marine ecosystems. Following a proposal by Hobday et al. (2016) MHW are often defined as an event when the sea surface temperature (SST) is larger than the 90^th percentile of the local climatology, during at least 5 consecutive days. The definition of MHWs presents a methodological challenge: at high temporal resolution, short-term excursions below the 90^th percentile threshold can interrupt the continuity of an event and thus invalidate it as a MHW. In contrast, when the data are averaged over longer timescales (e.g., 10 hours), such fluctuations are smoothed out, potentially allowing the same event to meet the MHW criteria. A relaxed version (allowing 2-day gap) of the definition consists in discarding short-time interruptions. We tested the influence of temporal resolution by calculating the number of marine heatwaves detected in 6 coastal stations situated along the French coast, across different temporal aggregations, from raw (e.g., 20-minute) resolution to daily averages. Because short timescale variability generates numerous brief threshold exceedances, finer resolutions yield more frequent but shorter events, whereas coarser resolutions reduce frequency, maximum intensity and inflate duration. Using these coastal stations, we show that marine heatwave (MHW) metrics are systematically sensitive to changes in temporal resolution and event detection criteria, with strict and relaxed definitions converging—and in some cases crossing—as data are aggregated to coarser timescales. Overall, the resolution dependence of MHW metrics is systematic across sites, underscoring the need to consider the time scale when estimating and comparing MHWs.