The need to adopt process-based or impact-based definitions for marine heatwaves

The oceanographic and climate communities are putting significant effort into reaching a consensus on a common definition for Marine Heatwaves (MHW). The absence of such a unified definition poses a substantial obstacle when it comes to making retrospective comparisons between various MHW studies. This hindrance is critical for achieving a mechanistic understanding of the role of MHWs in marine ecosystems.

However, why is it so challenging to characterize and define MHWs? The answer is straightforward: there isn't a single, distinct dynamical mechanism responsible for the persistence of heat anomalies in the ocean, which we refer to as MHWs. Unlike variability associated with phenomena such as large oceanic eddies, oceanic fronts, upwelling systems, tropical cyclones, or climate modes, prolonged heat anomalies do not exhibit characteristic time or spatial scales. As a result, common MHW definitions group together prolonged temperature anomalies lasting from days to years and spanning from a few kilometers to thousands of kilometers in scale.

Analyzing sea surface temperature anomalies through power spectra reveals a "red" power spectrum with no discernible time scales. A similar analysis in spatial dimensions similarly shows a lack of any specific scale. Given this absence of emergent scales, we suggest adopting a process-based definition for MHWs. Such an approach would classify all events into a smaller number of categories, each linked to a specific driver or dynamical process operating on certain spatiotemporal scales. This shift could significantly reduce the subjectivity involved in selecting the temporal and spatial scales required for current MHW definitions, ultimately advancing our understanding of these events.