Global propagation of marine heatwaves

Marine heatwaves (MHWs) are extreme warming events in the ocean that can significantly impact marine ecosystems and economies. While such extreme events have garnered significant research attention in recent years, most research to date took an Eulerian perspective, that is, they considered MHW as stationary objects. This disregards the fact that MHW are three dimensional objects that tend to propagate in space and time. 

In this study, we overcome this limitation and employ marEX, a novel method inspired by Ocetrac, to analyze and track MHWs globally. We limit outselves here to surface MHW, but the method is designed to extend to depth. Starting from a grid-level 95th percentile threshold-based detection following (A.J. Hobday et al., 2016), this method first creates coherent two-dimensional MHW objects by detecting contiguous regions of extreme sea-temperature anomalies at each time step. These regions are then linked over time using robust criteria that account for both merging (when two or more MHW regions combine into one) and splitting (when a single MHW divides into separate regions). This is achieved by tracking each piece of an event individually while maintaining links between them. This ensures that the full lifecycle of each MHW, including interactions between different events, is accurately recorded.

We test this method using ~40 years of simulated sea-surface data from the European Eddy RIch Earth System Models (EERIE) project. We find that most MHWs tend to form and dissipate in the same regions, indicating persistent hotspots for their development and termination. But there are also a substantial number of MHWs that persist for several months and propagate substantially, akin to the North Pacific “Blob event”. While the tracked MHW have, on average, a duration of ~14 days, the longest one propagated for as long as 520 days. These findings underscore the value of tracking MHWs as dynamic events, demonstrating that their propagation pathways and lifecycles hold crucial information.