Lagrangian attribution of marine heatwave drivers

Abstract Marine heatwaves drive ecosystem change across the globe and are, with ongoing global warming, key indicators for the evaluation of future climates. Marine heatwave drivers are typically compound events of teleconnections and local weather. Strictly locally, marine heatwaves are induced by heat fluxes through the surface of the investigated area, accumulating until the temperatures exceed a certain threshold. For large parts of the world ocean, drivers of the strongest of the marine heatwaves, were shown to have a dominant contribution from horizontal advection, however, whether the advected water was warmed up through surrounding surface forcing or originates from the interior ocean is unknown. Here, on an example from the extreme 2005 eastern Caribbean Sea marine heatwave, I demonstrate a method that allows spatial and temporal attribution of horizontal advection of heat anomalies to their sources. The Lagrangian back-tracking method, applied within the mixed layer, successfully attributes advective heat-anomaly sources to either the atmosphere (surface) or the ocean (grid-scale lateral mixing and mixed layer base exchange). Furthermore, the spatial origin of marine heatwave drivers is seen for the first time, showing the non-locality of marine heatwave drivers. The presented method greatly expands the possibilities to attribute drivers of marine heatwaves beyond the local effects and rather to their respective large-scale causes. Many ecosystems worldwide have already been repeatedly hit by marine heatwaves severe enough to lead to local extinction events. Even when ecosystems recover, the next global scale compound event is sure to come with rising temperatures due to climate change. Tools such as the presented method can greatly improve our understanding of past climate catastrophes, and thus, help understand future risks for our remaining ecosystems.