The rapid rise of severe marine heat wave systems

We introduce a new methodology to study marine heat waves (MHWs), extreme events in the sea surface temperature (SST) of the global ocean. Motivated by previously large and impactful MHWs and by theoretical expectation that the dominant heating processes coherently affect large regions of the ocean, we introduce a methodology from computer vision to construct marine heat wave systems (MWHSs) – the collation of SST extrema in dimensions of area and time. We identify 649 475 MHWSs in the 37 year period (1983–2019) of daily SST records and find that the duration tdur (days), maximum area Amax (km2), and total ‘volume’ VMHWS (days km2) for the majority of MHWSs are well-described by power-law distributions: tdur−3,Amax−2 and VMHWS−2 . These characteristics confirm SST extrema exhibit strong spatial coherence that define the formation and evolution of MHWs. Furthermore, the most severe MHWSs deviate from these power-laws and are the dominant manifestation of MHWs: extrema in ocean heating are driven by the ∼200 systems with largest area and duration. We further demonstrate that the previously purported rise in the incidence of MHW events over the past decade is only significant in these severe systems. A change point analysis reveals a rapid increase in days under a severe MHW in most regions of the global ocean over the period of 2000–2005. Understanding the origin and impacts of MHWs in the current and future ocean, therefore, should focus on the production and evolution of the largest-scale and longest-duration heating phenomena.