Characterisation of past marine heatwaves around South Pacific Island countries: what really matters?

Abstract. Marine heatwaves (MHWs) can have devastating and lasting impacts on marine ecosystems. We investigated past MHW characteristics around 12 southwestern Pacific Island countries and territories (PICTs) using two observed sea surface temperature products and an ocean reanalysis product. PICTs are highly dependent on their marine resources for their livelihoods: a better understanding of MHW characteristics is needed for planning and adaptation to risks associated with MHWs. Our research builds on previous studies where MHWs have been detected and described using a point-based definition. We first revisit past MHW characteristics based on their spatial extent, vertical extent and seasonality. We show that filtering MHWs by size (spatial extent) and seasonality can greatly affect their characterisation and help trace their physical drivers. We then characterise past events inside each Exclusive Economic Zone (EEZ) and at the coast with MHW indices tailored to benefit Pacific Island stakeholders. We consider two types of events: large-scale events, covering a large part of the EEZ, likely to affect pelagic fisheries, and events affecting coastal zones and ecosystems. We distinguish between events occurring in the hot season (November to April), and in the cold season (May to October). We show that all 12 PICTs experienced MHWs in the past 30 years that are getting more frequent with greater spatial extents, longer durations, explained by the long-term warming trend in sea surface temperature, but with lower maximum intensity. New Caledonia, Vanuatu, Fiji and Tonga appear to be more exposed to MHWs with longer duration, higher maximum intensity, and deeper extent compared to other countries.