Investigating the predictability of marine heatwaves at subseasonal to seasonal timescales in New Caledonia, South Pacific

Abstract

Marine Heatwaves (MHWs) have emerged as oneof the most important threat for marine ecosystems, with impacts such as coral bleaching, massive fish mortality and displacement of mobile fauna. In the context of climate change,it is urgent to develop strategies such as subseasonal to seasonal forecasting to help human societies adapt and reactto the increasing frequency, duration and intensity of theseevents. Here we evaluate the predictability of MHWs at thescale of a South Pacific island country, New Caledonia, using ensemble forecasts from a dynamical coupled oceanatmosphere model. We show that implementing a probabilistic approach where we extract information from the dispersion in the ensemble results in a higher skill than a deterministic approach where we simply compute the ensemble average. We find that longer, more intense, and wider MHWs,are more predictable than weaker, less intense, and shorterMHWs. We also find that the longest and widest MHWs occur in the cold season (June–October) during strong La Niñaepisodes, and that they can successfully be predicted up to7 months in advance. In contrast, MHWs occurring duringthe warm season have poor or no predictability of more thana few weeks in advance. We discuss how this information canbe efficiently transferred to marine stakeholders in terms ofthe usefulness and useability of the forecast. We recommendthat future research should focus on identifying the drivers ofdifferent types of MHWs in order to understand their sourcesof predictability.