Monitoring and predictions of Marine Heatwave events in the North East Pacific from ocean reanalyses and seasonal forecasts

A marine heatwave (MHW) is defined as a prolonged period (usually 5 days or more) of sea-surface temperature (SST) above the 90th climatological percentile, which is potentially devastating for marine ecosystems and economy. The available ocean information by Copernicus Marine (CMEMS) and Climate (C3S) Services allows the real-time detection and seasonal prediction of MHW. Reported 2020 MHW events in the North East Pacific happen in the context of increased frequency of long heatwaves. A positive feedback loop by which atmospheric conditions impact the upper ocean stratification making the ocean mixed layer more responsive to anomalous surface fluxes has been identified in reanalyses. The increased stratification at the base of the mixed layer seen since 2017 coincides with the resurgence of MHWs from 2018 onwards. Reliable predictions of developing MHW conditions could help advance planning and preparedness for such extreme variability events in the ocean. Seasonal forecasts showed skill in predicting the 2020 events at seasonal timescales, especially once the ocean was preconditioned after the first MHW of that year. The first order assessment of the forecast skill for MHW predictions presented here showed encouraging results, but for such information to be actionable in the future there is need to gain more confidence on the quality of the seasonal forecast information. Statistical forecast reliability quantification and further process understanding will be the subject of a follow-up study.