Predictability of Persistent Marine Heatwaves in the Extratropical Pacific

Marine heatwaves (MHWs) are extreme events that occur over a range of spatial and temporal scales, impacting ocean ecology, fisheries, and weather patterns. The largest, most intense, and most persistent MHWs occur at the 1000 km and seasonal-to-interannual scales, indicating the potential for predictability through slowly evolving climate drivers. Here, we investigate the atmospheric and oceanic processes responsible for predictability of the 2013-2015 Northeast Pacific Marine Heatwave (known colloquially as "The Blob") using the Community Earth System Model (CESM) Seasonal to Multi Year Large Ensemble (SMYLE) prediction system. We find that the rapid evolution of the Blob is not captured in any SMYLE ensemble hindcast initialized prior to 2014. However, after the January 2014 peak in Northeast Pacific sea surface temperature (SST) anomaly, hindcasts can reproduce observed SST anomaly magnitudes as well as North Pacific SST anomaly patterns. Furthermore, the SMYLE ensemble members most closely matching the observed 2014 phase transition of the Pacific Decadal Oscillation (PDO) also showed a marked improvement in Blob prediction. Likewise, SMYLE members matching observed SST in the Nino4 region showed similar improvement, although this did not hold true for SST the Nino3.4 region. Based on these results and prior work, we find that both the evolution of the PDO and teleconnections to the Western Tropical Pacific are essential for Northeast Pacific MHW predictability.