Multi-Model Ensemble Sea Level Forecasts for Tropical Pacific Islands

Sea level extremes impact tropical Pacific islands on wide-ranging timescales, often with too little warning to mitigate risks. With El Niño, such as the strong 2015–2016 event, comes weaker trade winds and sea level drops exceeding 30 cm in the western Pacific that expose shallow-water ecosystems at low tides. Nearly opposite climate conditions accompany La Niña events, which threaten coastlines with sea level high stands (10–20 cm) and cause more frequent storm-related inundations that in the past have been exacerbated by decadal sea level variability, as well as continuing global sea level rise. Climate models, which are increasingly better able to simulate past and future evolutions of phenomena responsible for these extremes (i.e., El Niño-Southern Oscillation, Pacific Decadal Oscillation, and greenhouse warming), are also able to describe, or even directly simulate, the sea level fluctuations. By compiling monthly sea level anomaly predictions from multiple statistical and dynamical (coupled ocean-atmosphere) models, which are typically skillful out to at least 6 months in the tropical Pacific, improved future outlooks are achieved. From this multi-model ensemble comes forecasts that are less prone to individual model errors and also an uncertainty measurement achieved by comparing retrospective forecasts with the observed sea level. This framework delivers a new real-time forecasting product of monthly mean sea level anomalies that is being served online to the Pacific island community.