Observation-based sea level forecasts for the Hawaiian Islands

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2021

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University of Hawaii at Manoa

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The Hawaiian Islands experienced record-high sea levels during 2017, which caused nuisance flooding in vulnerable coastal communities, especially when positive sea level anomalies coincided with high tides. To build toward solutions for mitigating inundation risk, the predictability of daily-averaged sea level anomalies is investigated. The investigation focused on leveraging observed westward propagation that open-ocean anomalies exhibit over a range of timescales to make sea level predictions. Daily near-real-time gridded altimetry was used to specify upstream sea level at each site with propagation speeds based on mode-one baroclinic Rossby wave speeds. An optimized observation-based forecast was created from weighted combinations of persistence and independent propagation-based forecasts for large ($>$300 km) and mesoscale ($<$300 km) open-ocean anomalies. Skill of the observation-based sea level forecasts exceeds persistence at long lead times out to 180 days. The utility of the observation-based sea level forecast was demonstrated by pairing the mean sea level forecast with a statistical model relating mean sea level to counts of flooding threshold exceedances. The combined model performed well in hindcasting seasonal periods of enhanced high-tide flooding at Hilo, underscoring the benefit of economical seasonal forecasts of mean sea level. Stakeholders can utilize the combined mean sea level and exceedance forecast model to assess flooding risks months in advance for facilitating preparedness across economic and coastal management sectors.

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Physical oceanography

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Hawaii

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