Advancing global hindcast of extreme sea levels: Insights from a 65-year study

Extreme sea levels (ESLs) are a leading cause of coastal hazards. Assessing risks and associated impacts requires reliable ESL statistics. These are typically derived from long but sparsely available tide-gauge records or through records obtained from long hindcasts. Here we present a 65-year global hindcast of hourly total sea levels that dynamically includes contributions from storm surges, tides, changes in water density (or baroclinicity) and their interactions. Evaluation shows good agreement between modelled and available observed sea levels, including extremes driven by extratropical and tropical cyclones. Significant improvements over other simulations result from our efforts in addressing underestimated reanalysis winds and incorporating baroclinicity, both of which have been overlooked in other global studies. The improvements can translate into reductions of return periods for given critical levels by decades. We therefore provide improved global estimates of ESL. In a first step toward developing seasonal forecast of flood risk, we also quantified ENSO-induced ESL modulations. The modulations show coherent spatial variabilities, consistent with ENSO-induced changes in the atmosphere and ocean. We also highlight the relevance of the often-overlooked neutral phase in regions where both El Niño and La Niña may suppress sea level variabilities.