Assessment of Future Nonstationary Changes in Extreme Sea Levels Considering Climate Change for Southeast Asia Using High‐Resolution Climate Forcing

Abstract This study investigates the projected nonstationary behavior of extreme total water levels (ETWL) and extreme coastal water levels (ECWL) along the Southeast Asian coastline under the high‐emission climate scenario. Using a high‐resolution seamless projection data set of storm surge and ocean waves forced by MRI‐AGCM3.2, combined with regional sea level anomaly, and tide data, we applied a nonstationary extreme value analysis (NEVA) framework to estimate future changes in magnitude, frequency, and component contributions. Four models of time‐dependent generalized extreme value were evaluated. Results indicate that approximately half of the study region exhibits significant nonstationary changes in extremes, with model choice influencing both trend direction and magnitude–particularly when scale parameter variation is included. Sea level rise (SLR) is the dominant contributor to future increases in ETWL and ECWL, with storm surge and wave components showing spatially heterogeneous changes driven by future shifts in tropical cyclones (TCs) frequency, intensity, and tracks. By the year 2100, present‐day 100‐year ETWL events in equatorial regions are projected to occur annually or more often, emphasizing the urgency for adaptation planning. These findings provide a comprehensive regional assessment of compound coastal hazards, offering insights to guide large‐scale risk management and coastal planning under climate change.