PhD Defense: Extratropical storm-generated swell induced vulnerability effects on the tropical islands of Hawai‘i

Yaprak Onat

The poleward shift of strong extratropical storms due to the change in the baroclinicity raises the question of how the storm intensification affects the susceptibility of distant remote islands under high wave energy environments. This study aims to identify the effective linkages between the intensification of extratropical storms and the corresponding swells in order to reduce the uncertainty in prioritizing vulnerable coastal systems in Hawai‘i. The minimum mean sea level pressure and geopotential height, and maximum vorticity are used as a criteria to define strong cyclonic activity from stationary points from an atmospheric reanalysis dataset to hindcast swell states of the North Pacific from 2007-2017. The de-seasonalized trend of the northwest swells and the spatial distribution of the wave exposure are visualized in an index-based coastal vulnerability GIS model to classify coastal exposure. The correlation between strong extratropical cyclones and swells showed an increase in the frequency of swells, which accounted for a quarter of the total swells reaching the Hawaiian Islands over the record period. The significant wave height and peak period of the associated swells at the northwest of O‘ahu displays a significant upward trend of up to 0.51 m and 1.72 s, respectively, while keeping a rather stable direction range of 325-330º during the record period. These swells contribute to the already alarming 34% of the medium to high vulnerability of the coastlines of the Hawaiian Islands. Understanding the dominant factors affecting shoreline vulnerability and the impact of strong extratropical storm-generated swells related to their susceptibility allows the formulation of better strategies to more effectively mitigate the potential risk for Pacific Island communities. The value of this work lies in both identifying the swell trends and customizing the proposed framework to determine crucial elements that increase the susceptibility of critically exposed shoreline segments. This work provides a guide for policymakers to promote public awareness and support deliberation, planning, and design of adaptation strategies.