Evaluation of Spectral Wave Models Physics as Applied to a 100‐Year Southern Hemisphere Extra‐Tropical Cyclone Sea State.

Extreme significant wave height estimates, and their probability of exceedance, are fundamental offshore and coastal engineering design parameters. These estimates are characterized by uncertainty due to an incomplete understanding of the atmosphere‐ocean energy and momentum exchanges during intense storms. This particularly affects extreme wave statistics of ocean regions exposed to large and frequent synoptic disturbances such as Extra‐Tropical Cyclones (ETCs). In this work, we assessed the performance of global phase‐averaged spectral wave models in representing the 1 in 100‐year sea state generated by a Southern Ocean ETC in April 2021. We collected in situ and remote sensing observations, from the storm generation region to its decaying phase and the impact on South‐East Australian coastlines. We compared the observations with a suite of reanalysis and hindcast global wave model data sets. While comparing well for wind speed up to 20 m/s, the models presented differences in solving the air‐sea momentum exchange between the atmosphere and the ocean for wind speed velocities between 20 and 35 m/s, which are a distinctive characteristic of ETCs. Despite marked differences in the storm generation region, the models converged to a similar representation of the swell systems impacting the South‐East Australian coastlines, as demonstrated by a comparison with deep‐water buoy observations close to the coastlines. Furthermore, we found that the energy of the ERA5 reanalysis, which assimilates satellite wave height measurements is quickly dispersed and, as such, of little advantage in representing the 9.9 m significant wave heights that impacted the South‐East Australian coastlines on 10 April 2021. Plain Language Summary: Ocean wind‐wave extreme events are a major driver of coastal erosion and a fundamental aspect in the design of offshore and coastal structures. Engineers rely on accurate one‐in‐N‐year significant wave height return period estimates, and in a changing climate, accurate estimates of wind‐wave extremes are also crucial to implement efficient and resilient coastal climate adaptation strategies. Global wave reanalysis data sets are commonly used to estimate wind and wave statistical properties and design parameters. However, despite the impressive accuracy of such data sets in representing average significant wave height conditions, models still underestimate wind‐wave extremes. In this work, we take advantage of a well‐documented one‐in‐100‐year sea state generated by a Southern Hemisphere Extra‐Tropical Cyclone (ETC) to analyze the performance of state‐of‐the‐art global wave models in representing wind‐wave extremes. The aim is to support informed extreme value analysis statistical studies and suggest improvements to wave models to better represent extreme climate in the Northern and Southern Hemisphere ocean regions exposed to ETCs. Key Points: The performance of phase‐averaged global wave models is evaluated for a 100‐year sea state generated by a Southern Hemisphere Extra‐Tropical Cyclone (ETC)Lack of understanding of air‐sea momentum exchange at wind speed between 20 and 35 m/s challenges the modeling of ETC sea statesNovel atmosphere‐ocean coupled models and new ETC sea state in situ measurements are needed to improve air‐sea momentum exchange modeling [ABSTRACT FROM AUTHOR]