Changes in the marine environment have been reported for over three decades in terms of mean annual wave heights, exceedance probabilities and extreme conditions. More recently, the existence of a link between these changes and anthropogenic climate change has been postulated. This is not unreasonable, as climatic changes in regional warming and cooling are likely to alter wind patterns, and therefore the wave climate as well. In an attempt to mitigate climate change and increase energy security, the offshore environment is being looked at to provide sustainable energy from wind, waves and tides. As a result the number of marine structures is only likely to increase. While survivability in this environment is essential for all such installations, some devices such as wave energy converters also rely on the environment for energy production. In designing these offshore structures to survive the harshest conditions as well as to ensure optimum operation, knowledge of the evolution of the wave climate is essential. This study aims to identify and evaluate any historical trends that may be exhibited by the wave climate in the North East Atlantic and North Sea region. The study also aims to investigate the link between any observed changes and atmospheric greenhouse gas levels and projected wave conditions for the 21st century. This is achieved by producing a long-term, high resolution hindcast of wave conditions for 1871-2010 using the third-generation spectral wave model WAVEWATCH III. A dataset of wave climate projections for the high, medium and low emissions scenarios is also prepared by forcing the model with GCM winds for 2001-2100. In addition to dynamically projecting the wave climate in the 21st century for different IPCC climate change scenarios, statistical methods were applied to historic data to estimate extreme events in terms of 100-year return values of significant wave height. These, together, provide some idea of the plausible wave climate up to 2100. The results of the work show the existence of long-term trends in the historical wave climate in the region from 1921 onwards. However, based on the findings of the study, it is unlikely that these are a result of changes in atmospheric greenhouse gas concentrations and are more likely due to internal variability in the system.

Rogue waves are unexpected, individual ocean surface waves that are disproportionately large compared to the background sea state. They present considerable risk to mariners and offshore structures when encountered in large seas. Rogue waves have gone from seafarer’s folktales to an actively researched and debated phenomenon. In this work an easily derived spectral parameter, as an indicator of rogue wave risk, is presented, and further evidence for the generation mechanism responsible for these abnormal waves is provided. With the additional goal of providing a practical rogue wave forecast, the ability of a standard wave model to predict the rogue wave probability is assessed. Current forecasts, like those at the European Centre for Medium-Range Weather Forecasts (ECMWF), rely on the Benjamin Feir Index (BFI) as a rogue wave predictor, which reflects the nonlinear process of modulation instability as the generation mechanism for rogue waves. However, this analysis finds BFI has little predictive power in the real ocean. From the analysis of long term sea surface elevation records in nearshore areas and hourly bulk statistics from open ocean and coastal buoys in the Northeast Pacific, crest-trough correlation shows the highest correlation with rogue wave probability. These results provide evidence in support of a probabilistic prediction of rogue waves based on random linear superposition and should replace forecasts based on modulation instability. Crest-trough correlation was then forecast by a regional WAVEWATCH III ® wave model with moderate accuracy compared with the high performance of forecasting significant wave height. Results from a case study of a large fall storm October 21-22, 2021, are presented to show that the regional wave model produces accurate forecasts of significant wave height at high seas and presents a potential rogue wave probability forecast. / Graduate

The need for a consistent and accurate production of long-term nearshore wave record is discussed. With multiple decades of offshore hindcasts and long, continuous data sets available, it is possible to create a nearshore synthetic wave record. The Brevard County coastline offers an area with a high quality 62-year long offshore hindcast, as well as an 11-year long and nearly continuous high-resolution nearshore wave record to compare with model performance. This thesis presents the steps in the development and performance of the synthetic nearshore wave record produced. A novel approach was used to compare, validate and calibrate this type of data which included using quantile-quantile plots and bin-averaged scatter plots. In a comparison between two reputable deep-water hindcasts (MSC50 and Wavewatch III), it was found that Wavewatch III significantly underpredicts wave heights in the higher range (>8m). At the nearshore STWAVE proves to be a simple, robust and fast way to create a nearshore wave record. Root mean squared error (0.272m-0.317m) and modified index of agreement (0.697-0.646) values for significant wave height show promising results for overall model performance with the currently available hindcast. Possible future improvements could be made by modifying the offshore hindcast to have finer grid resolution and further studying different friction models for the nearshore wave transformation model. Overall, the use of the MSC50 hindcast, to drive STWAVE at the nearshore, exhibits good agreement with ADCP data and analysis for significant wave heights can be used with confidence. Currently, no long-term trends can be resolved with the available record at the location used herein, yet more years of data/hindcasts in the future could provide more evident trends in wave climate change.