Global to coastal modulation of wind-wave climates by ocean surface currents

• Main Author: Echevarria, AR
• Format: Text
• Language: unknown
• Published: University of Tasmania 2021
• Subjects: Antarctic; Southern Ocean; The Antarctic; Pacific; Antarc*
• Online Access: https://dx.doi.org/10.25959/100.00039484; https://eprints.utas.edu.au/id/eprint/39484

This thesis examines the influence of the ocean surface circulation on the global wind-wave climate. Surface currents modulate wave properties through the Doppler shift effect, exchanges of energy mediated by the radiation stress, refraction, and through a modification of the relative wind. While the interaction between waves and currents has been thoroughly investigated in coastal locations with strong tidal flows, the impact of the global ocean circulation on the wave climate has received relatively little attention. In this study, spectral wind-wave simulations with and without current forcing were carried out using the WAVEWATCH III model. The current forcing data for the simulation with currents was taken from the Bluelink Reanalysis. Numerous wave observations taken from moored buoys, altimeter and Synthetic Aperture Radar (SAR) borne satellites were employed to carry out a comprehensive evaluation of both sets of wave simulations (with and without currents) to assess, describe and quantify the improvement in the representation of various wave parameters with the inclusion of currents. This dissertation examines and discusses the benefits of using directional wave spectra information for the description of the wave climate, as opposed to using integrated parameters. Drawbacks of the utilisation of bulk wave parameters are described, and circumstances where their usage leads to an imprecise description of the wave conditions are presented. First, a novel methodology is devised to investigate the seasonal variability of the global spectral wind-wave climate. By means of an empirical orthogonal function analysis, the main patterns of wave variability in the frequency/direction domain are extracted, together with their time evolution (Principal Component time series). This methodology that captures the spectral characteristics of the wave _eld provides a more comprehensive description of the wave climate variability in most areas of the world, with its largest influence found for low-latitude regions, especially in the Pacific Ocean, where various distinct wave systems (partitions) are present in the spectra. This approach also facilitates the quantification of changes in wave direction throughout the year. This analysis is also applied to evaluate the interannual variability of the global spectral wind-wave climate and to quantify how this variability is modulated by different large-scale climate modes. Importantly, we find that the Pacific South American modes (PSA-1 and PSA-2, the second and third modes of the Southern Hemisphere atmospheric circulation variability) have a remarkable impact on modelled wave heights and directions, especially across the South Pacific Ocean. The sensitivity of wind-wave modelling to the inclusion of surface current forcing is analysed by comparing results from simulations forced with and without currents. A significant improvement in significant wave height estimates in most areas of the world is attained in the current-forced simulation, especially in the Southern Ocean. Here, the positive bias in wave heights is lessened due to a reduced relative wind, a product of the Southern Hemisphere westerly winds and the Antarctic Circumpolar Current travelling in the same direction. The Equatorial currents and Counter-Current, as well as the western boundary currents around the world, induce broad and large changes in wave heights, periods and directions. The East Australian Current has a significant impact on the wave climate measured at the coast. Further, a comprehensive comparison with moored buoy observations around Australia and the United States shows statistically significant improvements in estimates of different wave parameters in the current-forced simulation. Finally, estimates of wind friction velocity, atmosphere-to-ocean energy and momentum fluxes, whitecap coverage, and Stokes drift speed and direction are substantially modified in Equatorial regions with the inclusion of currents into wave simulations. Numerous implications of these results on other wave processes are discussed. SAR borne satellites measure the directional wave spectra for long waves (swell). A thorough inter-comparison between Sentinel-1 SAR wave observations and modelled wave spectra from the Centre for Australian Weather and Climate Research (CAWCR) waves hindcast for the greater Australian region is presented. In particular, an exhaustive regional-scale evaluation of swell wave periods and directions is presented for the first time. SAR observations are divided by platform, incidence angles and orbit directions, which enables the identification of inherent errors in the model simulations and the observations. In addition, directional wave spectra from SAR are used to provide a detailed description of the seasonal variability of the spectral wave climate around Australia. Finally, the influence and importance of the ocean circulation on swell waves in the Australian region is assessed by comparing SAR observed directional wave spectra with wave simulations with and without surface current forcing. SAR measurements from Sentinel-1 satellites are co-located against a dense spectral output grid from the wave model. This comparison shows that swell significant wave height, mean period and peak direction estimates are generally improved in the current-forced simulation. Measurements of directional wave spectra from SAR are used to provide more clarity into the differences observed. In particular, swell wave periods are broadly improved, especially southwest of Australia. Overall, the results presented in this thesis demonstrate that surface currents are an essential consideration for the modelling of wind-waves in many areas of the world. Additionally, this work illustrates the benefits of describing the wind-wave climate using directional wave spectra data, which leads to a deeper understanding of the characteristics of the wind-wave fi_eld, and can enable further improvements in spectral wave models.