Elsevier

Ocean Modelling

Volume 172, April 2022, 101980
Ocean Modelling

The wave climate of Bass Strait and South-East Australia

https://doi.org/10.1016/j.ocemod.2022.101980Get rights and content

Highlights

  • High-resolution wave model for SE Australia validated against extensive buoy data.

  • Mean, seasonal, and extreme wave conditions are examined.

  • Trends over the last 40-years show an increasing wave climate.

  • Changes in Southern Ocean swell have a major impact on the local wave climate.

  • Sheltering in the region results in high spatial gradients in wave climate.

Abstract

A high-resolution third-generation wave model based on unstructured grids, WAVEWATCH III, was used to investigate the wave climate of Bass Strait and South-East Australia over the period 1981 to 2020. The model results are extensively validated against a network of coastal buoys and demonstrate that the model can capture the overall wave characteristics in this region. Analyses of model outputs across the 40-year period show that significant wave height has increased by approximately 5% and a slight counterclockwise rotation of peak wave direction has occurred with likely implications for coastal processes. Seasonal variations show higher significant wave height in winter compared to summer, which is driven by dominant Southern Ocean swell. The peak wave direction in the eastern region shifts from south-westerly in winter to south-easterly in summer. In autumn and winter, there is a statistically significant correlation between wave conditions and the Southern Annular Mode. During these seasons, a southward movement of Southern Ocean low pressure systems is associated with increased significant wave height, an increase in the peak wave period and a counterclockwise rotation of the peak wave direction.

Introduction

Understanding regional ocean wave climate is critical for a range of reasons. Beach morphology and shoreline erosion are highly influenced by ocean waves (Kirezci et al., 2020, Leach et al., 2021, McSweeney and Shulmeisterb, 2018) and nearshore breaking waves play an important role in the stability and erosion/accretion of beaches. Wave climate also influences the distribution and productivity of coastal and marine communities, with wave energy influencing the morphology, community structure and spatial organization of taxa in the marine coastal zone through a number of direct and indirect processes (Rattray et al., 2015, Young et al., 2020b). Ocean waves also represent a major design input to the construction and operation of coastal facilities (Akpinar et al., 2016, Jenkins et al., 2012, Lv et al., 2014). The increase in the offshore renewable energy industry has opened up the development of nearshore regions where an understanding of wave climate becomes an important design consideration (Ching-Piao et al., 2012, Cornett, 2008, Flocard et al., 2016, Morim et al., 2014, Morim et al., 2016, Ribal et al., 2020). Ocean waves also define the aerodynamic roughness of the air–water interface which impacts the flux of energy and gases across the interface and defines the shape of the atmospheric boundary layer (Jenkins et al., 2012).

Numerous studies have investigated the wave climate at both global and regional scales using satellite observations (Lin et al., 2019, Liu et al., 2016, Young, 1999, Young et al., 2020a, Young and Ribal, 2019), reanalysis datasets (Harley et al., 2010, Hemer et al., 2010), and/or ocean wave model hindcasts (Liang et al., 2019, Mori et al., 2017, Shimura and Mori, 2019, Young et al., 2020a). The wave models used have included WAVEWATCH III (WW3) (WW3DG, 2019), SWAN (Booij et al., 1999, SWAN Team, 2019) and WAM (WAMDI WAMDI Group, 1998). Relationships between observed or modelled wave parameters and climate indices have also been studied, for example, El Niño-Southern Oscillation (Harley et al., 2010, Hemer et al., 2010, McSweeney, 2020, Tsai et al., 2012), Southern Annular Mode (Cuttler et al., 2020, Harley et al., 2010, Hemer et al., 2010, McSweeney, 2020), Arctic Oscillation (Liu et al., 2016) and subtropical high-pressure regime and intensity (Cuttler et al., 2020).

Based on a 31-year long satellite dataset from 1985 to 2018, Young and Ribal (2019) found extreme conditions of global oceanic wind speed and significant wave height had increased over the study period, with the largest impact in the Southern Ocean, while the changes in the corresponding mean values were relatively small. Liu et al. (2016) reported the Arctic Oscillation and Arctic dipole anomaly have clear impacts on the wind and wave climate of the Arctic Ocean based upon 20-years of satellite observations. By nesting a regional high-resolution WAVEWATCH III (WW3) model within a global model, Shimura and Mori (2019) distinguished different wave spectra types around Japan and attributed their variations to atmospheric conditions through empiric orthogonal function analysis.

The Victorian coast in south-east Australia (Fig. 1) is one of the most densely populated regions in Australia (Morim et al., 2016). Understanding wave climate in this region is crucial for coastal applications. The wave climate of this region is heavily influenced by Southern Ocean swell (McSweeney, 2020). Global scale observations (Young and Ribal, 2019, Young et al., 2011) and model hindcasts (Meucci et al., 2020a, Morim et al., 2019) have indicated that the wave climate in the Southern Ocean is changing faster than any other region on Earth. Hence, an understanding of the changes which have occurred in this nearshore region has both local and global significance.

Over the past few decades, satellite altimeters have provided global scale measurements of significant wave height (Ribal and Young, 2019), but such applications are limited to the deep oceans, due to the finite footprint size of such satellite systems (of order 10 km) and the large satellite cross-track separation (of order 200 km) (Liu et al., 2018, Ribal and Young, 2019). In situ buoy observations provide valuable point observations, but they are typically of relatively short duration and spatially too sparse to quantify the wave climate across long and complex coastal areas (McSweeney, 2020). Third-generation wave models have shown their ability to model ocean waves on global and regional scales (Abdolali et al., 2020, Ching-Piao et al., 2012, Cuttler et al., 2020, Harley et al., 2010, Hemer et al., 2010, Liang et al., 2019, Shimura and Mori, 2019). However, there are still a limited number of studies of long-term wave climate in complex coastal regions exposed to the Southern Ocean, such as Victoria. McSweeney (2020) studied spatio-temporal variations of wave parameters at 10 offshore locations along the Victorian coast (offshore distances: 33–41 km) by extracting directional wave hindcast data from NOAA’s global WW3 model. Although valuable, such large-scale, regionally unvalidated model studies may raise downscaling issues when applied to coastal regions.

The present paper aims to provide a comprehensive analysis of the wave climate of Bass Strait and South-East Australia over the 40-year period 1981–2020. The analysis utilizes a high-resolution regional ocean wave model (the WW3 model) based on unstructured grids for Bass Strait and South-East Australia, nested within a global WW3 model. The model and the validation buoy data sources are described in Section 2. Section 3 describes the validation results against the extensive network of coastal buoys. The wave climate of the region is discussed in Section 4, including mean conditions and seasonal variations, long-term trends, extreme conditions and the impact of multi-decadal climate variability. Section 5 draws conclusions from the study.

Section snippets

Research domain

The model domain selected for the study is shown in Fig. 1, covering the area 137–155°E, 35–45°S. Three Australian coastal states span this domain, Victoria, Tasmania, and southern New South Wales. Victoria and Tasmania are separated by the relatively shallow Bass Strait. There are over 50 islands between Victoria and Tasmania including King Island, Flinders Island and Cape Barren Island, which greatly increase the bathymetric complexity of this region.

Wave model

The WW3 model (WW3DG, 2019) is a

Model validation

Before considering the WW3 model outputs to investigate the wave climate of Bass Strait and South-East Australia, it is essential to validate the model against nearshore in situ observations. Therefore, all available buoy observations (Table 1 and Fig. 1) along the coast were used to validate the WW3 model. The bulk wave parameters used for validation include significant wave height, peak wave period and peak wave direction. Hourly values of co-located buoy and model data were considered for

Mean conditions

The 40-year time series of significant wave height, peak wave period and peak wave direction were each averaged to obtain the corresponding mean conditions over the computational domain (Fig. 7). Both mean significant wave height (Fig. 7a) and mean peak wave period (Fig. 7b) show the largest magnitudes in the south-western and southern regions of the domain. The sheltering provided by Tasmania is clear with a significantly reduced wave climate within Bass Strait and the eastern regions of the

Conclusions

The Southern Ocean remains a region where the wave climate is poorly understood. In addition, it is unique in that the extremely long fetches give rise to both large waves but also conditions with longer peak periods than any other oceanic basin. As such, the region is an interesting test of present-day physics in spectral wave models, such as WW3. The southern coast of Australia is directly exposed to the Southern Ocean and Tasmania, Bass Strait and Victoria represent a complex coastal region.

CRediT authorship contribution statement

Jin Liu: Methodology, Software, Data curation, Writing – original draft, Visualization, Writing – review & editing. Alberto Meucci: Software, Writing – review & editing. Qingxiang Liu: Data curation, Writing – review & editing. Alexander V. Babanin: Writing – review & editing. Daniel Ierodiaconou: Data curation, Writing – review & editing. Ian R. Young: Conceptualization, Methodology, Data curation, Writing – original draft, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the WW3 development group for model development and providing source codes and the University of Melbourne for providing the high-performance computer cluster. We also thank Huy Quang Tran for sharing the combined bathymetric data. Wave buoy data were provided by the Port of Melbourne and Gippsland Ports. Wind data were provided by Climate Data Store. AODN wave data were sourced from Australia’s Integrated Marine Observing System (IMOS) – IMOS is enabled by the National

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