@article {bnh-5166, title = {The response of the southwest Western Australian wave climate to Indian Ocean climate variability}, journal = {Climate Dynamics}, volume = {50}, year = {2018}, month = {03/2018}, pages = {1533-1557}, chapter = {1533}, abstract = {

Knowledge of regional wave climates is critical for coastal planning, management, and protection. In order to develop a regional wave climate, it is important to understand the atmospheric systems responsible for wave generation. This study examines the variability of the southwest Western Australian (SWWA) shelf and nearshore wind wave climate and its relationship to southern hemisphere climate variability represented by various atmospheric indices: the southern oscillation index (SOI), the Southern Annular Mode (SAM), the Indian Ocean Dipole Mode Index (DMI), the Indian Ocean Subtropical Dipole (IOSD), the latitudinal position of the subtropical high-pressure ridge (STRP), and the corresponding intensity of the subtropical ridge (STRI). A 21-year wave hindcast (1994{\textendash}2014) of the SWWA continental shelf was created using the third generation wave model Simulating WAves Nearshore (SWAN), to analyse the seasonal and inter-annual wave climate variability and its relationship to the atmospheric regime. Strong relationships between wave heights and the STRP and the STRI, a moderate correlation between the wave climate and the SAM, and no significant correlation between SOI, DMI, and IOSD and the wave climate were found. Strong spatial, seasonal, and inter-annual variability, as well as seasonal longer-term trends in the mean wave climate were studied and linked to the latitudinal changes in the subtropical high-pressure ridge and the Southern Ocean storm belt. As the Southern Ocean storm belt and the subtropical high-pressure ridge shifted southward (northward) wave heights on the SWWA shelf region decreased (increased). The wave height anomalies appear to be driven by the same atmospheric conditions that influence rainfall variability in SWWA.

}, doi = {https://doi.org/10.1007/s00382-017-3704-z}, url = {https://link.springer.com/article/10.1007/s00382-017-3704-z}, author = {Mortiz Wandres and Charitha Pattiaratchi and Yasha Hetzel and E.M.S Wijeratne} } @article {bnh-2587, title = {Predicting continental shelf waves in Australia}, number = {160}, year = {2016}, month = {02/2016}, institution = {Bushfire and Natural Hazards CRC}, address = {Melbourne}, abstract = {

A range of processes can cause extreme water levels that endanger human life and cause property damage. Storm surges caused by tropical cyclones are well known to pose a risk for coastal inhabitants.\  However, a lesser-known effect of landfalling cyclones is the generation of coastally trapped waves that can propagate along the coast and influence water levels thousands of kilometres away. Examples of such waves include continental shelf waves (CSWs) that travel to the left along the coastline in the southern hemisphere and are common along the Australian coastline (Church et al., 1986; Eliot and Pattiaratchi, 2010).

This study investigated the generation and propagation characteristics of continental shelf waves (CSWs) in Western Australia, a hotspot of continental shelf wave activity in Australia, utilising the Regional Ocean Modelling System (ROMS).\  An idealised model of the North West Shelf (NWS) with ~ 4 km spatial resolution and 30 vertical sigma layers was forced by simulated tropical cyclones that varied in speed, direction (parallel, perpendicular or at 45{\textdegree} to the coastline) and category.\  Realistic cyclone simulations were performed for Tropical Cyclone Bianca in 2011 using an Australia-wide model and comparisons made to observations.

Results from the idealised numerical model showed that a cyclone{\textquoteright}s path, speed and category affect continental shelf wave generation and propagation.\  Greater cyclone category resulted in higher amplitude continental shelf waves.\  Cyclones that travelled parallel to the coastline generated continental shelf waves that were faster than the cyclone forward speed and produced higher amplitude waves than those generated by different trajectories.\  Increased parallel cyclone forward speed corresponded to an increase in the continental shelf wave height. If the parallel cyclone forward speed was too fast (i.e. greater than 14 ms-1), the continental shelf wave height decreased.\  Parallel moving cyclones travelling near to the continental shelf break with a forward speed of ~ 7.5 ms-1 generated the highest amplitude waves. If the cyclone discontinued travelling parallel, the continental shelf wave gained similar properties to continental shelf waves generated by perpendicular or 45{\textdegree} travelling cyclones.\  These type of cyclones generated continental shelf waves that had a lower wave height, which were even lower for faster moving cyclones.\  The propagation speed of these waves scaled to the Coriolis parameter and characteristic length scale of the continental shelf.\  The lower wave height of these waves can be explained by the continental shelf wave travelling as a "free wave".

Propagation characteristics of continental shelf waves generated by Cyclone Bianca and Carlos from realistic cyclone simulations showed cyclone path and continental shelf bathymetry had an influence on CSW height and propagation speed. The calculated approximate travel speed from lag correlation analysis was 5.8 ms-1 for both continental shelf waves.\  Tropical Cyclone Bianca travelled parallel to the Western Australian coastline which reinforced the continental shelf wave height. Tropical Cyclone Carlos continued offshore from Exmouth so the continental shelf wave had little energy to propagate around the curved coastline in the Capes Region.\  Both continental shelf waves decreased in propagation speed between Exmouth and Carnarvon where the continental shelf width decreased rapidly.\  The continental shelf wave generated by Tropical Cyclone Bianca rapidly decreased at Albany which may be explained by the curvature of the coastline which resulted in wave scattering.\ \ \ 

Results from this study demonstrate how particular cyclone properties influence the generation of continental shelf waves and through this improves our ability to predict the influence of CSWs on extreme water levels along the Australian coast. .\  It is recommended that design criteria, modelling studies and inundation risk assessments for coastal regions in Western Australia need to consider the effects of continental shelf waves.\ \ 

}, issn = {160}, author = {Charitha Pattiaratchi and E.M.S Wijeratne and Yasha Hetzel and Ivica Janekovic} } @article {bnh-3429, title = {Are meteotsunamis an underrated hazard?}, journal = {Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences}, volume = {373}, year = {2015}, month = {10/2015}, abstract = {

Meteotsunamis are generated by meteorological events, particularly moving pressure disturbances due to squalls, thunderstorms, frontal passages and atmospheric gravity waves. Relatively small initial sea-level perturbations, of the order of a few centimetres, can increase significantly through multi-resonant phenomena to create destructive events through the superposition of different factors. The global occurrence of meteotsunamis and the different resonance phenomena leading to amplification of meteotsunamis are reviewed. Results from idealized numerical modelling and field measurements from southwest Australia are presented to highlight the relative importance of the different processes. It is shown that the main influence that leads to amplification of the initial disturbance is due to wave shoaling and topographic resonance. Although meteotsunamis are not catastrophic to the extent of major seismically induced basin-scale events, the temporal and spatial occurrence of meteotsunamis are higher than those of seismic tsunamis as the atmospheric disturbances responsible for the generation of meteotsunamis are more common. High-energy events occur only for very specific combinations of resonant effects. The rareness of such combinations is perhaps the main reason why destructive meteotsunamis are exceptional and observed only at a limited number of sites globally.

}, doi = {10.1098/rsta.2014.0377}, url = {http://rsta.royalsocietypublishing.org/content/373/2053/20140377}, author = {Charitha Pattiaratchi and E.M.S Wijeratne} } @conference {bnh-2389, title = {Storm surge risk from transitioning tropical cyclones in Australia}, booktitle = {Australasian Coasts \& Ports Conference 2015}, year = {2015}, month = {11/2015}, publisher = {ResearchGate}, organization = {ResearchGate}, address = {Auckland, NZ}, abstract = {

When tropical cyclones come ashore strong winds can set up storm surges that devastate coastal communities through erosion and inundation of low lying areas. In Australia, storm surge risk from tropical cyclones is generally restricted to warmer northern regions whilst in the southern regions damaging storm surges tend to arise from extratropical low pressure systems. Often, tropical cyclones weaken as they move into higher latitudes passing over cooler waters and experiencing higher wind shear. However, in some cases tropical cyclones can interact with mid-latitude weather systems causing the storms to lose tropical characteristics and become extratropical in nature, often intensifying and accelerating{\textendash} this is known as Extratropical Transition (ET). Storms that undergo ET pose a serious threat to life and property by extending tropical cyclone-like conditions over a larger area in latitudes that do not normally experience such events. Here we aim to describe the storm surge risk from tropical cyclones transitioning into the higher latitudes around Australia using observations, atmospheric reanalyses, and a coupled storm surge-wave model. We highlight modelling challenges and examine two contrasting case studies: (1) the worst case scenario of Cyclone Alby (1978) that underwent ET causing large waves, flooding, erosion and $50 million in damage in southwest Australia; and, (2) a {\textquoteleft}near miss{\textquoteright} event from Cyclone Bianca (2011) that weakened just before making landfall in the same region. The simulated surge from Alby peaked at 1.1 m in Busselton, with approximately 10\% of the height resulting from wave effects included in the model. Predicted wave heights exceeded 10 m offshore. Cyclone Bianca weakened dramatically before landfall and major damage did not occur. We propose that tropical cyclones moving into higher latitudes need to be given special consideration due to their unpredictable nature and the potential for damaging storm surges.

}, author = {Yasha Hetzel and Ivica Janekovic and Charitha Pattiaratchi and E.M.S Wijeratne} } @article {bnh-3452, title = {Estimating present day extreme water level exceedance probabilities around the coastline of Australia: tropical cyclone-induced storm surges}, journal = {Climate Dynamics}, volume = {42}, year = {2014}, month = {01/2014}, pages = {139-157}, chapter = {139}, abstract = {

The incidence of major storm surges in the last decade have dramatically emphasized the immense destructive capabilities of extreme water level events, particularly when driven by severe tropical cyclones. Given this risk, it is vitally important that the exceedance probabilities of extreme water levels are accurately evaluated to inform risk-based flood and erosion management, engineering and for future land-use planning and to ensure the risk of catastrophic structural failures due to under-design or expensive wastes due to over-design are minimised. Australia has a long history of coastal flooding from tropical cyclones. Using a novel integration of two modeling techniques, this paper provides the first estimates of present day extreme water level exceedance probabilities around the whole coastline of Australia, and the first estimates that combine the influence of astronomical tides, storm surges generated by both extra-tropical and tropical cyclones, and seasonal and inter-annual variations in mean sea level. Initially, an analysis of tide gauge records has been used to assess the characteristics of tropical cyclone-induced surges around Australia. However, given the dearth (temporal and spatial) of information around much of the coastline, and therefore the inability of these gauge records to adequately describe the regional climatology, an observationally based stochastic tropical cyclone model has been developed to synthetically extend the tropical cyclone record to 10,000\ years. Wind and pressure fields derived for these synthetically generated events have then been used to drive a hydrodynamic model of the Australian continental shelf region with annual maximum water levels extracted to estimate exceedance probabilities around the coastline. To validate this methodology, selected historic storm surge events have been simulated and resultant storm surges compared with gauge records. Tropical cyclone induced exceedance probabilities have been combined with estimates derived from a 61-year water level hindcast described in a companion paper to give a single estimate of present day extreme water level probabilities around the whole coastline of Australia. Results of this work are freely available to coastal engineers, managers and researchers via a web-based tool (www.sealevelrise.info). The described methodology could be applied to other regions of the world, like the US east coast, that are subject to both extra-tropical and tropical cyclones.

}, doi = {10.1007/s00382-012-1653-0}, url = {https://link.springer.com/article/10.1007\%2Fs00382-012-1653-0}, author = {Ivan D. Haigh and Leigh R. MacPherson and M. S. Mason and E.M.S Wijeratne and Charitha Pattiaratchi and R Crompton and Steve George} } @article {bnh-3451, title = {Estimating present day extreme water level exceedance probabilities around the coastline of Australia: tides, extra-tropical storm surges and mean sea level}, journal = {Climate Dynamics}, volume = {42}, year = {2014}, month = {01/2014}, pages = {121-138}, chapter = {121}, abstract = {

The occurrence of extreme water levels along low-lying, highly populated and/or developed coastlines can lead to considerable loss of life and billions of dollars of damage to coastal infrastructure. Therefore it is vitally important that the exceedance probabilities of extreme water levels are accurately evaluated to inform risk-based flood management, engineering and future land-use planning. This ensures the risk of catastrophic structural failures due to under-design or expensive wastes due to over-design are minimised. This paper estimates for the first time present day extreme water level exceedence probabilities around the whole coastline of Australia. A high-resolution depth averaged hydrodynamic model has been configured for the Australian continental shelf region and has been forced with tidal levels from a global tidal model and meteorological fields from a global reanalysis to generate a 61-year hindcast of water levels. Output from this model has been successfully validated against measurements from 30 tide gauge sites. At each numeric coastal grid point, extreme value distributions have been fitted to the derived time series of annual maxima and the several largest water levels each year to estimate exceedence probabilities. This provides a reliable estimate of water level probabilities around southern Australia; a region mainly impacted by extra-tropical cyclones. However, as the meteorological forcing used only weakly includes the effects of tropical cyclones, extreme water level probabilities are underestimated around the western, northern and north-eastern Australian coastline. In a companion paper we build on the work presented here and more accurately include tropical cyclone-induced surges in the estimation of extreme water level. The multi-decadal hindcast generated here has been used primarily to estimate extreme water level exceedance probabilities but could be used more widely in the future for a variety of other research and practical applications.

}, doi = {10.1007/s00382-012-1652-1}, url = {https://link.springer.com/article/10.1007/s00382-012-1652-1}, author = {Ivan D. Haigh and E.M.S Wijeratne and Leigh R. MacPherson and Charitha Pattiaratchi and M. S. Mason and R Crompton and Steve George} } @article {bnh-3468, title = {Observations of meteorological tsunamis along the south-west Australian coast}, journal = {Natural Hazards}, volume = {74}, year = {2014}, month = {10/2014}, pages = {281-303}, chapter = {281}, abstract = {

Tide gauge data were used to identify the occurrence, characteristics, and cause of tsunamis of meteorological origin (termed {\textquoteright}meteotsunamis{\textquoteright}) along the Western Australian coast. This is the first study to identify meteotsunamis in this region, and the results indicated that they occur frequently. Although meteotsunamis are not catastrophic to the extent of major seismically induced basin-scale events, the wave heights of meteotsunamis examined at some local stations in this study were higher than those recorded through seismic tsunamis. In June 2012, a meteotsunami contributed to an extreme water-level event at Fremantle, which recorded the highest water level in over 115 years. Meteotsunamis (wave heights \> 0.4 m, when the mean tidal range in the region is similar to 0.5 m) were found to coincide with thunderstorms in summer and the passage of low-pressure systems during winter. Spectral analysis of tide gauge time series records showed that existing continental seiche oscillations (periods between 30 min and 5 h) were enhanced during the meteotsunamis, with a high proportion of energy transferred to the continental shelf oscillation period. Three recent meteotsunami events (22 March 2010, 10 June 2012, and 7 January 2013) two due to summer thunderstorms and one due to a winter frontal system were chosen for detailed analysis. The meteotsunami amplitudes were up to a factor 2 larger than the local tidal range and sometimes contributed up to 85 \% of the non-tidal water signal. A single meteorological event was found to generate several meteotsunamis along the coast, up to 500 km apart, as the air pressure disturbance propagated over the continental shelf; however, the topography and local bathymetry of the continental shelf defined the local sea-level resonance characteristics at each location. With the available data (sea level and meteorological), the exact mechanisms for the generation of the meteotsunamis could not be isolated.

}, doi = {10.1007/s11069-014-1263-8}, url = {https://link.springer.com/article/10.1007\%2Fs11069-014-1263-8}, author = {Charitha Pattiaratchi and E.M.S Wijeratne} }