Are meteotsunamis an underrated hazard? Pattiaratchi, Charitha B.; Wijeratne, E. M. S.
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
10/2015, Letnik:
373, Številka:
2053
Journal Article
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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.
Western Australia is susceptible to tsunamis from seismic sources that originate from distant sources including the Sunda Arc. Many surface and subsurface topographic ocean features are located ...between the Australian continent and locations where tsunamigenic earthquakes occur. These include the Venin Meinesz Seamounts (including Christmas Island) and Horizon Ridge, Exmouth, Zenith and Cuvier Plateaus. Numerical simulations of idealised tsunamigenic earthquakes along the Sunda Arc revealed that these topographic features have a large influence on the distribution of tsunami heights, propagating speeds and energy distribution. The interaction between tsunami waves and Venin Meinesz Seamounts and Horizon Ridge, located close to the earthquake locations, scatter the tsunami energy into several beams. Exmouth Plateau acts as a focusing feature to increase wave heights between North West Cape and Barrow Island whilst Cuvier Plateau deflects energy towards Shark Bay. Although Zenith Plateau has a local effect, it does not influence tsunami waves along the coast. Southwest Australia is “sheltered” from the direct effect of tsunami waves from Sunda Arc due to the combined effects of the Seamounts and Cuvier Plateau in the scattering and refraction of tsunami waves.
Extensive partial mortality of intertidal corals was observed at the Houtman Abrolhos Islands, Western Australia, associated with extended and recurrent daytime low water levels from September to ...November 2018. Branching
Acropora
corals on shallow leeward reef platforms were emersed during the middle of day during the Austral spring due to diurnal tidal influences, seasonal mean sea-level minima associated with the southward flowing Leeuwin Current and interannual variability related to the El Niño–Southern Oscillation. Pre-emersion total live coral cover was estimated to be ~ 35%, dominated by
Acropora
(~ 24%). Post-emersion, live
Acropora
cover decreased to 11.1% (± 1.5 SE) and total cover to 21.7% (± 0.8 SE). Mortality of non-acroporid corals which were lower in the water column was not observed. Partial coral mortality associated with seasonal low tides and interannual variability in mean sea level is a universal process and under-reported driver of temporal variability in coral cover on intertidal reef platforms.
Transport of water between the coast and the deeper ocean, across the continental shelf, is an important process for the distribution of biota, nutrients, suspended and dissolved material on the ...shelf. Presence of denser water on the inner continental shelf results in a cross-shelf density gradient that drives a gravitational circulation with offshore transport of denser water along the sea bed that is defined as Dense Shelf Water Cascade (DSWC). Analysis of field data, collected from multiple ocean glider data missions around Australia, confirmed that under a range of wind and tidal conditions, DSWC was a regular occurrence during autumn and winter months over a coastline spanning > 10,000 km. It is shown that even in the presence of relatively high wind- and tidal-induced vertical mixing, DSWCs were present due to the strength of the cross-shelf density gradient. The occurrence of DSWC around Australia is unique with continental scale forcing through air-sea fluxes that overcome local wind and tidal forcing. It is shown that DSWC acts as a conduit to transport suspended material across the continental shelf and is a critical process that influences water quality on the inner continental shelf.
Plastics represent the vast majority of human-made debris present in the oceans. However, their characteristics, accumulation zones, and transport pathways remain poorly assessed. We characterised ...and estimated the concentration of marine plastics in waters around Australia using surface net tows, and inferred their potential pathways using particle-tracking models and real drifter trajectories. The 839 marine plastics recorded were predominantly small fragments ("microplastics", median length = 2.8 mm, mean length = 4.9 mm) resulting from the breakdown of larger objects made of polyethylene and polypropylene (e.g. packaging and fishing items). Mean sea surface plastic concentration was 4256.4 pieces km(-2), and after incorporating the effect of vertical wind mixing, this value increased to 8966.3 pieces km(-2). These plastics appear to be associated with a wide range of ocean currents that connect the sampled sites to their international and domestic sources, including populated areas of Australia's east coast. This study shows that plastic contamination levels in surface waters of Australia are similar to those in the Caribbean Sea and Gulf of Maine, but considerably lower than those found in the subtropical gyres and Mediterranean Sea. Microplastics such as the ones described here have the potential to affect organisms ranging from megafauna to small fish and zooplankton.
Large epipelagic fishes (> 30 kg maximum size) are known to display a variety of patterns of vertical movement. Although advances in the affordability and sophistication of electronic tags now allows ...researchers to routinely document these patterns, there is no standardised approach to classify these behaviours and investigate their physical and biological drivers. This paper reviews the existing knowledge of the vertical movements of large, epipelagic fishes and the evidence for the underlying factors that structure this behaviour. The review focuses on behaviours occurring at a range of temporal scales, from seconds to years. We propose that patterns of vertical movement in gill-breathing animals of the epipelagic are best characterised by the need to move continuously in a three-dimensional environment while optimising food encounter and energy expenditure, avoiding predators, searching for mates and remaining within the limits imposed by the physical environment on their physiology (notably water temperature and oxygen). Modern biologging technologies that record both the internal (body temperature, heart rate) and external physical environment coupled with direct recording of behaviour from tri-axial sensors and animal-borne cameras offer a new approach to the analysis of drivers of vertical movement. Ultimately, this can provide insights into the evolution of the behaviour and morphology of these animals.
Coastal protection design heights typically consider the superimposed effects of tides, surges, waves, and relative sea-level rise (SLR), neglecting non-linear feedbacks between these forcing ...factors. Here, we use hydrodynamic modelling and multivariate statistics to show that shallow coastal areas are extremely sensitive to changing non-linear interactions between individual components caused by SLR. As sea-level increases, the depth-limitation of waves relaxes, resulting in waves with larger periods, greater amplitudes, and higher run-up; moreover, depth and frictional changes affect tide, surge, and wave characteristics, altering the relative importance of other risk factors. Consequently, sea-level driven changes in wave characteristics, and to a lesser extent, tides, amplify the resulting design heights by an average of 48-56%, relative to design changes caused by SLR alone. Since many of the world's most vulnerable coastlines are impacted by depth-limited waves, our results suggest that the overall influence of SLR may be greatly underestimated in many regions.
Millimeter-sized plastics are abundant in most marine surface waters, and known to carry fouling organisms that potentially play key roles in the fate and ecological impacts of plastic pollution. In ...this study we used scanning electron microscopy to characterize biodiversity of organisms on the surface of 68 small floating plastics (length range = 1.7-24.3 mm, median = 3.2 mm) from Australia-wide coastal and oceanic, tropical to temperate sample collections. Diatoms were the most diverse group of plastic colonizers, represented by 14 genera. We also recorded 'epiplastic' coccolithophores (7 genera), bryozoans, barnacles (Lepas spp.), a dinoflagellate (Ceratium), an isopod (Asellota), a marine worm, marine insect eggs (Halobates sp.), as well as rounded, elongated, and spiral cells putatively identified as bacteria, cyanobacteria, and fungi. Furthermore, we observed a variety of plastic surface microtextures, including pits and grooves conforming to the shape of microorganisms, suggesting that biota may play an important role in plastic degradation. This study highlights how anthropogenic millimeter-sized polymers have created a new pelagic habitat for microorganisms and invertebrates. The ecological ramifications of this phenomenon for marine organism dispersal, ocean productivity, and biotransfer of plastic-associated pollutants, remains to be elucidated.
Periods of high astronomically generated tides contribute to the occurrence of extreme sea levels. Over interannual time scales, two precessions associated with the orbit of the Moon cause systematic ...variation of high tides. A global assessment of when these tidal modulations occur allows for the prediction of periods when the enhanced risk of coastal flooding is likely in different parts of the world. This paper uses modeled tides to assess the influence of the 18.61 year lunar nodal cycle and the 8.85 year cycle of lunar perigee (which affects high tidal levels as a quasi 4.4 year cycle) on high tidal levels on a global scale. Tidal constituents from the TPXO7.2 global tidal model are used, with satellite modulation corrections based on equilibrium tide expectations, to predict multidecadal hourly time series of tides on a one‐quarter degree global grid. These time series are used to determine the amplitude and phase of tidal modulations using harmonic analysis fitted to 18.61, 9.305, 8.85, and 4.425 year sinusoidal signals. The spatial variations in the range and phase of the tidal modulations are related to the global distribution of the main tidal constituents and tidal characteristics (diurnal or semidiurnal and tidal range). Results indicate that the 18.61 year nodal cycle has the greatest influence in diurnal regions with tidal ranges of >4 m and that the 4.4 year cycle is largest in semidiurnal regions where the tidal range is >6 m. The phase of the interannual tidal modulations is shown to relate to the form of the tide.
Key Points
Interannual tidal modulations influence extreme sea levels and coastal flooding
This paper maps the range and phase of the tidal modulations on a global scale
Determine where the modulations contribute largest to high tidal levels
A 15 year (2000–2014) simulation of the oceans around Australia, with the shelf‐scale model ozROMS, was used to estimate the mean, seasonal, and interannual variability of the surface and subsurface ...boundary currents and associated inflows. The simulation clarified some previous points of uncertainty and provided new information previously unknown and this is listed here. In the Indian Ocean, flow through the Timor Passage was linked to southeast Australia through the Holloway (HLC), Leeuwin (LC), South Australian (SAC), and Zeehan (ZC) Currents. The main inflows were from the Indonesian Throughflow and Eastern Gyral Current in the north whilst the central and southern branches of the South Indian Counter Current (SICC) provided major (>60%) inflows to the LC in the west. The HLC at North‐west Cape was at a maximum in April–May and its annual cycle accounted for 70% of the seasonal variance of LC, SAC, and ZC. In the Pacific Ocean, the northern branches of the South Equatorial Current were the main inputs to initiate the Hiri and East Australian (EAC) Currents flowing north and south, respectively, at ∼15°S. Inflow from the South Caledonia Jet to the EAC was ∼35%. The Flinders Current (FC) contributed to the Leeuwin Undercurrent (LU) directly as a northward flow and LU was enhanced from inflow from the subsurface southern SICC in the west (∼32–33°S). The majority of LU flowed westward offshore between 24 and 29°S while ∼25% continued northward to the northwest shelf. All Australian surface boundary currents systems were enhanced during the 2011–2013 La Niña.
Key Points
Major inflows to the Leeuwin Current and Leeuwin Undercurrent are provided by the South Indian Counter Current
The main inflow (65%) to the East Australian Current is through the northern branches of the South Equatorial Current
All Australian boundary current transport was enhanced during the 2011–2013 La Niña
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