Tsunami observations by coastal ocean radar

When tsunami waves propagate across the open ocean, they are steered by the Coriolis effect and refraction due to gentle gradients in the bathymetry on scales longer than the wavelength. When the wave encounters steep gradients at the edges of continental shelves and at the coast, the wave becomes nonlinear and conservation of momentum produces squirts of surface current at the head of submerged canyons and in coastal bays. High frequency (HF) coastal ocean radar is well conditioned to observe the surface current bursts at the edge of the continental shelf and give a warning of 40 minutes to 2 hours when the shelf is 50 to 200km wide. The period of tsunami waves is invariant over changes in bathymetry and is in the range 2 to 30 minutes. Wavelengths for tsunamis (in 500 to 3000m depth) are in the range 8.5 to over 200 km, and on a shelf where the depth is about 50m (as in the Great Barrier Reef (GBR)) the wavelengths are in the range 2.5 to 30 km. In the use of HF radar technology, there is a trade-off between the precision of surface current speed measurements and time resolution. It is shown that the phased array HF ocean surface radar being deployed in the GBR and operating in a routine way for mapping surface currents, can resolve surface current squirts from tsunamis in the wave period range 20 to 30 minutes and in the wavelength range greater than about 6 km. An advantage in signal-to-noise ratio can be obtained from the prior knowledge of the spatial pattern of the squirts at the edge of the continental shelf, and it is estimated that, with this analysis, the time resolution of the GBR radar may be reduced to about 2.5 minutes, which corresponds to a capability to detect tsunamis at the shelf edge in the period range 5 to 30 minutes. It is estimated that the lower limit of squirt velocity detection at the shelf edge would correspond to a tsunami with water elevation of about 2.5 cm in the open ocean. This means that the GBR HF radar is well conditioned for use as a monitor of small, as well as larger, tsunamis and has the potential to contribute to the understanding of tsunami genesis research

The relentless march of mass coral bleaching: a global perspective of changing heat stress

The global coral bleaching event of 2014-2017 resulted from the latest in a series of heat stress events that have increased in intensity. We assessed global- and basin-scale variations in sea surface temperature-based heat stress products for 1985-2017 to provide the context for how heat stress during 2014-2017 compared with the past 3 decades. Previously, undefined "Heat Stress Year" periods (used to describe interannual variation in heat stress) were identified for the Northern and Southern Hemispheres, in which heat stress peaks during or shortly after the boreal and austral summers, respectively. The proportion of reef pixels experiencing bleaching-level heat stress increased through the record, accelerating during the last decade. This increase in accumulated heat stress at a bleaching level is a result of longer stress events rather than an increase in the peak stress intensity. Thresholds of heat stress extent for the three tropical ocean basins were established to designate "global" events, and a Global Bleaching Index was defined that relates heat stress extent to that observed in 1998. Notably, during the 2014-2017 global bleaching event, more than three times as many reefs were exposed to bleaching-level heat stress as in the 1998 global bleaching

Social–environmental drivers inform strategic management of coral reefs in the Anthropocene

Without drastic efforts to reduce carbon emissions and mitigate globalized stressors, tropical coral reefs are in jeopardy. Strategic conservation and management requires identification of the environmental and socioeconomic factors driving the persistence of scleractinian coral assemblages—the foundation species of coral reef ecosystems. Here, we compiled coral abundance data from 2,584 Indo-Pacific reefs to evaluate the influence of 21 climate, social and environmental drivers on the ecology of reef coral assemblages. Higher abundances of framework-building corals were typically associated with: weaker thermal disturbances and longer intervals for potential recovery; slower human population growth; reduced access by human settlements and markets; and less nearby agriculture. We therefore propose a framework of three management strategies (protect, recover or transform) by considering: (1) if reefs were above or below a proposed threshold of >10% cover of the coral taxa important for structural complexity and carbonate production; and (2) reef exposure to severe thermal stress during the 2014–2017 global coral bleaching event. Our findings can guide urgent management efforts for coral reefs, by identifying key threats across multiple scales and strategic policy priorities that might sustain a network of functioning reefs in the Indo-Pacific to avoid ecosystem collapse

SATELLITE BATHYMETRY USE IN NUMERICAL MODELS OF OCEAN THERMAL STRESS

Techniques for deriving estimated bathymetry from satellite data are well established; however, use of this product in complex terrains is limited. Accurate bathymetry is essential in the construction of hydrodynamic models and satellite-derived bathymetry is a strong candidate for use in coastal and shallow waters. A case study of Palau is presented which uses satellite-derived bathymetry as input to a hydrodynamic model. Palau underwent widespread coral bleaching during 1998, thought to be due to thermal stress, and existing satellite products observed anomalous increases in temperature. The numerical model is used to evaluate sea surface temperature patterns during such a bleaching event. Comparisons between the model and thermal indicators derived from satellite data are made, and the results used to suggest improvements for satellite monitoring of thermal stress event

Mitigation of coral bleaching on the reef front by wave mixing

Coral bleaching becomes particularly likely when the water becomes stratified, with warm water absorbing solar radiation and sitting in a stable fashion on top of underlying cooler water. The physical conditions that cause vertical mixing of the water column often mitigate coral bleaching. Generally these are thought to be: low current speeds (low turbulent kinetic energy); low wave heights (small degree of micro-breakers and non-linearities); low wind speeds (low stress and shears). In this work we evaluate the mixing along the exposed 'weather edge' of a reef where even small-amplitude waves break and cause vertical mixing. Given that low wind speeds increase the likelihood of bleaching, the waves that might influence the reef-front are generally swell waves produced by the wind at a distant location. The combination of non-linearities in the waves and dispersion mean that the swell is the main contributor to vertical mixing at the reef-front. We adopt linear wave theory to bring the wave across the shelf to the reef front. Within about one wavelength of a steep reef front, the wave becomes non-linear and its surface elevation increases until it breaks. In this breaking zone, wave dynamics do not apply and the physics is best dealt with by momentum and energy considerations. Some of the energy is reflected as a propagating wave; some energy is carried over the reef-top into a lagoon or reef-flat as a momentum bore or a re-generated wave train; and some is converted to turbulence at the reef front. The energy that produces turbulent mixing at the reef front is compared with the potential energy of stratification, and it is found that even small-amplitude waves produce significant mixing. The partitioning of energy depends predominantly on wave height and sea water level. For most reefs, the elevation of the reef top is the level of the Lowest Astronomical Tide and this is used as the datum. The partition indices are presented as functions of Hs/D, where Hs is the significant wave height, and D is the water depth. Other parameters of water depth, wavelength, and width of the reeftop have smaller effects on the energy partition. The calculations show that for small Hs/D values (high tide) the mixing is high. As Hs/D increases towards low tide, more energy is spilled across the reef and less is spent on vertical mixing at the reef front. The application of this work to reef ecology is in identifying sections of a reef that are more resilient to coral bleaching. The results here show that even the smallest of swell produces sufficient vertical mixing to remove stratification and mitigate coral bleaching along the exposed front of a reef. The side of the reef normally exposed to swell will also normally be the windward side where prevailing wind-driven currents are likely to be driven across the reef flat. This means that corals on the reef front are likely to be a prime source of larvae in any recovery process if the reef suffers coral loss by bleaching, crown of thorns or disease

HF coastal ocean radar in an observing system

The HF coastal ocean radar in the southern part of the Great Barrier Reef is part of a national integrated marine observing system, and the data will be available for research applications from a national archive. The primary archived product is hourly values of surface current vectors, significant wave heights and wind directions at over 1000 grid points at 4 km spacing over the shelf and adjacent ocean. A review of currently associated research projects includes validation of hydrodynamic modelling, tsunami detection, connectivity of reefs and islands, empirical physical process modeling, and combining radar surface current data with satellite-based sea surface temperature data to produce an index of mixing in the vertical column of water

Improvements to and continuity of operational global thermal stress monitoring for coral bleaching

Mass coral bleaching results from periods of elevated sea temperature. Satellite monitoring of thermal stress has enhanced the capacity for the management of coral bleaching events worldwide. Satellite-based monitoring tools provide reef managers with cost-effective observations of temperature conditions to monitor the risk of bleaching and to target in situ observations in areas under stress. This paper describes improvements to satellite remote sensing products from NOAA's Coral Reef Watch to enhance product coverage and to correct identified errors in the production of coral reef-specific metrics for thermal stress. In addition, threats to the operational production of the thermal stress metrics are considered and a contingency plan is described to ensure continuity of operations

Advancing ocean monitoring near coral reefs

[Extract] Because of the sensitivity of corals to environmental factors, bleaching events are seen\ud as indicators of the changing state of the world’s oceans as climate warms. In 1997–1998, a worldwide bleaching event occurred. Since then, bleaching has been observed in\ud every ocean basin with increased frequency and severity. For example, in 2010 reefs in Southeastern Asia bleached at record levels, parts of the Hawaiian archipelago have\ud experienced mild bleaching, and current monitoring and forecasts indicate significant to severe bleaching in the Caribbean.\ud \ud Observing the physical properties of ocean waters around coral reefs provides important insights for linking environmental changes to such biological consequences.\ud Remotely sensed data are particularly valuable, providing broad spatial coverage in a timely fashion to inform\ud actions by reef managers. However, there remain significant gaps in monitoring capability and scientific understanding of coral ecosystem responses to changes in the physical environment.\u

Dynamics of the coastal zone

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