FIELD OBSERVATION SITE FOR AIR-SEA INTERACTIONS IN TROPICAL CYCLONES

Accurate predictions of winds, waves and currents within extreme tropical cyclones are critical for shipping, offshore oil and gas, ports and harbours, coastal erosion, tourism and fishing. The paper will describe a unique field observation programme intended to gather in situ data about air-sea interactions in tropical cyclones. The site has been established on the Woodside-operated North Rankin Complex, an offshore gas production facility located off the north-west coast of Western Australia. The facility is multi-purpose. It will assist Woodside to manage platform operations during the cyclone season and to make advances in the estimate of extreme wave crest heights for platform loading while enabling academic researchers to measure air-sea interactions. Concurrent measurements are conducted in the atmospheric boundary layer, on the ocean surface and below the surface all the way to the bottom at 120 m depth. The measurements include fluxes of momentum and energy across the air-sea interface, spray production, directional wave spectra up to high wavenumbers, and will allow us to close the balance of the air-sea exchanges for the first time in extreme field conditions

The effect of wave-induced turbulence on the ocean mixed layer during tropical cyclones: Field observations on the Australian North-West Shelf

Field observations of water temperature on the Australian North-West Shelf (Eastern Indian Ocean) with the support of numerical simulations are used to demonstrate that the injection of turbulence generated by the wave orbital motion substantially contributes to the mixing of the upper ocean. Measurements also show that a considerable deepening of the mixed layer occurs during tropical cyclones, when the production of wave-induced turbulent kinetic energy overcomes the contribution of the current-generated shear turbulence. Despite a significant contribution to the deepening of the mixed layer, the effect of a background current and atmospheric forcing are not on their own capable of justifying the observed deepening of the mixed layer through most of the water column. Furthermore, variations of a normally shallow mixed layer depth are observed within a relatively short timescale of approximately 10 hours after the intensification of wave activity and vanish soon after the decay of storm surface waves. This rapid development tends also to exclude any significant contribution by wave breaking, as small rates of vertical diffusivity for wave breaking-induced turbulence would require longer timescales to influence the depth of the mixed layer.status: publishe

Detritus as food for grazing fishes on coral reefs

Algal turf assemblages of the northern Great Barrier Reef, Australia, were sampled to determine the nutritional value of detritus and algae. Samples were collected with a suction apparatus across an exposure gradient from (1) the reef crest at highly exposed outer barrier reefs, (2) the reef crest of moderately exposed midshelf reefs, and (3) the reef slope of sheltered midshelf reefs. The biomass of algae and detritus decreased from sheltered midshelf reefs to moderately exposed midshelf reefs to highly exposed outer barrier reefs. This decrease was significant only for detritus (P < 0.005). Wave energies were calculated across the exposure gradient with the wave model WAMGBR. Detrital mass was inversely correlated with predicted wave energies and fitted a polynomial relationship (P < 0.001) and explained 52.8% of the variation. A similar relationship was also found between algal mass and wave energy (P < 0.001) but only explained 30.0% of the variation. The nutritional value of samples in protein amino acids and starch was assessed. The amino acid composition of detritus and algae was similar and not considered nutritionally different, whereas the concentration of protein amino acids was significantly (P < 0.001) higher in detritus (21.2 ± 2.0 mg g-1) than in algae (11.8 ± 1.0 mg g-1). Starch content was significantly (P < 0.05) higher in algae (7.7 ± 0.9 mg g-1) than in detritus (6.0 ± 1.0 mg g-1). These results demonstrate that detritus is a potentially valuable food source to grazing fishes on coral reefs

Estimating present day extreme water level exceedance probabilities around the coastline of Australia: tropical cyclone-induced storm surges

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 o