Field observations of the mixed layer depth in the Indian Ocean: effect of wave-induced turbulence

Recent theoretical and experimental studies seem to suggest that turbulence generated by non-breaking surface waves may substantially affect the ocean mixed layer depth. Here, field observations of water temperature profiles in the Indian Ocean off the North-West coast of Australian (provided by Woodside Energy Ltd.) are used to corroborate this conjecture. Measurements show that the generally thin mixed layer depth is subjected to a rapid and substantial deepening during intense tropical storms. This appears to be consistent with a rapid and significant intensification of wave-induced turbulence through the water column, which overcomes the turbulence production from background currents by few orders of magnitude. Under these circumstances, the observed mixed layer depth is in good agreement with estimates of a wave-induced mixed layer depth, which is calculated with a waveamplitude-based Reynolds number. Deepening of the mixed layer are observed to occur within a relatively short time scale of a few tens of hours after harshening of the wave activity and vanish soon after the decay of storm activities. This rapid development excludes any significant contribution of wave-breaking in the observed phenomenon. Vertical diffusivity of wave-breaking-induced turbulence would in fact require much longer time scales (few days) to justify the observed mixing layer depth

Modeling suspended sediment during construction in Great Barrier Reef world heritage area

A marina was constructed in the Great Barrier Reef World Heritage Area in close proximity to coral reefs that could be damaged by excess turbidity generated during construction. Since there was uncertainty about both the fate of suspended sediments and their effect on corals, initial water quality constraints were set very conservatively. In order to better understand the movement of suspended sediment during construction, a numerical model study was commissioned using three-dimensional, numerical, hydrodynamic, and Lagrangian particle tracking models. The study was successful in: (1) increasing the understanding of and reducing the uncertainty of sediment dispersal patterns under a range of common forcing conditions; (2) testing the variation in suspended sediment concentrations over sensitive areas for two different outfall locations; (3) offering evidence that a good choice in outfall locations will reduce the threat to corals; and importantly (4) presenting the results in a way that enhanced understanding by nontechnical reef managers. This final result was achieved by creating movies of sediment movement that clearly demonstrated the complex hydrodynamic processes involved with near-coastal water currents. Specific model results showed: (1) that a more seaward outfall increases effluent dispersal away from sensitive areas; (2) the highest concentrations of effluent over sensitive sites occur during no wind and neap tide conditions; and (3) prevailing southeast winds advect effluent offshore, away from sensitive sites

Modelling tropical cyclone over-water wind and pressure fields

A parametric wind field model and an interactive computer tool are used to develop time histories of 2D wind field (surface wind speed and direction) during the 64 historical tropical cyclones that threatened the east coast of Queensland, Australia, during the 33 years (1969–2002) after the advent of meteorological satellite data for the region. These wind fields are suitable to be applied to applications such as wave, storm surge and circulation modelling. The parametric wind field model includes a double vortex and a representation of the synoptic winds in which the tropical cyclone is embedded. The process used all easily accessible meteorological data, including wind speeds, directions and pressures to determine the best-fit wind fields. Detailed comparisons between model and measurements for three individual storms are shown. These highlight the difficulties associated with fitting the parametric model to the very sparse measurements. If the storm did not pass close to a measurement station, then estimates of parameter values for radius of the storm and spatial shape of the wind field make the calibration process difficult and subjective. Nevertheless, results from the whole ensemble show that the modelled wind fields are unbiased with very good error statistics. These calibrations provide information for the distributions of values for radius of maximum winds and Holland B, parameters for which data are nonexistent in the Coral Sea region. An example of the use of the calibrated wind fields for the forcing of storm surge and wave models shows a very accurate comparison between modelled and measured surge and waves

Modelling tropical cyclone wave population of the Great Barrier Reef

Wave information is generated for the population of tropical cyclones that affects the Great Barrier Reef region. Several models are combined to create a wave database at 150,000 locations at a resolution of 1,500 m throughout this large and remote region. An autoregressive time series model, based on historical data, generated time series of position and central pressure for a very large ensemble of synthetic tropical cyclones. An empirical model was used to create the wind field throughout the wave generating area. The wind field had three components: The primary cyclone vortex, a secondary vortex used to model the low-pressure trough in which a cyclone is often embedded, and the external synoptic scale wind field created between the cyclone and the mid latitude high-pressure systems. WAMGBR, a wave generation model especially adapted for conditions in the Great Barrier Reef, was used to simulate a very large ensemble of storms on a nested grid system in transformed spherical coordinates. The results of the study are significant wave heights, peak periods, and mean directions for return periods between 20 and 1,000 years

A wave model for the Great Barrier Reef

A new wind wave generation model, WAMGBR, is presented that has been adapted from WAM especially for use in the complex geometry of the Great Barrier Reef. A technique (reef parameterization) has been presented that incorporates sub-grid scale dissipation caused by coral reefs. Three other improvements to WAM have been proposed. An explicit/implicit finite difference scheme has been implemented that allows for more efficient modelling (longer time steps) while maintaining diffusive characteristics that are at least as good as those of WAM. An offset in discrete angles creates more uniform diffusive characteristics. And, a transformed spherical coordinate system allows for more efficient grid sizes and smaller grid dependent refraction. Comparisons between modelling techniques and between model and measured data show that WAMGBR produces very good results in the difficult challenge of modelling both non-cyclonic and tropical cyclone waves in the geographically complex environment of the Great Barrier Reef

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 turbulence 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 time scale 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 time scales to influence the depth of the mixed layer

Source term balance in a severe storm in the Southern North Sea

This paper presents the results of a wave hindcast of a severe storm in the Southern North Sea to verify recently developed deep and shallow water source terms. The work was carried out in the framework of the ONR funded NOPP project (Tolman et al. 2013) in which deep and shallow water source terms were developed for use in third-generation wave prediction models. These deep water source terms for whitecapping, wind input and nonlinear interactions were developed, implemented and tested primarily in the WAVEWATCH III model, whereas shallow water source terms for depth-limited wave breaking and triad interactions were developed, implemented and tested primarily in the SWAN wave model. So far, the new deep-water source terms for whitecapping were not fully tested in shallow environments. Similarly, the shallow water source terms were not yet tested in large inter-mediate depth areas like the North Sea. As a first step in assessing the performance of these newly developed source terms, the source term balance and the effect of different physical settings on the prediction of wave heights and wave periods in the relatively shallow North Sea was analysed. The December 2013 storm was hindcast with a SWAN model implementation for the North Sea. Spectral wave boundary conditions were obtained from an Atlantic Ocean WAVEWATCH III model implementation and the model was driven by hourly CFSR wind fields. In the southern part of the North Sea, current and water level effects were included. The hindcast was performed with five different settings for whitecapping, viz. three Komen type whitecapping formulations, the saturation-based whitecapping by Van der Westhuysen et al. (2007) and the recently developed ST6 whitecapping as described by Zieger et al. (2015). Results of the wave hindcast were compared with buoy measurements at location K13 collected by the Dutch Ministry of Transport and Public Works. An analysis was made of the source term balance at three locations, the deep water location North Cormorant, the inter-mediate depth location K13 and at location Wielingen, a shallow water location close to the Dutch coast. The results indicate that at deep water the source terms for wind input, whitecapping and nonlinear four-wave interactions are of the same magnitude. At the inter-mediate depth location K13, bottom friction plays a significant role, whereas at the shallow water location Wielingen also depth-limited wave breaking becomes important.Environmental Fluid Mechanic

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