Derek M. Burrage

Trapping and dispersion of coral eggs around Bowden Reef, Great Barrier Reef, following mass coral spawning

Bowden Reef is a 5 km long kidney-shaped coral reef with a lagoon, located on the mid-shelf of the central region of the Great Barrier Reef. Field studies were carried out, in November 1986, at the time of mass coral spawning, of the water circulation around Bowden Reef and in the surrounding inter-reefal waters. The near-reef water circulation was strongly three-dimensional although the stratification was weak. In calm weather, coral eggs were aggregated in slicks along topographically controlled fronts. In the absence of a longshore current, water and coral eggs were trapped in the lagoon and in a boundary layer around Bowden Reef, by tidally driven recirculating motions. In the presence of a longshore current, some trapping occurred in the lagoon, but the bulk of the coral eggs was advected away from Bowden Reef and reached downstream reefs in a few days. This implies a likelihood of both self-seeding of reefs, and connectivity between reefs.

Mesoscale circulation features of the great barrier reef region inferred from NOAA satellite imagery

Abstract The commissioning of a NOAA satellite receiving station at Townsville in North Queensland in 1988 greatly expanded the A VHRR coverage of the northeast Australian region to include the entire Great Barrier Reef system and marginal seas. Selected imagery from this and a southern station installed previously at Aspendale, Victoria provide a valuable new perspective on oceanographic phenomena occurring in this ecologically significant region. This perspective could not be attained using conventional ship-board and in situ oceanographic sampling techniques. A rich spectrum of mesoscale oceanographic features is revealed in the analyzed imagery, and various features such as western boundary current meanders, frontal shear waves, eddies, and jets are described. The temporal and spatial variability of these features appears strongly linked to that of the larger-scale Coral Sea current circulation. Several of the features identified are unique to the region; others resemble features observed in other western boundary current systems, but are significantly modified by the complex regional topography, and by the presence of the Great Barrier Reef (GBR). Evidence has been found for a number of processes which have significant implications for the origin and maintenance of GBR ecosystems, including shelf edge exchange processes, stratified slope water intrusions onto the shelf, and boundary layer mixing around reefs. Such processes provide a mechanism for injection of cool nutrient-rich waters into the reef matrix. The imagery provides a clear picture of a well-organized, but spatially complex, frontal system existing in the southern Coral Sea, which is associated with enhanced commercial and recreational fishing activity in the region. The AVHRR imagery has thus proven to be a valuable tool for spatial mapping of oceanographic features throughout the GBR region, for hypothesis formation in dynamical and modeling studies, and for ship-board reconnaissance operations.

Structure and influence of tropical river plumes in the Great Barrier Reef: application and performance of an airborne sea surface salinity mapping system

Input of freshwater from rivers is a critical consideration in the study and management of coral and seagrass ecosystems in tropical regions. Low salinity water can transport natural and manmade river-borne contaminants into the sea, and can directly stress marine ecosystems that are adapted to higher salinity levels. An efficient method of mapping surface salinity distribution over large ocean areas is required to address such environmental issues. We describe here an investigation of the utility of airborne remote sensing of sea surface salinity using an L-band passive microwave radiometer. The study combined aircraft overflights of the scanning low frequency microwave radiometer (SLFMR) with shipboard and in situ instrument deployments to map surface and subsurface salinity distributions, respectively, in the Great Barrier Reef Lagoon. The goals of the investigation were (a) to assess the performance of the airborne salinity mapper; (b) to use the maps and in situ data to develop an integrated description of the structure and zone of influence of a river plume under prevailing monsoon weather conditions; and (c) to determine the extent to which the sea surface salinity distribution expressed the subsurface structure. The SLFMR was found to have sufficient precision ( 1 psu) and accuracy (∼3 psu) to provide a useful description of plumes emanating from estuaries of moderate discharge levels with a salinity range of 16 to 32 psu in the open sea. The aircraft surveys provided a means of rapidly assessing the spatial extent of the surface salinity distribution of the plume, while in situ data revealed subsurface structure detail and provided essential validation data for the SLFMR. The combined approach allowed us to efficiently determine the structure and zone of influence of the plume, and demonstrated the utility of sea surface salinity remote sensing for studying coastal circulation in tropical seas.

Linear systems analysis of momentum on the continental shelf and slope of the central Great Barrier Reef

Current meter records from a mooring transect deployed across the continental shelf and slope of the central Great Barrier Reef, Australia during 1985 have been analyzed in a study of the subtidal momentum balance. In the 3–20 day wave band, a single-input linear systems model of the subtidal along-shelf flow, driven by across-shelf pressure gradient (i.e., assuming semi-geostrophic balance), explained over 70% of the variance on the shelf, but only 30% at the shelf break and upper slope. A two-input model driven by along-shelf horizontal pressure gradient and wind stress, and incorporating along-shelf acceleration and bottom stress, explained approximately 60% of the variance on both the shelf and upper slope. The model responses evidently combine local wind-driven circulation and freely-propagating continental shelf waves. Linear resistance coefficients estimated from the two-input model averaged 0.07 cm s−1, but were higher (0.09) within the reef matrix and lower (0.06) near the coast.

Ocean Surface Roughness Spectrum in High Wind Condition for Microwave Backscatter and Emission Computations

AbstractOcean surface roughness plays an important role in air–sea interaction and ocean remote sensing. Its primary contribution is from surface waves much shorter than the energetic wave components near the peak of the wave energy spectrum. Field measurements of short-scale waves are scarce. In contrast, microwave remote sensing has produced a large volume of data useful for short-wave investigation. Particularly, Bragg resonance is the primary mechanism of radar backscatter from the ocean surface and the radar serves as a spectrometer of short surface waves. The roughness spectra inverted from radar backscatter measurements expand the short-wave database to high wind conditions in which in situ sensors do not function well. Using scatterometer geophysical model functions for L-, C-, and Ku-band microwave frequencies, the inverted roughness spectra, covering Bragg resonance wavelengths from 0.012 to 0.20 m, show a convergent trend in high winds. This convergent trend is incorporated in the surface rough...

Long-term current prediction in the central Great Barrier Reef

Abstract Long-term current records in the Great Barrier Reef region are needed to address major ecological problems such as periodic outbreaks of the coral predator, Crown of Thorns Starfish (COTS, Acanthaster sp.). In situ current meter data are sparsely distributed and typically available for deployments shorter than 1 year. A suite of linear systems models has therefore been developed which allow low-frequency along shelf currents to be specified using readily available meteorological and oceanographic forcing data. The models which are essentially statistical, nevertheless reflect our understanding of regional hydrodynamics. Using optimally lagged multilinear regression they allow predictions to be made quickly and economically from input time series and a few specified parameters. Models were calibrated using current meter mooring data obtained from a transect across the central Great Barrier Reef in 1985 and validated using data from similar deployments in 1987 and 1990. The models which are simplest to implement, perform well and are statistically efficient are those based on the geostrophic across-shelf momentum balance. Using as inputs coastal sea levels or, when available, offshore sea level differences, they can precisely predict currents over time spans of up to 25 years. They accurately respond to fluctuations at weather time scales and, when offshore differences are used, at seasonal and inter-annual scales. The predicted currents are being used to drive advective models of COTS larvae dispersal and to set boundary conditions for more complex numerical hydrodynamic current simulation models.

Simulating passive Microwave radiometer designs using SIMULINK

The authors have developed a simulation system for a passive microwave radiometer using MATLAB and SIMULINK from The MathWorks, Inc. The system comprises a high-level block diagram simulation for a modern instrument, the Scanning Low Frequency Microwave Radiometer, or SLFMR. The hardware prototype SLFMR, which was designed to map sea surface salinity, was implemented using the classic pulsed noise injection Dicke radiometer design. A suite of lower-level user-written components arranged in user block libraries has also been developed. The simulations functionally model the signal flow of the prototype, with random fluctuations representing the equivalent noise temperature of various signals propagating through the system, so the fundamental physics of random noise processes are preserved. Examples of several simulations are presented, and the computational feasibility, performance, and possible enhancements are assessed.

Optimizing Performance of a Microwave Salinity Mapper: STARRS L-Band Radiometer Enhancements*

Abstract Airborne microwave radiometers for salinity remote sensing have advanced to a point where operational surveys can be conducted over the inner continental shelf to observe the evolution of freshwater plumes emanating from rivers and estuaries. To determine seawater microwave emissivity, and hence conductivity and salinity, precisely and accurately demands high instrument sensitivity, stability, and sampling rates; such requirements involve significant design trade-offs. The Salinity, Temperature, and Roughness Remote Scanner (STARRS) was developed to enhance these features relative to existing instruments. The authors describe here key elements of the STARRS design and the results of early performance assessments and deployments. During early deployments, the instrument performed well in areas of moderate to high salinity signal-to-noise ratio, but more homogenous areas revealed band-limited random signal fluctuations on the order of a 6-min period and ∼1-K amplitude that were of internal origin. ...

Observing sea surface salinity in coastal domains using an airborne surface salinity mapper

Sea Surface Salinity directly affects the status of coastal ecosystems and serves as a tracer for seawater constituents associated with freshwater runoff. As part of an NRL-sponsored study of the dynamics of coastal buoyancy jets (CoJet), which began in July, 2000, the original Scanning Low Frequency Microwave Radiometer (SLFMR) was deployed in various coastal locations to evaluate its performance for mapping sea surface salinity, and demonstrate its application to studies of coastal plumes and buoyant jets. In a sequence of three campaigns, the radiometer was flown repeatedly over the Cheseapeake and Mobile Bay plumes and over the northern Gulf of Mexico and Florida Bay using a twin-engine Piper Navajo aircraft. Extensive surveys of sea surface salinity distributions were conducted on time scales of a few hours. The instrument was field calibrated using in situ data from oceanographic research vessels and the resulting salinity maps were corrected for known environmental influences. The logistical convenience and broad dynamic range of the instrument allowed surface maps to be generated quickly over waters that were either significantly fresher or more saline than standard seawater. The instrument performance and resulting map quality were thus found to meet the requirements of coastal oceanographic studies that are characterized by large buoyancy signals, and a variety of forcing effects that evolve relatively rapidly in time and space. The instrument and data processing system are first described and two new methods of field calibration method are presented. Examples of surface salinity maps of rapidly evolving coastal plume features are then described and interpreted using supporting in situ data. Finally, the overall capability and utility of the system is evaluated, and recent advances in the technology and future prospects are briefly considered.

Long-term sea-level variations in the central Great Barrier Reef

Abstract Low frequency sea-level variations and associated geostrophic currents in the central Great Barrier Reef (GBR) region near Townsville are studied using optimally-lagged multivariate regression. The analyses show that pressure-adjusted coastal sea levels and mid-shelf geostrophic currents are influenced predominantly by local along-shelf wind stress at the weather time-scale, and by climatic variables, such as atmospheric pressure and temperature, at seasonal and inter-annual time-scales. These forcing variables can specify sea levels over annual and inter-annual time-scales with a forecasting skill of 0.53 and 0.22, respectively (where 1.0 is perfect skill). Associated along-shelf geostrophic currents can be forecast with a skill of 0.57 over an annual time scale. If, instead, absolute coastal sea levels or offshore sea-level differences are used to specify the along-shelf geostrophic current, the forecasting skill is 0.75. A characteristic El Nino/Southern Oscillation (ENSO) response is detected for time periods up to 25 years in monthly sea-level both at Townsville and at western Pacific island sea-level stations. This spatially coherent response varies in intensity and phase within the Coral Sea. Sea-level differences show a pattern which characterizes known features of the large-scale circulation of the Coral Sea. These very low frequency sea-level variations in the Coral Sea must be taken into account to obtain accurate predictions of along-shelf geostrophic current variations on seasonal and inter-annual time scales. Regression analysis and a diagnostic river plume model show that the influence of the major rivers can produce sea-level changes due to buoyancy of order 5 cm. The corresponding errors in geostrophic velocities estimated using pressure-adjusted Townsville sea-level data alone are of order 5 cm s−1 rms.