Craig R. Steinberg

The Southwest Pacific Ocean circulation and climate experiment (SPICE)

The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions.

IMOS National Reference Stations: A Continental-Wide Physical, Chemical and Biological Coastal Observing System

Sustained observations allow for the tracking of change in oceanography and ecosystems, however, these are rare, particularly for the Southern Hemisphere. To address this in part, the Australian Integrated Marine Observing System (IMOS) implemented a network of nine National Reference Stations (NRS). The network builds on one long-term location, where monthly water sampling has been sustained since the 1940s and two others that commenced in the 1950s. In-situ continuously moored sensors and an enhanced monthly water sampling regime now collect more than 50 data streams. Building on sampling for temperature, salinity and nutrients, the network now observes dissolved oxygen, carbon, turbidity, currents, chlorophyll a and both phytoplankton and zooplankton. Additional parameters for studies of ocean acidification and bio-optics are collected at a sub-set of sites and all data is made freely and publically available. Our preliminary results demonstrate increased utility to observe extreme events, such as marine heat waves and coastal flooding; rare events, such as plankton blooms; and have, for the first time, allowed for consistent continental scale sampling and analysis of coastal zooplankton and phytoplankton communities. Independent water sampling allows for cross validation of the deployed sensors for quality control of data that now continuously tracks daily, seasonal and annual variation. The NRS will provide multi-decadal time series, against which more spatially replicated short-term studies can be referenced, models and remote sensing products validated, and improvements made to our understanding of how large-scale, long-term change and variability in the global ocean are affecting Australias coastal seas and ecosystems. The NRS network provides an example of how a continental scaled observing systems can be developed to collect observations that integrate across physics, chemistry and biology.

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.

Do Clouds Save the Great Barrier Reef? Satellite Imagery Elucidates the Cloud-SST Relationship at the Local Scale

Evidence of global climate change and rising sea surface temperatures (SSTs) is now well documented in the scientific literature. With corals already living close to their thermal maxima, increases in SSTs are of great concern for the survival of coral reefs. Cloud feedback processes may have the potential to constrain SSTs, serving to enforce an “ocean thermostat” and promoting the survival of coral reefs. In this study, it was hypothesized that cloud cover can affect summer SSTs in the tropics. Detailed direct and lagged relationships between cloud cover and SST across the central Great Barrier Reef (GBR) shelf were investigated using data from satellite imagery and in situ temperature and light loggers during two relatively hot summers (2005 and 2006) and two relatively cool summers (2007 and 2008). Across all study summers and shelf positions, SSTs exhibited distinct drops during periods of high cloud cover, and conversely, SST increases during periods of low cloud cover, with a three-day temporal lag between a change in cloud cover and a subsequent change in SST. Cloud cover alone was responsible for up to 32.1% of the variation in SSTs three days later. The relationship was strongest in both El Niño (2005) and La Niña (2008) study summers and at the inner-shelf position in those summers. SST effects on subsequent cloud cover were weaker and more variable among study summers, with rising SSTs explaining up to 21.6% of the increase in cloud cover three days later. This work quantifies the often observed cloud cooling effect on coral reefs. It highlights the importance of incorporating local-scale processes into bleaching forecasting models, and encourages the use of remote sensing imagery to value-add to coral bleaching field studies and to more accurately predict risks to coral reefs.

A National Reference Station infrastructure for Australia - Using telemetry and central processing to report multi-disciplinary data streams for monitoring marine ecosystem response to climate change

As part of a broader Integrated Marine Observing System (IMOS), the marine community in Australia is developing a National Reference Station (NRS) network to monitor coastal processes. IMOS is an Australian Government initiative established under the National Collaborative Research Infrastructure Strategy (NCRIS). The aim of NCRIS is to provide researchers with access to the infrastructure and networks to build automated and ongoing in situ observing systems necessary to undertake world-class research. The NRS network fulfils this role as part of the Australian National Mooring Network, which is one of eleven IMOS facilities. The nine stations around Australia continue and expand the three existing sites where monthly water quality data have been collected since the 1940s. The overall aim of the NRS network is to provide the data to examine interactions between major coastal boundary currents and continental shelf ecosystems, especially in the context of climate change. To do this each NRS will provide long-term data series of physical and chemical parameters alongside community composition and primary (phytoplankton) and secondary (zooplankton) biological production and diversity. This will be achieved using a combination of in situ measurements (moored sensors) and monthly visits to collect samples for laboratory analysis. The NRS will provide critical baseline data to examine the impact of human stresses (such as climate change and eutrophication) on Australian marine ecosystems.

Evaluation of a New Airborne Microwave Remote Sensing Radiometer by Measuring the Salinity Gradients Across the Shelf of the Great Barrier Reef Lagoon

Over the last ten years, some operational airborne remote sensing systems have become available for mapping surface salinity over large areas in near real time. A new dual-polarized Polarimetric L-band Multibeam Radiometer (PLMR) has been developed to improve accuracy and precision when compared with previous instrument generations. This paper reports on the first field evaluation of the performance of the PLMR by measuring salinity gradients in the central Great Barrier Reef. Before calibration, the raw salinity values of the PLMR and conductivity-temperature-depth (CTD) differed by 3-6 psu. The calibration, which uses in situ salinity data to remove long-term drifts in the PLMR as well as environmental effects such as surface roughness and radiation from the sky and atmosphere, was carried out by equating the means of the PLMR and CTD salinity data over a subsection of the transect, after which 85% of the salinity values between the PLMR and CTD are within 0.1 psu along the complete transect. From offshore to inshore across the shelf, the PLMR shows an average cross-shelf salinity increase of about 0.4 psu and a decrease of 2 psu over the inshore 20 km at -19deg S (around Townsville) and -18deg S (around Lucinda), respectively. The average cross-shelf salinity increase was 0.3 psu for the offshore 100 km over all transects. These results are consistent with the in situ CTD results. This survey shows that PLMR provided an effective method of rapidly measuring the surface salinity in near real time when a calibration could be made.

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.

Validation of radar-based Lagrangian trajectories against surface-drogued drifters in the coral sea, Australia

Surface current velocity fields measured every 10 min by a high-frequency ocean radar system (HF radar) located at the southern Great Barrier Reef (GBR), Australia, were used to compute Lagrangian trajectories (radar-based trajectories). The radar-based trajectories were validated against surface-drogued satellite tracked drifting buoys released on the shelf inside the reef lagoon and on the continental slope. Current speeds estimated from the drifters were typically within 5% to 8% of the HF radar currents extracted at the exact position occupied by the drifters for each sampled time, but some large current velocity biases occurred over short periods of time in shallow areas. Maximum separation distances between the drifter and radar-based tracks ranged from 5 km to 14 km on the shelf and from 4 km to 35 km on the slope for 24 tracks tracked up to one week. The reduction of the predictability of the radar-based trajectories occurred mostly in the vicinity of the islands of the reef matrix, suggesting the influence of small scale processes in the drifter paths not seen at the HF radar spatial resolution. A filtering procedure was applied a posteriori to the u and v components of the current velocity vectors calculated from non-quality controlled radial data obtained from the IMOS archive. Interpolation was used to cover temporal gaps less than 3 h allowing radar-based trajectories to be followed for up to 7 days. The use of radial data reprocessed and quality controlled by the Australian Coastal Radar Network (ACORN) extended the tracking period up to 13 days, reprocessing allows retrieval of some information lost in the non-quality controlled radial data.