Deep Sea Research Part II: Topical Studies in Oceanography
Observed and simulated Lagrangian and eddy characteristics of the East Australian Current and the Tasman Sea
Introduction
Godfrey et al. (1980) make the following observation of the overall picture emerging from the observations, at that time, of the East Australian Current, “In contrast (to other western boundary currents), the flow patterns in the East Australian Current are so complex and variable that it is often difficult even to decide whether a single continuous current exists”. The EAC is prominent amongst western boundary currents for the dominance of eddy kinetic energy (Stammer, 1997). Seasonal variability therefore accounts for a comparatively smaller fraction of the total variance in this region. An understanding of the instabilities and transient dynamical processes taking place is therefore necessary for a complete understanding of the EAC.
It has been established that the EAC has a distinct seasonal cycle (Ridgway and Godfrey, 1997). The EAC is characterised by relatively warm/fresh-water masses emanating from the Coral Sea. During the Austral summer the warm-water masses are sourced from the South Equatorial Current (Church, 1987), forming an observable shelf current along the shelf break of the Great Barrier Reef. During the Austral winter the coherent portion of the EAC weakens and retreats to a narrower region from southern Queensland to northern New South Wales. This is similarly confirmed by Schiller et al. (2008), which show the ratio of eddy kinetic energy to total kinetic energy for January and July (see Figure 7 of that article) in the Australian region based on the BLUElink ocean reanalysis (described in more detail in Section 2).
Historical observations of XBT’s (Nilsson and Cresswell, 1981) and drifting buoys (Cresswell and Golding, 1979, Chiswell and Rickard, 2008), discussed in more detail in the next section, have demonstrated the EAC is frequently composed of eddies that obscure the interpretation of the EAC as a boundary current. Ridgway and Dunn (2003) developed a high-resolution climatology for the Australian region from historical in situ observations. The average maximum alongshore transport of the EAC shows a local minima between 27 and 29°S. The local minimum lies to the north of the classical separation point between 30 and 34°S (Godfrey et al., 1980) and indicates a secondary EAC separation point. This is also associated with a local extremum in the mean dynamic topography indicating a recirculation (Ridgway and Dunn, 2003).
Australian marine science has established a record for observing the ocean through innovative technologies and a golden period of observing the EAC and the Tasman Sea took place in the 1970s and 1980s. A collaboration between the Royal Australian Navy and the CSIRO undertook world leading studies of anticyclonic eddies defining many of the properties of this now classical type of eddy that is generated from the EAC flow separation. Early studies involved largely ship based hydrographic sections (Andrews and Scully-Power, 1976, Nilsson et al., 1977, Nilsson and Cresswell, 1981). In these early days eddies were individually named in an analogous way to atmospheric cyclone, for example Maria, Mario and Leo. A tradition that did not persist with the realisation of the complexity of eddy interactions (Airey, 1983) and the introduction of altimetry showing the abundance of eddies. Nonetheless, the EAC does generate only a handful of large-scale eddies each year, which have a dominant and persistent influence over the Tasman Sea circulation and perhaps there is a basis for reintroducing this practice amongst ocean forecasters.
Critical to early observational campaigns of the mesoscale were the use of drogued buoys. In 1972 a 3 m long transponder designed to be balloon-borne for high-altitude studies was put into a drifter and tracked by the French satellite EOLE. The drifter worked for 2 months and revealed meanders of the EAC. The drifter was a PVC/fiberglass spar buoy 5 m long. In 1977, drifters were used to track anticyclonic eddies in the EAC by the Nimbus satellite. These included eddies A–C, with B being studied for an year (Cresswell and Golding, 1979, Chiswell and Rickard, 2008, Nilsson and Cresswell, 1981). In 1978 eddy J was studied for over a year with drifters and ship based hydrographic surveys (Cresswell, 1980 and others in a special issue of the Australian Journal of Marine and Freshwater Research). This early work was at the leading edge of demonstrating the instabilities of the EAC and the dominance of geostrophic turbulence in the Tasman Sea. The complexity of eddy–eddy interactions in the Tasman Sea was also observed during this period with the identification of a stratified merger of a new and an old anticyclonic eddy. In 1980/81 drifting buoys were used to track and study eddies Leo and Maria—and subsequently Mario after these two eddies coalesced (Cresswell, 1982, Cresswell and Legeckis, 1986). The coalescence was captured by the drifters and two ship surveys. The resulting eddy, Mario, was perhaps unusual in that it moved northward from a southern NSW location. CLS Argos was used for drifter tracking from this time onwards.
The deployment of drifting buoys in the Tasman Sea declined in the 1990s just as the satellite remote sensing and Argo profilers were emerging. Buoy deployments in the Indian and Southern Ocean over this period were supported by the Bureau of Meteorology to support meteorological forecasting. Operational buoy deployments sample the sea-surface temperature and sea-level pressure. Deployments are designed to sample the prevailing frontal systems. A similar operational schedule of buoys was deployed by the New Zealand meteorological service in the southern Tasman Sea for similar meteorological motivations.
The introduction of remote sensing of the sea-surface temperature and altimetry provided the first images of the global ocean variability and demonstrated the dominance of eddy kinetic energy over much of the ocean. Over the past decade estimates for the mean dynamic topography, the global tides and other atmospheric corrections have provided sea-surface height anomalies from altimetry with an estimated precision of 5 cm (Laing and Challenor, 1999). The range of surface height anomalies for mesoscale eddies in the Tasman Sea is <1 m. Therefore eddies with surface height anomalies between 20 cm and 1 m are observable with variability larger than the expected error. The use of narrow-swath altimeters with repeat track periods of 10 days (Topex-Poseidon, Jason and Jason2) and 35 days (ERS1, ERS2 and Envisat) provides low temporal sampling which limits the wavelength of eddies that can be observed to greater than ∼150 km (Ducet et al., 2000). In the Tasman Sea this represents the large, deep and stable vortices that persist and dominate the local circulation. Features such as fronts and the secondary circulation of eddies are poorly observed from the available narrow-swath altimeters. A wide-swath altimeter (Fu et al., 2009) or constellations of narrow swath altimeters are required to adequately observe these features.
Drifting buoys approximately propagate with the ocean circulation thereby tracing a Lagrangian trajectory. Lagrangian trajectories can provide estimates of the mean velocity over different averaging periods from the mesoscale through climate scales. The precision of the estimates of surface circulation obtained is independent of the surface topography and is therefore complementary to altimetry observations (Hernandez et al., 1995). During the past decade the objectives of drifting buoys have targeted the mean upper-ocean circulation with a global distribution of 1250 buoys (Niiler, 2001), which represents an approximate density of one buoy for every 5°×5° surface area. This target density is adequate to capture mean circulation such as the Pacific Ocean tropical current system. In the Tasman Sea throughout the 1990s and the 2000s the drifting buoy deployments contributing to the global array declined and were shown to be below this target density (Summons et al., 2006). It was determined from a Lagrangian analysis of the BLUElink ocean Reanalysis (Schiller et al., 2008) that observing the EAC would require a targeted sampling strategy. A series of pilot experiments through a collaboration between the Bureau of Meteorology and the NOAA was initiated in 2007 (Brassington et al., 2007b) and designed to test a sustainable deployment strategy for the East Australian Current using volunteer ships.
A renewal of drifting buoy deployments has several motivations. The standard buoy design for the Surface Velocity Program (SVP; Lumpkin and Pazos, 2007) and analysis tools are now mature. Drifting buoys offer a reliable and high-yield observing system in terms of cost per observation and are suitable for sustained deployments. The drifting buoys provide a high quality in situ measurement of ocean currents and are a useful calibration/validation for the quality of other platforms such as the analysis of altimetry and HF radars. In addition, Lagrangian trajectories observe the total current and therefore provide in situ measurements of the mesoscale ocean circulation such as eddies, fronts and instabilities of boundary currents. In places where the circulation is closed, such as eddies, drifting buoys can undergo multiple orbits to provide sufficient observations to interpret their dynamics (Richardson et al., 2000, Font et al., 2004, Sangra et al., 2005, Brassington, 2010).
In places where the circulation is not closed, where the buoys will have a relatively short residence time, a higher density or repeated sampling is required to provide sufficient observations to interpret the dynamics. Many oceanic processes such as heat and momentum transport, and biological processes of larval dispersal, are controlled by Lagrangian processes. Lagrangian observations provide essential observations for evaluation of these models (e.g., Chiswell, 2009, Chiswell and Rickard, 2008). Further, Lagrangian observations are also critical to evaluation of models for use in Search and Rescue (Davidson et al., 2009) and Marine Accident and Emergency Service (Hackett et al., 2009) applications. The standard buoy also includes a temperature sensor that provides a bulk measurement of sea surface temperature with a known error characteristic. The temperature observations are routinely used for calibration/validation of satellite SST products that improve the detection of biases and time-dependent errors (Donlon et al., 2002, Donlon et al., 2009).
This paper organises results from recent observational experiments and model/observation assimilated analyses, focusing particularly upon the mesoscale variability and processes that are a permanent feature of this region. The observations discussed and the model simulations used are presented, and the specific methods that have been developed and recent techniques in pattern matching have been adapted to aid in interpreting the observations and model simulations are described.
Section snippets
Observations
A multi-year drifting buoy experiment was designed to determine if the volunteer observing ship (VOS) program could be used as a cost-effective method for seeding the EAC and the Tasman Sea (Brassington et al., 2007b, Brassington et al., 2008). It is difficult to sustain observations in a boundary current with surface drifting buoys due to the relatively short period that the buoys are resident within the current and a regular deployment strategy was explored. The experiment made use of two
Transient EAC separation through a vortex dipole
A pair of drifting buoys was deployed on 21 February 2007 into the surface circulation of an anticyclonic eddy within the EAC (Brassington et al., 2007b, Brassington, 2010). If we assume the analysed sea level anomaly (not shown) is in geostrophic balance the geostrophic speed is estimated to be −0.53 m s−1, which compares well to the observed tangential speed of the two buoys of 0.61 and 0.64 m s−1, respectively. The difference is partly accounted for by the mean eddy propagation speed estimated
Conclusion
A transient separation event of the EAC was observed to occur during March 2007. The event involved a warm-core anticyclonic eddy propagating within the EAC mean flow between 26 and 29°S where it encountered a cyclonic circulation anomaly over the continental shelf. The anticyclonic eddy surface circulation was observed by a pair of drifting buoys throughout the separation event. The cyclonic circulation was observed as a cool surface anomaly at ∼29.5°S and was simulated by the BRANv2.2 as a
Acknowledgments
The authors thank the BLUElink science team, Bureau of Meteorology and CSIRO for use of the BLUElink model products. We gratefully acknowledge Lisa Cowen and Graeme Ball from the Bureau of Meteorology, Rick Lumpkin from the NOAA and the crew of both the Forum Samoa II and the Capitaine Tasman for assisting in the coordination of the drifting buoy experiments described. We gratefully acknowledge Dr. George Cresswell’s communications on the history of drifting buoys and mesoscale studies in the
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