East Australian Current System Research Articles

Mean‐State and Seasonal Variability in Temperature Structure and Heat Transport in the East Australian Current System From a Multi‐Decadal Regional Ocean Model

AbstractWestern Boundary Currents (WBCs) such as the East Australian Current (EAC) are vital for moving heat from low to high latitudes, controlling regional weather and global climate. Previous EAC System studies have provided a general overview of temperature variability and heat transport, but they lacked spatial and seasonal detail. Using a high‐resolution, 26‐year‐long ocean model simulation, we systematically characterize the seasonal variability and structure in key temperature metrics and heat transport in the EAC System. Our findings reveal a clear seasonal cycle with a poleward expansion of key variables in summer (mean and eddy kinetic energy, upper ocean heat content, heat transport) and contraction equatorward with reductions in winter. We show that the dynamical regime transition, from jet to eddy‐dominated at the EAC separation zone (S), modifies the net meridional heat transport. Upstream of separation, transport is poleward, with some zonal input of heat. Downstream of EAC separation, there is an increase in heat recirculation by mesoscale eddies combined with a smaller zonal export of heat. This study highlights the significance of resolving spatio‐temporal variability in WBC systems driven by mesoscale dynamics, which will have implications for the representation of critical dynamics in future climate models.

Comparison of 4-dimensional variational and ensemble optimal interpolation data assimilation systems using a Regional Ocean Modeling System (v3.4) configuration of the eddy-dominated East Australian Current system

Abstract. Ocean models must be regularly updated through the assimilation of observations (data assimilation) in order to correctly represent the timing and locations of eddies. Since initial conditions play an important role in the quality of short-term ocean forecasts, an effective data assimilation scheme to produce accurate state estimates is key to improving prediction. Western boundary current regions, such as the East Australia Current system, are highly variable regions, making them particularly challenging to model and predict. This study assesses the performance of two ocean data assimilation systems in the East Australian Current system over a 2-year period. We compare the time-dependent 4-dimensional variational (4D-Var) data assimilation system with the more computationally efficient, time-independent ensemble optimal interpolation (EnOI) system, across a common modelling and observational framework. Both systems assimilate the same observations: satellite-derived sea surface height, sea surface temperature, vertical profiles of temperature and salinity (from Argo floats), and temperature profiles from expendable bathythermographs. We analyse both systems' performance against independent data that are withheld, allowing a thorough analysis of system performance. The 4D-Var system is 25 times more expensive but outperforms the EnOI system against both assimilated and independent observations at the surface and subsurface. For forecast horizons of 5 d, root-mean-squared forecast errors are 20 %–60 % higher for the EnOI system compared to the 4D-Var system. The 4D-Var system, which assimilates observations over 5 d windows, provides a smoother transition from the end of the forecast to the subsequent analysis field. The EnOI system displays elevated low-frequency (>1 d) surface-intensified variability in temperature and elevated kinetic energy at length scales less than 100 km at the beginning of the forecast windows. The 4D-Var system displays elevated energy in the near-inertial range throughout the water column, with the wavenumber kinetic energy spectra remaining unchanged upon assimilation. Overall, this comparison shows quantitatively that the 4D-Var system results in improved predictability as the analysis provides a smoother and more dynamically balanced fit between the observations and the model's time-evolving flow. This advocates the use of advanced, time-dependent data assimilation methods, particularly for highly variable oceanic regions, and motivates future work into further improving data assimilation schemes.

Energetics of Eddy–Mean Flow Interactions in the East Australian Current System

Abstract Examining eddy–mean flow interactions in western boundary currents is crucial for understanding the mechanisms of mesoscale eddy generation and the role of eddies in the large-scale circulation. However, this analysis is lacking in the East Australian Current (EAC) system. Here we show the detailed three-dimensional structure of the eddy–mean flow interactions and energy budget in the EAC system. The energy reservoirs and conversions are greatest in the upper 500 m, with complex vertical structures. Strong mean kinetic energy is confined within a narrow band (24.5°–32.5°S) in the EAC jet. Most energy is contained in the eddy fields instead of the mean flow in the EAC typical separation and extension regions (south of 32.5°S). Strong barotropic instability is the primary source of eddy kinetic energy north of 36°S, while baroclinic instability dominates the eddy kinetic energy production in the EAC southern extension, which peaks in the subsurface. The mean flow transfers 5.22 GW of kinetic energy and 3.33 GW of available potential energy to the eddy field in the EAC typical separation region. The largest conversion term is from available potential energy conversion from the mean flow to the eddy field through baroclinic instability, dominating between 29° and 35.5°S. Nonlocal eddy–mean flow interactions also play a role in the energy exchange between the mean flow and the eddy fields. This study provides the mean state of the eddy–mean flow interactions in the EAC system, paving the way for further studies exploring seasonal and interannual variability and provides a baseline for assessing the impact of environmental change.

Quantifying Cross-Shelf Transport in the East Australian Current System: A Budget-Based Approach

Abstract Cross-shelf transport plays an important role in the heat, salt, and nutrient budgets of the continental shelf. In this study, we quantify cross-shelf volume transport and explore its dynamics within a high-resolution (2.5–6 km) regional ocean model of the East Australian Current (EAC) System, a western boundary current with a high level of mesoscale eddy activity. We find that the largest time-mean cross-shelf flows (>4 Sv per 100 km; 1 Sv ≡ 106 m3 s−1) occur inshore of the coherent western boundary current, between 26° and 30°S, while the strongest time-varying flows occur in the EAC southern extension, poleward of 32°S, associated with mesoscale eddies. Using a novel diagnostic equation derived from the momentum budget we show that the cross-shelf transport is dominated by the baroclinic and geostrophic component of the velocities, as the EAC jet is relatively free to flow over the variable shelfbreak topography. However, topographic interactions are also important and act through the bottom pressure torque term as a secondary driver of cross-shelf transport. The importance of topographic interaction also increases in shallower water inshore of the coherent jet. Downstream of separation, cross-shelf transport is more time-varying and associated with the interaction of mesoscale eddies with the shelf. The identification of the change in nature and drivers of cross-shelf transport in eddy versus jet dominated regimes may be applicable to understanding cross-shelf transport dynamics in other boundary current systems. Significance Statement Cross-shelf transport, i.e., the movement of water from the open ocean on or off the continental shelf, is not reported often as it is difficult to measure and model. We demonstrate a simple but effective method to do this and, using an ocean model, apply it to the East Australian Current System and show what drives it. The results show two distinct regimes, which differ depending on which part of the current system you are in. Our results help to place observations of cross-shelf transport in better context and provide a framework within which to consider the transport of other things such as heat and carbon from the open ocean to the continental shelf.

Dynamics of Interannual Eddy Kinetic Energy Modulations in a Western Boundary Current

AbstractAmong Western Boundary Currents, the East Australian Current (EAC) has a more energetic eddy field relative to its mean flow, however, the relationship between upstream transport and downstream eddy kinetic energy (EKE) is still unclear. We investigate the modulation of downstream EKE in the EAC's typical separation region (Tasman EKE Box) (33.S–36.S) based on a long‐term (22‐year), high‐resolution (2.5–6 km) model simulation and satellite altimeter observations from 1994 to 2016. Our results show that the poleward EAC transport at S leads the EKE in the Tasman EKE Box by 93–118 days. Barotropic instabilities are the primary source of EKE, and they control EKE variability in the EAC system. Anticyclonic eddies shed from the EAC dominate from S–S during high‐EKE periods, but in low‐EKE periods anticyclonic eddies penetrate even further south by .

Oceanic Circulation Drives the Deepest and Longest Marine Heatwaves in the East Australian Current System

AbstractAlthough the impacts of marine heatwaves (MHWs) can extend well below the ocean surface, little is known about how oceanic and atmospheric forcings control their vertical structure. Here, we relate the MHW drivers to their sub‐surface characteristics in different dynamical regimes including the East Australia Current. We detect MHWs in the depth‐dependent surface mixed layer over 30‐years and use a heat budget to identify the dominant mechanisms driving them. We show that MHWs in the Western Boundary Current (WBC) jet are predominantly driven by air‐sea heatflux whilst in the WBC extension, MHWs are advection‐driven. The deepest and longest MHWs are advection‐driven and are more prevalent in autumn and winter. Surface (latent) flux‐driven MHWs are shallower and occur predominantly in summer. Demonstrating how MHW characteristics are linked to their drivers facilitates their prediction through driver diagnosis, especially below the surface where observations are sparse and ecological impact is high.

Entrainment and development of larval fish assemblages in two contrasting cold core eddies of the East Australian Current system

Cyclonic eddies are diverse in their size, age, upwelling and behaviour, which has significant implications for fisheries production and connectivity when they interact with the continental shelf. To ascertain coastal entrainment by eddies, we compared the larval fish community of 2 contrasting cyclonic eddies in 3 depth strata (0-5, 5-50, 50-100 m), and with the adjacent shelf community. The frontal cyclonic eddy was smaller and younger than the mesoscale cyclonic eddy. A larval fish entrainment index, based on the ratio of coastal to oceanic taxa, revealed the relative abundance of coastal larvae entrained into the upper mixed layer of the frontal eddy, consistent with published numerical modelling studies of similar eddies. The frontal eddy had a high abundance of commercially important coastal taxa entrained from the inner shelf. However, the adjacent inner shelf water and putative location for frontal eddy formation had recently been displaced by the East Australian Current, resulting in the larval fish community being dominated by oceanic taxa. The spatial and temporal dynamics of coastal entrainment into the larger, older cyclonic eddy and the adjacent shelf region were revealed by mixtures of coastal and oceanic taxa in each of the depth strata. The larger cyclonic eddy had a higher biomass of zooplankton, indicating the cumulative effects of eddy age and production. Eddies which interact with the shelf enable cross-shelf mixing and may contribute to coastal fisheries.

Quantitative Mapping of the East Australian Current Encroachment Using Time Series Himawari‐8 Sea Surface Temperature Data

AbstractThe East Australian Current (EAC) is a highly dynamic western boundary current that exerts significant influences on the marine ecosystem off southeast margin of Australia. For the first time, a quantitative mapping of the EAC system was conducted using Himawari‐8 sea surface temperature data and a Topographic Position Index (TPI)‐based image processing technique. The validation using Bluelink ReANalysis data suggested good reliability of our mapping results. Subsequent quantitative examination provides new insights into the EAC's cross‐shelf movement which is a driving force of adjacent slope‐shelf circulations. Along the coast of New South Wales (NSW), large‐scale and high‐frequency EAC encroachment was observed, being every 60–80 days upstream (30–32°S) and every 90–100 days downstream (33–35°S), which is associated with the EAC's intrinsic oscillation and eddy shedding. Downstream, the EAC encroachment exhibits a 20‐day longer period and a double amplitude. Such dephasing is most likely due to the abrupt change of regime at the EAC separation point (32–33°S). Upstream, the EAC encroachment also exhibits seasonality, being 10–15 km closer to the coast in austral summer, which is due to the seasonal EAC broadening. Higher‐frequency (16–32 days) and smaller‐magnitude EAC intrusion was observed along the entire NSW coast (28–37°S), which is possibly associated with EAC's meanders and frontal eddies. In the extension zone (37.30–44°S), we observed maximum EAC encroachment in summer, which is an expression of the seasonal boundary flow off eastern Tasmania. A data set of EAC encroachment (2015–2017) is available (see Data Availability Statement).

Downstream Evolution of the East Australian Current System: Mean Flow, Seasonal, and Intra‐annual Variability

AbstractThe East Australian Current (EAC) varies on both seasonal and mesoscale eddy related timescales, with important impacts for weather, primary productivity, and fisheries. While the EAC and its eddies have been the subject of many synoptic studies, a consistent three‐dimensional picture of the current, its downstream evolution, and the associated seasonal and intra‐annual variability is lacking. We use a 22‐year model simulation to quantify the EAC's temporal and latitudinal variability, from where it flows as a mostly coherent jet (27° S) to its separation from the coast (31–33° S), its energetic eddy field in the Tasman Sea and its southern extension. We show that the current intensifies and deepens poleward toward the separation latitude; the mean poleward transport reaches a maximum of 26.8 Sv and the seasonal cycle amplitude peaks at 6.2 Sv. Over the separation region, the EAC core broadens and shallows, mean poleward transport is lower and the transport standard deviation peaks at 22.0 Sv. The meandering EAC jet is modulated at interannual timescales, while the eddy driven transport over the separation region has a strong annual modulation, related to the annual cycle in eddy kinetic energy. SSH variability at intra‐annual frequencies is described primarily by an alongshore propagating mode and an onshore propagating mode, demonstrating that mesoscale SSH variability stems from a combination of intrinsic instabilities of the EAC jet and westward propagating Rossby waves. The results provide the first holistic analysis of the EAC system along its latitudinal range.

Eddy‐Driven Cross‐Shelf Transport in the East Australian Current Separation Zone

AbstractIn western boundary current systems, sharp velocity gradients between the poleward flowing jet and coastal waters generally act to inhibit cross‐shelf exchange. Downstream of jet separation, dynamic mesoscale eddies dominate the flow. In the East Australian Current System, counter‐rotating eddy dipoles are often present which, in the appropriate configuration, have potential to drive cross‐shelf transport. However, this eddy dipole mode is poorly understood in the framework of cross‐shelf exchange and the effect of these structures on shelf waters is uncertain. Using 25 years of satellite altimetry, as well as in situ sampling of a typical dipole event, we investigate the characteristics of eddy‐driven cross‐shelf exchange. We show that the maximum onshore velocity is driven by an eddy dipole structure and occurs in a defined latitudinal band between 33°S and 34°S more than 50% of the time. We sample a typical eddy dipole and find a strong onshore jet, 37 km wide, with velocities up to 1.78 m s and a transport of at least 16 Sv. Hydrographic data from an autonomous underwater glider show that this jet manifests on the shelf as a subsurface intrusion of warm salty water extending from offshore up onto the midshelf. In the light of climatic changes in western boundary current transport and the increase in their eddy kinetic energy, understanding eddy‐driven cross‐shelf exchange is important to predict future changes to the shelf water mass.

Retention and Leakage of Water by Mesoscale Eddies in the East Australian Current System

AbstractMesoscale eddies are ubiquitous in the ocean, transporting semi‐isolated water masses as well as advecting tracers and biota. The extent to which eddies impact the environment depends on the time they retain water parcels. Here we quantify retention times of mesoscale eddies in a (1/10)° model of the East Australian Current and its extension along the southeast coast of Australia. We find that retention times vary widely, between 3 and 357 days, but peak around 24 and 27 days for anticyclones and cyclones, respectively. Changes in eddy shape, though not in eddy size, relate to water exchange between the eddy and the background flow. An increase in eccentricity (eddy elongation) often leads to water leakage, while a decrease is associated with water retention. Thus, the change in eddy eccentricity can be used as a diagnostic of the eddy's likelihood to exchange water with its surrounding. We find that water within a region of the eddy that is close to uniform rotation and rotating faster than uniform vorticity is more likely to be retained. Typical retention times are long enough for eddies to transport water across regions of contrasting hydrographic properties, develop a biogeochemical response, and influence connectivity patterns.

A high-resolution biogeochemical model (ROMS 3.4 + bio_Fennel) of the East Australian Current system

Abstract. Understanding phytoplankton dynamics is critical across a range of topics, spanning from fishery management to climate change mitigation. It is particularly interesting in the East Australian Current (EAC) system, as the region's eddy field strongly conditions nutrient availability and therefore phytoplankton growth. Numerical models provide unparalleled insight into these biogeochemical dynamics. Yet, to date, modelling efforts off southeastern Australia have either targeted case studies (small spatial and temporal scales) or encompassed the whole EAC system but focused on climate change effects at the mesoscale (with a spatial resolution of 1/10∘). Here we couple a model of the pelagic nitrogen cycle (bio_Fennel) to a 10-year high-resolution (2.5–5 km horizontal) three-dimensional ocean model (ROMS) to resolve both regional and finer-scale biogeochemical processes occurring in the EAC system. We use several statistical metrics to compare the simulated surface chlorophyll to an ocean colour dataset (Copernicus-GlobColour) for the 2003–2011 period and show that the model can reproduce the observed phytoplankton surface patterns with a domain-wide RMSE of approximately 0.2 mg Chl a m−3 and a correlation coefficient of 0.76. This coupled configuration will provide a much-needed framework to examine phytoplankton variability in the EAC system providing insight into important ecosystem dynamics such as regional nutrient supply mechanisms and biogeochemical cycling occurring in EAC eddies.

Observation Impact in a Regional Reanalysis of the East Australian Current System

AbstractThe East Australia Current dominates the circulation along the east coast of Australia; therefore, identifying observations that best constrain its transport and eddies may help improve circulation estimates. Observational data sets are sparse in time and space and numerical models are unable to predict the timing and location of eddies due to their chaotic nature. Data assimilation combines observations with a numerical model such that the model better represents the observations and provides the dynamic context. This study uses variational methods to quantify how oceanic observations from various platforms impact model estimates of transport and eddy kinetic energy in the East Australia Current. The most influential observations are, in this order, satellite‐derived sea surface temperature; radial components of sea surface velocity from an high‐frequency radar array midway along the coast; satellite‐derived sea surface height, temperature, salinity, and velocity observations from a full‐depth mooring array in the upstream portion of the domain; and subsurface hydrographic data measured by ocean gliders. Not only do the high‐frequency radar observations have high impact on transport estimates at the array location, but also they have significant impact both upstream and downstream. Likewise, the impact of the mooring array observations is far reaching, contributing to transport estimates hundreds of kilometers downstream. The observation impact of deep gliders deployed into eddies is particularly high. Significantly, we find that observations taken in regions with greater natural variability contribute most to constraining the model estimates.

Estimating the Velocity and Transport of Western Boundary Current Systems: A Case Study of the East Australian Current near Brisbane

AbstractWestern boundary currents (WBCs) are highly variable narrow meandering jets, making assessment of their volume transports a complex task. The required high-resolution temporal and spatial measurements are available only at a limited number of sites. In this study a method is developed for improving estimates of the East Australian Current (EAC) mean transport and its low-frequency variability, using complementary modern datasets. The present calculation is a case study that will be extended to other subtropical WBCs. The method developed in this work will reduce uncertainties in estimates of the WBC volume transport and in the interannual mass and heat budgets of the meridional overturning circulations, improving our understanding of the response of WBCs to local and remote forcing on long time scales. High-resolution expendable bathythermograph (HR-XBT) profiles collected along a transect crossing the EAC system near Brisbane, Australia, are merged with coexisting profiles and parking-depth trajectories from Argo floats, and with altimetric sea surface height data. Using HR-XBT/Argo/altimetry data combined with Argo trajectory-based velocities at 1000 m, the 2004–15 mean poleward alongshore transport of the EAC is 19.5 ± 2.0 Sv (1 Sv ≡ 106 m3 s−1) of which 2.5 ± 0.5 Sv recirculate equatorward just offshore of the EAC. These transport estimates are consistent in their mean and variability with concurrent and nearly collocated moored observations at 27°S, and with earlier moored observations along 30°S. Geostrophic transport anomalies in the EAC system, including the EAC recirculation, show a standard deviation of ±3.1 Sv at interannual time scales between 2004 and 2015.

Eddy surface properties and propagation at Southern Hemisphere western boundary current systems

Abstract. Oceanic eddies exist throughout the world oceans, but are more energetic when associated with western boundary currents (WBC) systems. In these regions, eddies play an important role in mixing and energy exchange. Therefore, it is important to quantify and qualify eddies associated with these systems. This is particularly true for the Southern Hemisphere WBC system where only few eddy censuses have been performed to date. In these systems, important aspects of the local eddy population are still unknown, like their spatial distribution and propagation patterns. Moreover, the understanding of these patterns helps to establish monitoring programs and to gain insight in how eddies would affect local mixing. Here, we use a global eddy data set to qualify eddies based on their surface characteristics in the Agulhas Current (AC), the Brazil Current (BC) and the East Australian Current (EAC) systems. The analyses reveal that eddy propagation within each system is highly forced by the local mean flow and bathymetry. Large values of eddy amplitude and temporal variability are associated with the BC and EAC retroflections, while small values occur in the centre of the Argentine Basin and in the Tasman Sea. In the AC system, eddy polarity dictates the propagation distance. BC system eddies do not propagate beyond the Argentine Basin, and are advected by the local ocean circulation. EAC system eddies from both polarities cross south of Tasmania but only the anticyclonic ones reach the Great Australian Bight. For all three WBC systems, both cyclonic and anticyclonic eddies present a geographical segregation according to radius size and amplitude. Regions of high eddy kinetic energy are associated with the eddies' mean amplitudes, and not with their densities.

Statistical description of the East Australian Current low-frequency variability from the WOCE PCM3 array

The in situ dataset used in the current study consists of the Pacific Current Meter 3 (PCM3) array, which was a significant part of the Australian contribution to the World Ocean Circulation Experiment to study the variability of the East Australian Current (EAC), and was operational between September 1991 and March 1994. Area-preserving spectral analysis has been used to investigate the typical time scales observed by the current meters. As a general rule, the spectra from the top layers of the shallow (1, 2 and 3) and the deep (4, 5 and 6) moorings have a distinct peak in the temporal mesoscale band (periods between 70 and 170 days), with a general redistribution of energy towards the higher-frequencies near the ocean floor. This peak has been linked with eddy variability of the EAC system, which influences the fluctuations of the current main jet. The vertical modes of the velocity profile show that the strong surface-intensified baroclinic signal of the EAC dominated the variability at mooring 4 location. Further offshore the predominant configuration resembles more closely the barotropic mode. Ultimately, spatial empirical orthogonal functions (EOF) analysis point out the impact of the presence/absence of the EAC jet in the array.

The Dynamics of the East Australian Current System: The Tasman Front, the East Auckland Current, and the East Cape Current

The dynamics of the flow field surrounding New Zealand are investigated using a series of global ocean models. The physical mechanisms governing the direction, magnitude, and location of the East Australian Current (EAC), the Tasman Front, the East Auckland Current (EAUC), and the East Cape Current (ECC) are studied using numerical simulations whose complexity is systematically increased. As new dynamics are added to each successive simulation, their direct and indirect effects on the flow field are examined. The simulations have horizontal resolutions of 1/8°, 1/16°, or 1/32° for each variable, and vertical resolutions ranging from 1.5-layer reduced gravity to 6-layer finite depth with realistic bottom topography. All simulations are forced by the Hellerman and Rosenstein monthly wind stress climatology. Analysis of these simulations shows that several factors play a critical role in governing the behavior of the examined currents. These factors include 1) mass balance of water pathways through the region, 2) gradients in the wind stress curl, 3) nonlinear flow instabilities, and 4) upper-ocean–topographic coupling due to mixed baroclinic and barotropic instabilities. Transport streamfunctions of a linear reduced gravity model reproduce the large-scale features well but produce an EAUC that flows counter to the observed direction. The residual of the mass balance of the transport through the Tasman Sea, the basinwide transport at 32°S, and the transport of the South Pacific subtropical gyre east of New Zealand determines the direction of the EAUC. The 6-layer nonlinear model allows isopycnal outcropping, which changes the transport through the Tasman Sea and produces an EAUC flowing in the observed direction. Gradients in the zonally integrated wind stress curl field determine the coastal separation points of the EAC, the EAUC, and the ECC, while a combination of nonlinear flow instabilities and upper-ocean–topographic coupling contribute to the formation of meanders in the Tasman Front. Increased resolution results in greater mixed baroclinic–barotropic instabilities and thus more upper-ocean–topographic coupling and surface variability, giving a more accurate simulation of topographically controlled mean meanders in the Tasman Front.

Variability of the East Australian Current over most of its depth and a comparison with other regions

A knowledge of the structure of current systems over their total depth, especially of western boundary currents such as the East Australian Current, is important in assessing global transports. Many measurements have been performed on these currents and the generation of climatological statistics for them should assist in assessing the representativeness of conditions obtained in any particular study. To examine the structure, in a statistical sense, of the East Australian Current system over most of its depth, archived hydrological station data are analyzed to produce profiles of various statistics of temperature and dynamic height perturbations (e.g., means, standard deviations, and correlation coefficients as a function of vertical separation). These are compared with the same statistics derived for two other locations with less vigorous mesoscale perturbations. Nondimensionalized geostrophic velocity profiles are also obtained. The method uses all the individual profiles from a given area and does not need series of oceanographic sections or areal surveys. It is found that the magnitude of perturbations and their vertical length scales are larger in the East Australian Current region than in either of the other two locations.

Eddies off southeastern Australia

Satellite infrared images and satellite drifters, together with some supporting ship data, have been used to follow complex and rapidly evolving events in the East Australian Current system. The episodic extension of a geopotential ridge into the southern Tasman Sea was seen to be arrested and diverted by a substantial anticyclonic eddy. Later the ridge and the eddy partially coalesced, and this provided a pathway for the East Australian Current to travel southward and encircle the eddy. A new eddy was formed in summer when this eddy coalesced with another. The new eddy initially had two subsurface thermostads with temperatures of 18.8 and 17.0°C, respectively, that were the result of mixing within the two component eddies the previous winter. It was kept under continuous surveillance by drifters for 8 months and by occasional ship surveys for a further 5 months. The drifters showed the motion of the eddy center to comprise an overall northward component upon which were occasionally superimposed anticlockwise loops of 100 km diameter, which were described in about 30 days. The maximum speed of translation of the eddy center exceeded 0.3 m s −1. The loops brought the eddy in near the continental shelf edge, with the result that it was transiently distorted into a precessing ellipse. The unusual northward move of the eddy took place over the winter, during which a new mixed layer cooled and deepened until it incorporated the upper thermostad in June and the deeper thermostad in September; no further cooling occurred. The motion of the drifters relative to the eddy center was used to calculate angular momentum, taken to be twice the rate at which the area was swept by the radius to a drifter. In the short term, angular momentum was found to be quasi-conservative around individual elliptical loops and overall showed roughly a r 1.8 dependence. It was used to calculate the radial distribution of angular velocity, azimuthal velocity, and relative vorticity for the eddy in its undisturbed circular form.

Vertical electric field fluctuations at the floor of the Tasman Abyssal Plain

A record of vertical electric field fluctuations occuring naturally in the ocean has been obtained at a site (36°14′S, 152°15′E) off the coast of easter Australia, on the floor of the Tasman Abyssal Plain in water of depth 4836 m. The free-fall and self-contained sea-floor instrument measured ambient voltage fluctuations between two low-noise silver-silver chloride electrodes connected by a vertical insulated wire 160 m long. The record returned is 107 days in duration, with readings taken every 28 1 8 s. The data obtained are interpreted in terms of east-west fluid current motion past the wire, causing electromagnetic induction with the horizontal north component of the earth's steady magnetic field. These motional fields are observed on a wide range of time scales. Spectral analysis of the data shows inertial oscillations and shorter period internal waves, with good agreement between observation and the theory of Chave (1984, Journal of Geophysical Research, 89, 10519–10528). There is also a peak in the power spectrum at the frequency of the semi-diurnal tides. The shortest period signals occurring are interpreted as due to some form of turbulence. Aperiodic motions on time scales of days are attributed to mesoscale activity in the East Australian Current system.