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Here we present the first open-access long-term 3D hydrodynamic ocean hindcast for the New Zealand ocean estate. The 28-year 5 km×5 km resolution free-running ocean model configuration was developed under the umbrella of the Moana Project, using the Regional Ocean Modeling System (ROMS) version 3.9. It includes an improved bathymetry, spectral tidal forcing at the boundaries and inverse-barometer effect usually absent from global simulations. The continuous integration provides a framework to improve our understanding of the ocean dynamics and connectivity, as well as identify long-term trends and drivers for particular processes. The simulation was compared to a series of satellite and in situ observations, including sea surface temperature (SST), sea surface height (SSH), coastal water level and temperature stations, moored temperature time series, and temperature and salinity profiles from the CORA5.2 (Coriolis Ocean database for ReAnalysis) dataset – including Argo floats, XBTs (expendable bathythermographs) and CTD (conductivity–temperature–depth) stations. These comparisons show the model simulation is consistent and represents important ocean processes at different temporal and spatial scales, from local to regional and from a few hours to years including extreme events. The root mean square errors are 0.11 m for SSH, 0.23 ∘C for SST, and

Accurate estimates and forecasts of ocean eddies in key regions such as western boundary currents are important for weather and climate, biology, navigation, and search and rescue. The dynamic nature of mesoscale eddies requires data assimilation to produce accurate eddy timings and locations in ocean model simulations. However, data assimilating models are rarely assessed below the surface due to a paucity of observations; hence it is not clear how data assimilation impacts the subsurface eddy structure. Here, we use a suite of observing system simulation experiments to show how the subsurface representation of eddies is changed within data assimilating simulations even when assimilating nearby observations. We examine in detail two possible manifestations of how the data assimilation process impacts three-dimensional eddy structure, namely, by producing overly active baroclinic instability and through inaccurate vertical mode structure. Therefore, in DA simulations, subsurface temperature structures can be too deep and too warm, particularly in dynamic eddy features. Our analyses demonstrate the need for further basic research in ocean data assimilation methodologies to improve the representation of the subsurface ocean structure.

Western boundary current (WBC) extensions such as the East Australian Current (EAC) southern extension are warming 2–3 times faster than the global average. However, there are nuances in the spatial and temporal variability of the warming that are not well resolved in climate models. In addition, the physical drivers of ocean heat content (OHC) extremes are not well understood. Here, using a high-resolution ocean model run for multiple decades, we show nonuniform warming trends in OHC in the EAC, with strong positive trends in the southern extension region (∼36°–38°S) but negative OHC trends equatorward of 33°S. The OHC variability in the EAC is associated with the formation of anticyclonic eddies, which is modulated by transport ∼880 km upstream (EAC mode) and the westward propagation of Rossby waves (eddy mode). Diagnosing the drivers of temperature extremes has implications for predictability both in the EAC and in WBCs more broadly, where ocean warming is already having considerable ecological impacts.

Western boundary currents (WBCs) form the narrow, fast-flowing poleward return flows of the great subtropical ocean gyres and are sources of rapidly varying mesoscale eddies. Accurate simulation of the vertical structure, separation latitude, and ocean heat content of WBCs is important for understanding the poleward transport of heat in the global ocean. However, state estimation and forecasting in WBC regions, such as the East Australian Current (EAC), the WBC of the South Pacific subtropical gyre, is challenging due to their dynamic nature and lack of observations at depth. Here we use observing system simulation experiments to show that subsurface temperature observations in a high eddy kinetic energy region yield large improvement in representation of key EAC circulation features, both downstream and ∼ 600 km upstream of the observing location. These subsurface temperature observations (in concert with sea surface temperature and height measurements) are also critical for correctly representing ocean heat content along the length of the EAC. Furthermore, we find that a more poleward separation latitude leads to an EAC and eddy field that is represented with far reduced error, compared to when the EAC separates closer to the Equator. Our results demonstrate the importance of subsurface observations for accurate state estimation of the EAC and ocean heat content that can lead to marine heatwaves. These results provide useful suggestions for observing system design under different oceanographic regimes, for example, adaptive sampling to target high energy states with more observations and low energy states with fewer observations.

Accurate estimates and forecasts of ocean eddies in key regions such as Western Boundary Currents are important for weather and climate, biology, navigation and search and rescue. The dynamic nature of mesoscale eddies requires data assimilation to produce accurate eddy timings and locations in ocean model simulations. However, data assimilating models are rarely assessed below the surface due to a paucity of observations, hence it is not clear how data assimilation impacts the subsurface eddy structure. Here, we use a suite of Observing System Simulation Experiments to show how the subsurface representation of eddies is changed within data assimilating simulations even when assimilating nearby observations. We examine in detail two possible manifestations of how the data assimilation process impacts 3-dimensional eddy structure, namely, by producing overly active baroclinic instability and through inaccurate vertical mode structure. Therefore in DA simulations, subsurface temperature structures can be too deep and too warm, particularly in dynamic eddy features. Our analyses demonstrate the need for further basic research in ocean data assimilation methodologies to improve representation of subsurface ocean structure.

Here we present the first open access long term 3D hydrodynamic ocean hindcast for the New Zealand ocean estate. The 28 years 5 km x 5 km resolution free running ocean model configuration was developed under the umbrella of the Moana Project, using the Regional Ocean Model System (ROMS) version 3.9. It includes an improved bathymetry, spectral tidal forcing at the boundaries, and inverse barometer effect usually absent from global simulations. The continuous integration provides a framework to improve our understanding of the ocean dynamics and connectivity, as well as identify long-term trends and drivers for particular processes. The simulation was compared to a series of satellite and in-situ observations, including sea surface temperature (SST), sea surface height (SSH), coastal water level and temperature stations, moored temperature time series, and temperature and salinity profiles from the CORA5.2 dataset – including Argo floats, XBT and CTD stations. These comparisons show the model simulation is consistent and represents important ocean processes at different temporal and spatial scales, from local to regional and from a few hours to years including extreme events. The root-mean-squared errors are 0.11m for SSH, 0.23 °C for SST, and < 1 °C and 0.15 g/kg for temperature and salinity profiles. Coastal tides are simulated well, and both high skill and correlation are found between modelled and observed sub-tidal sea level and water temperature stations. Moreover, cross-sections of the main currents around New Zealand show the simulation is consistent with transport, velocity structure, and variability reported in the available literature. This first multi-decadal, high resolution, open access hydrodynamic model represents a significant step forward for ocean sciences in the New Zealand region.

Transport of larvae by ocean currents is an important dispersal mechanism for many species. The timing and location of spawning can have a large influence on settlement location. Shifts in the known spawning habitat of fish, whether due to climate or the discovery of new spawning stock, can influence the distribution of juveniles and our understanding of connectivity. The globally distributed species; Pomatomus saltatrix, is one such example where a previously unrecognised summer spawning event and a more southern latitudinal extent was recently reported for the southwest Pacific population. Although restrictions are in place to protect the traditional spawning event, the importance of the newly recognised summer spawning event is uncertain. Here, we investigate larval dispersal of P. saltatrix using particle tracking simulations to identify the contributions of the different spawning events to settlement. By modelling dispersal of larvae released in northern and mid-latitude regions over the Austral spring and summer, we show that the newly recognised mid-latitude summer spawning event contributes over 50% of the larvae reaching southern latitudes. This is due to a reduced (1–2 days) pelagic larval duration (associated with temperature), resulting in reduced larval mortality, and the seasonal (summer) strengthening of the East Australian Current (EAC) transporting particles ~50 km further south. These findings demonstrate that in dynamic boundary current systems such as the EAC, the final settlement location of larvae that are transported by ocean currents can vary considerably depending on the timing and location of spawning and that multiple spawning events are important for maximum dispersal.

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.

Western boundary currents (WBCs) have intensified and become more eddying in recent decades due to the spin-up of the ocean gyres, resulting in warmer open ocean temperatures. However, relatively l...

The Hawkesbury Bioregion located off southeastern Australia (31.5–34.5oS) is a region of highly variable circulation. The region spans the typical separation point of the East Australian Current (EAC), the western boundary current that dominates the flow along the coast of SE Australia. It lies adjacent to a known ocean warming hotspot in the Tasman Sea, and is a region of high productivity. However, we have limited understanding of the circulation, temperature regimes and shelf transport in this region, and the drivers of variability. We configure a high resolution (750m) numerical model for the Hawkesbury Shelf region nested inside 2 data assimilating models of decreasing resolution, to obtain the best estimate of the shelf circulation and transport over a 2-yr period (2012–2013). Here we show that the transport is driven by the mesoscale EAC circulation that strengthens in summer and is related to the separation of the EAC jet from the coast. Transport estimates show strong offshore export is a maximum between 32-33oS. Median offshore transports range 2.5–8.4Sv seasonally and are a maximum during in summer driven by the separation of the EAC jet from the coast. The transport is more variable downstream of the EAC separation, driven by the EAC eddy field. Onshore transport occurs more frequently off Sydney 33.5–34.5oS; seasonal medians range -1.7 to 2.3Sv, with an onshore maximum in winter. The region is biologically productive, and it is a known white shark nursery area despite the dominance of the oligotrophic western boundary current. Hence an understanding of the drivers of circulation and cross-shelf exchange is important.

The baroclinic tides are a crucial source of mixing energy into the deep ocean; however, the incoherent portion of the spectrum is not well examined because it is difficult to observe. This study estimates the coherent and incoherent M2 internal tide energy fluxes in the Philippine Sea using a primitive equation model that resolves the M2 barotropic and baroclinic tides and the time-evolving atmospherically forced eddying circulation. A time-mean, incoherent, internal tide energy flux of 25% of the coherent energy flux is found to emanate eastward into the Philippine Sea from the Luzon Strait and a time-mean incoherent portion of 30% of the coherent energy flux propagates westward into the South China Sea (SCS). The incoherent internal tide energy results from baroclinic tide generation and propagation variability. Quantifying the incoherent portion estimates the energy missing from altimeter-derived or line-integral acoustic measurements and places short-lived, in situ observations in the context of variability.

Ocean currents are an energy resource that could be harnessed to increase renewable energy yields and reduce humanity's dependence on fossil fuels. The first challenge is to identify regions of significant kinetic energy flux that are a viable distance from the shore for the extraction and transport of energy. Western boundary currents (WBCs) have been identified as the most promising resource in this regard due to their strong current speeds (typically 1–2 m s−1). However, such a resource is by necessity regional, so each prospective region requires thorough investigation. While global studies of WBCs (including the East Australian Current, hereafter the EAC) have been conducted using numerical models, they have coarse resolutions and often underestimate kinetic energy, which is the specific resource that needs accurate assessment. Here we address this knowledge gap, using a range of current velocity observations from moorings, high frequency radar and satellite altimetry to investigate the potential of the EAC as a future renewable energy resource, at various latitudes along the east Australian coast. Finally, in combination with a high-resolution data assimilating model, we propose the optimal location for future energy extraction opportunities with relatively high hydrokinetic flux (average of ~ 500 W m−2 at 50 m depth), constant direction and suitable location (25 km from shore in 400 m seawater depth).

Submesoscale flows dominate the vertical transport in the upper ocean and play an important role in air-sea fluxes, the distribution of nutrients and biota, the connectivity of populations. Yet submesoscale predictability is a new frontier and presents additional challenges to that of mesoscale flows. This study assesses the capacity of an operational system to predict the circulation along the landward edge of the East Australian Current where the cyclonic band of vorticity allows small scale instabilities to grow. We use a downscaling approach in which a fine-scale resolution (750–1000 m) coastal model is one-way nested within a coarser resolution (2.5–6 km) mesoscale eddy-resolving regional model that assimilates all available data, including coastal high-frequency radar radial velocities. Assimilation of the surface radial velocities into the regional model provides improved representation of the cyclonic band inshore of the current, however forecasts at this resolution perform poorly in representing this cyclonic vorticity. Downscaling to the higher resolution coastal model produces a sharper across-current vorticity gradient within the EAC and leads in turn to more intense submesoscale features growing on its inshore side. Boundary forcing error from the regional model propagates into the nested coastal model. The results motivate improved representation of fine-scale flows in regional forecasts for downscaling purposes.

Understanding phytoplankton dynamics is critical across a range of topics, spanning from fisheries 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 1/10º). 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 chla 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.

This study uses a primitive equation model to estimate the time-varying M2 internal tide dissipation in the Philippine Sea in the presence of the subtidal circulation. The time-mean diapycnal diffusivity due to the M2 internal tide is estimated to be 4.0–4.8 × 10−4 m2 s−1 at the Luzon Strait and 2–9 × 10−5 m2 s−1 in the Philippine Sea basin. The variability in internal tides and their interactions with the subtidal ocean circulation results in significant spatial and temporal variability in the energy available for mixing. The subtidal circulation influences internal-tide-induced mixing in two ways: by introducing variability in internal tide generation and by increased dissipation of baroclinic energy associated with greater velocity shear. Close to the generation site, mixing is dominated by high-mode internal tide dissipation, while in the far field the influence of the mesoscale energy on internal tide dissipation is significant, resulting in increased dissipation. This study presents model-based estimates of the important and relatively unknown effect of mesoscale circulation on internal-tide-induced mixing away from internal tide generation sites in a region of high eddy kinetic energy.

This study examines the effects of the subtidal circulation on the generation and propagation of the M2 internal tide in the Philippine Sea using a primitive equation model. Barotropic to baroclinic conversion at the Luzon Strait is found to vary due to the background circulation changes over the generation site and the changing influence of remotely generated internal tides from the Mariana Arc. The varying effect of remotely generated waves results from both changing generation energy levels at the Mariana Arc and variability in the propagation of the internal tides across the Philippine Sea. The magnitude and direction of the depth-integrated baroclinic energy fluxes vary temporally, due to a combination of changing generation, propagation, and dissipation. Spatial patterns of internal tide propagation near the Luzon Strait are influenced by the locations of mesoscale eddies to the east and west of the strait. The results provide insight into the mechanisms of variability of the baroclinic tides and highlight the importance of considering both the remotely generated internal tides and the subtidal dynamics to estimate internal tide energetics.

The traditional view of the East Australian Current (EAC), as depicted in many schematics, is of a continuous boundary current that flows along the shelf off eastern Australia, between approximately 18°S and about 32.5°S, where it separates from the coast and continues either towards New Zealand, along the Tasman Front; or towards Tasmania, as the EAC Extension. Additionally, it is widely recognised that eddies are prevalent in the EAC region – particularly south of the EAC separation. We revisit this long-standing paradigm and suggest that the EAC is perhaps better viewed as a continuous, meandering stream, flowing adjacent to the coast that “feeds” a field of mesoscale eddies. Observations show that EAC eddies are prevalent over a broad region of the western Tasman Sea, as far north as 25°S, typically with maximum intensity between 30 and 35°S. At any instant in time the EAC jet is usually evident as a poleward stream adjacent to the continental shelf edge. Other commonly accepted components of the EAC System, including the Tasman Front and EAC Extension, are rarely seen as distinct, identifiable features. Rather, these features are evident only in time-mean fields, when the eddy-variability is filtered out. It is also common for the EAC to be spatially discontinuous – due to the presence of eddies – often with multiple short streams that sometimes separate and re-attach to the coast. Recognition of the EAC as an eddy-dominated current system has seen many recent studies focus on various aspects of eddies in the EAC System, providing new insights into mesoscale ocean dynamics. Recent studies of individual eddies have shown that the circulation within eddies, including tilting and vertical motion, is more complex than previously understood. A summary of these studies, along with a review of the EAC System, particularly its path, separation, and eddy field is presented here.

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea using the adjoint of a numerical model. A one year series of five day backward integrations of the adjoint model quantify the sensitivity of travel-times to varying dynamics that can alter the travel-time of a 611 km ray by 200 ms. The early evolution of the sensitivities reveals high-mode internal waves that dissipate quickly, leaving the lowest three modes, providing a connection to variations in the internal tide generation prior to the sample time. They are also strongly sensitive to advective effects that alter density along the ray path. These sensitivities reveal how travel-time measurements are affected by both nearby and distant waters. Temporal nonlinearity of the sensitivities suggests that prior knowledge of the ocean state is necessary to exploit the travel-time observations.

This study investigates the impact of remotely generated internal tides on model estimates of barotropic to baroclinic tidal conversion for two generation sites bounding the Philippine Sea: the Luzon Strait and the Mariana Island Arc. A primitive equation model is used to characterize the internal tides generated by the principal semidiurnal tide (M2) over a domain encompassing the two generation sites. Energetic internal tides are generated at the Luzon Strait where nearly 17 GW of barotropic tide energy is converted to baroclinic energy, of which 44% (4.78 GW) is radiated eastward into the Philippine Sea. From the Mariana Arc, baroclinic energy propagates westward into the Philippine Sea as a result of 3.82 GW of barotropic to baroclinic energy conversion. Simulations that focus on each generation site without influence of the other are performed, and comparisons show that remotely generated internal tides have a significant effect on local conversion at the two sites. Total conversion is greater in the absence of remotely generated internal tides at both sites: 11% greater at the Luzon Strait and 65% greater at the Mariana Arc. The first three modes of the remotely generated internal tides traverse the basin and alter the amplitude and phase of bottom pressure. The arrival of the remote internal tides varies significantly with changing stratification and mesoscale circulation. The results suggest that an important source of variability in local conversion around the globe is due to remotely generated internal tides.

As with other western boundary currents globally, the East Australian Current (EAC) is inherently dynamic making it a challenge to model and predict. For the EAC region, we combine a high-resolution state-of-the-art numerical ocean model with a variety of traditional and newly available observations using an advanced variational data assimilation scheme. The numerical model is configured using the Regional Ocean Modelling System (ROMS 3.4) and takes boundary forcing from the BlueLink ReANalysis (BRAN3). For the data assimilation we use an Incremental Strong-Constraint 4-Dimensional Variational (IS4D-Var) scheme. This paper describes the data assimilative model configuration that achieves an optimised minimisation of the difference between the modelled solution and the observations to give a dynamically-consistent `best-estimate' of the ocean state over a 2-year period. The reanalysis is shown to represent both assimilated and non-assimilated observations well. It achieves mean spatially-averaged RMS residuals with the observations of 7 cm for SSH and 0.4 °C for SST over the assimilation period. The time-mean RMS residual for subsurface temperature measured by Argo floats is a maximum of 1 °C between water depths of 100–300 m and smaller throughout the rest of the water column. Velocities at several offshore and continental shelf moorings are well represented in the reanalysis with complex correlations between 0.8–1 for all observations in the upper 500 m. Surface radial velocities from a high-frequency radar array are assimilated and the reanalysis provides surface velocity estimates with complex correlations with observed velocities of 0.8–1 across the radar footprint. Comparison with independent (non-assimilated) shipboard CTD cast observations shows a marked improvement in the representation of the subsurface ocean in the reanalysis, with the RMS residual in potential density reduced to about half of the residual with the free-running model in the upper eddy-influenced part of the water column. This shows that information is successfully propagated from observed variables to unobserved regions as the assimilation system uses the model dynamics to determine covariance, such that the ocean state better fits and is in balance with the observations. This is the first study to generate a reanalysis of the region at such a high resolution, making use of an unprecedented observational data set and using an assimilation method that uses the time-evolving model physics to adjust the model in a dynamically consistent way. As such, the reanalysis potentially represents a marked improvement in our ability to capture important circulation dynamics in the EAC. The reanalysis is being used to study EAC dynamics, observation impact in state-estimation and as forcing for a variety of downscaling studies.

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. Using an advanced numerical model, we can identify both the ocean dynamics that control the ocean state along a ray path and quantify the informational content of the ray travel-time observation. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea over a one year experiment. The travel-times are found to be sensitive to the internal tide generation prior to the sample time and to advective effects that alter density along the ray path. Temporal nonlinearity of these sensitivities suggest that prior knowledge of the ocean state is necessary to exploit the travel-time observations. After assimilating the travel-time observations, the contribution of the travel-time information to our estimation of the ocean state is quantified and evaluated to further identify what the travel-time observation reveals about the ocean.