East Australian Current Research Articles

Observing system simulation experiments reveal that subsurface temperature observations improve estimates of circulation and heat content in a dynamic western boundary current

Abstract. 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.

Spatial Analysis of a Rapid Intrusion Event of the East Australian Current Using High Frequency Radar Data

The East Australian Current (EAC) is a highly dynamic western boundary current of the South Pacific Gyre. The EAC frequently encroaches shoreward, drives upwelling, changes coastal bio-physical dynamics, and thus exerts significant impacts on coastal marine ecosystems. In this study, with high frequency (HF) radar and mooring data, for the first time accurate daily mapping and tracking of a rapid EAC intrusion event was conducted and the impacts of the EAC intrusion on the shelf water off Coffs Harbor were monitored. The results show that, during the event, the EAC was highly dynamic with a mean daily onshore/offshore movement of ~5 km/day. In addition, we found that the bottom ocean temperature and the surface current speed on the shelf varied linearly with the EAC-to-coast distance. This study thus demonstrates the value of HF remotely sensed data for the ongoing quantitative monitoring of the highly dynamic EAC fluctuations.

Staying in place and moving in space: Contrasting larval thermal sensitivity explains distributional changes of sympatric sea urchin species to habitat warming.

For marine ectotherms, larval success, planktonic larval duration and dispersal trajectories are strongly influenced by temperature, and therefore, ocean warming and heatwaves have profound impacts on these sensitive stages. Warming, through increased poleward flow in regions with western boundary currents, such as the East Australia Current (EAC), provides opportunities for range extension as propagules track preferred conditions. Two sea urchin species, Centrostephanus rodgersii and Heliocidaris tuberculata, sympatric in the EAC warming hotspot, exhibit contrasting responses to warming. Over half a century, C. rodgersii has undergone marked poleward range extension, but the range of H. tuberculata has not changed. We constructed thermal performance curves (TPC) to determine if contrasting developmental thermal tolerance can explain this difference. The temperatures tested encompassed present-day distribution and forecast ocean warming/heatwave conditions. The broad and narrow thermal optimum (Topt) ranges for C. rodgersii and H. tuberculata larvae (7.2 and 4.7°C range, respectively) matched their realized (adult distribution) thermal niches. The cool and warm temperatures for 50% development to the feeding larva approximated temperatures at adult poleward range limits. Larval cool tolerances with respect to mean local temperature differed, 6.0 and 3.8°C respectively. Larval warm tolerances were similar for both species as are the adult warm range edges. The larvae of both species would be sensitive to heatwaves. Centrostephanus rodgersii has stayed in place and shifted in space, likely due to its broad cold-warm larval thermal tolerance and large thermal safety margins. Phenotypic plasticity of the planktonic stage of C. rodgersii facilitated its range extension. In contrast, larval cold intolerance of H. tuberculata explains its restricted range and will delay poleward extension as the region warms. In a warming ocean, we show that intrinsic thermal biology traits of the pelagic stage provide an integrative tool to explain species-specific variation in range shift patterns.

Variability and Drivers of Ocean Temperature Extremes in a Warming Western Boundary Current

Abstract 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.

Shelf Transport Pathways Adjacent to the East Australian Current Reveal Sources of Productivity for Coastal Reefs

The region where the East Australian Current (EAC) separates from the coast is dynamic and the shelf circulation is impacted by the interplay of the western boundary current and its eddy field with the coastal ocean. This interaction can drive upwelling, retention or export. Hence understanding the connection between offshore waters and the inner shelf is needed as it influences the productivity potential of valuable coastal rocky reefs. Near urban centres, artificial reefs enhance fishing opportunities in coastal waters, however these reefs are located without consideration of the productivity potential of adjacent waters. Here we identify three dominant modes of mesoscale circulation in the EAC separation region (~31.5−34.5°S); the ‘EAC mode’ which dominates the flow in the poleward direction, and two eddy modes, the ‘EAC eddy mode’ and the ‘Eddy dipole mode’, which are determined by the configuration of a cyclonic and anticyclonic eddy and the relationship with the separated EAC jet. We use a Lagrangian approach to reveal the transport pathways across the shelf to understand the impact of the mesoscale circulation modes and to explore the productivity potential of the coastal waters. We investigate the origin (position and depth) of the water that arrives at the inner-mid shelf over a 21-day period (the plankton productivity timescale). We show that the proportion of water that is upwelled from below the euphotic zone varies spatially, and with each mesoscale circulation mode. Additionally, shelf transport timescales and pathways are also impacted by the mesoscale circulation. The highest proportion of upwelling (70%) occurs upstream of 32.5°S, associated with the EAC jet separation, with vertical displacements of 70–120 m. From 33 to 33.5°S, water comes from offshore above the euphotic layer, and shelf transport timescales are longest. The region of highest retention over the inner shelf is immediately downstream of the EAC separation region. The position of the EAC jet and the location of the cyclonic eddy determines the variability in shelf-ocean interactions and the productivity of shelf waters. These results are useful for understanding productivity of temperate rocky reefs in general and specifically for fisheries enhancements along an increasingly urbanised coast.

Habitat limits the poleward establishment of anemonefishes in a climate change hotspot

Climate change is modifying marine species' distribution with consequences for the structure and function of ecosystems. The warming and strengthening East Australian Current (EAC) is driving the tropicalisation of subtropical marine ecosystems through changing overwinter survival of tropical species. However, this can be complex for obligate symbionts, such as anemonefishes. Here, we documented fine-scale temporal changes in the density of anemonefishes and assessed the influence of latitude, temperature, and host sea anemone availability. Juvenile, subadult and adult Amphiprion akindynos and Amphiprionlatezonatus occurred at all study locations: North Solitary Island (equatorward), South Solitary Island, and Fish Rock (poleward). While the sea anemone Heteractis crispa was readily available at Fish Rock, we found that a lack of another sea anemone Entacmaea quadricolor hindered the establishment of adult A. akindynos and A. latezonatus populations. Minimum water temperature was also a bottleneck for the tropical anemonefish A. akindynos but not for the subtropical A. latezonatus. Our findings suggest anemonefishes are capable of contributing to the tropicalisation of the east coast of Australia. However, poleward establishment is currently limited by the absence of the preferred host sea anemone.

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 .

Tracing iron along the flowpath of East Australian Current using iron stable isotopes

The mechanisms delivering iron (Fe) to Southern Ocean surface waters and resulting capacity for the oceanic drawdown of CO2 are important to constrain in a changing climate. The East Australian Current (EAC) is a major western boundary current flowing south along the eastern margin of Australia, hypothesized to play an important role in entraining Fe from a variety of potential sources and supplying Fe to large annual spring phytoplankton blooms in HNLC waters. We report Fe concentration and Fe isotope (δ56Fe) profiles for both dissolved and particulate phases at stations within the EAC, in the frontal mixing zone, and in subantarctic waters along the southward flowpath of the EAC in early spring. Mixed layer dFe concentrations declined from EAC source waters (0.32–0.53 nmol kg−1) into subantarctic waters (0.27–0.42 nmol kg−1) with a concurrent deepening of the ferricline. Particulate trace metal concentrations indicate robust inputs of lithogenic particles to subtropical EAC waters, potentially impacted by advection of shelf sedimentary particles and evidence of a significant supply from local aerosol deposition. The isotopic composition of pFe in the EAC (0.3 ± 0.1‰) is similar to prior reports of aerosol δ56Fe and upper ocean δ56pFe in the western Pacific and isotopically heavier than average values for the bulk continental crust. In EAC surface waters, isotopically light δ56dFe values (0.11–0.16‰) are observed likely due to reductive release of dFe from lithogenic particles in the photic zone with a particle-dissolved phase fractionation (Δ56FedFe–pFe) of −0.1 to −0.4‰. At depth, elevated dFe concentrations and a heavy δ56dFe signature suggest a non-reductive release of dFe from lithogenic particles via desorption or dissolution processes with a Δ56FedFe–pFe of +0.3‰. During southward transit of EAC waters, mixed layer dFe pools become increasingly heavy due to biological uptake through the frontal mixing zone and into subantarctic waters. We also observe differences in particulate trace metal concentrations in intermediate waters between subantarctic stations that could result from entrainment of sedimentary particles during transit over the relatively shallow South Tasman Rise.

Imprint of chaotic ocean variability on transports in the southwestern Pacific at interannual timescales

Abstract. The southwestern Pacific Ocean sits at a bifurcation where southern subtropical waters are redistributed equatorward and poleward by different ocean currents. The processes governing the interannual variability of these currents are not completely understood. This issue is investigated using a probabilistic modeling strategy that allows disentangling the atmospherically forced deterministic ocean variability and the chaotic intrinsic ocean variability. A large ensemble of 50 simulations performed with the same ocean general circulation model (OGCM) driven by the same realistic atmospheric forcing and only differing by a small initial perturbation is analyzed over 1980–2015. Our results show that, in the southwestern Pacific, the interannual variability of the transports is strongly dominated by chaotic ocean variability south of 20∘ S. In the tropics, while the interannual variability of transports and eddy kinetic energy modulation are largely deterministic and explained by the El Niño–Southern Oscillation (ENSO), ocean nonlinear processes still explain 10 % to 20 % of their interannual variance at large scale. Regions of strong chaotic variance generally coincide with regions of high mesoscale activity, suggesting that a spontaneous inverse cascade is at work from the mesoscale toward lower frequencies and larger scales. The spatiotemporal features of the low-frequency oceanic chaotic variability are complex but spatially coherent within certain regions. In the Subtropical Countercurrent area, they appear as interannually varying, zonally elongated alternating current structures, while in the EAC (East Australian Current) region, they are eddy-shaped. Given this strong imprint of large-scale chaotic oceanic fluctuations, our results question the attribution of interannual variability to the atmospheric forcing in the region from pointwise observations and one-member simulations.

Remotely Sensed Seasonal Shoreward Intrusion of the East Australian Current: Implications for Coastal Ocean Dynamics

For decades, the presence of a seasonal intrusion of the East Australian Current (EAC) has been disputed. In this study, with a Topographic Position Index (TPI)-based image processing technique, we use a 26-year satellite Sea Surface Temperature (SST) dataset to quantitatively map the EAC off northern New South Wales (NSW, Australia, 28–32°S and ~154°E). Our mapping products have enabled direct measurement (“distance” and “area”) of the EAC’s shoreward intrusion, and the results show that the EAC intrusion exhibits seasonal cycles, moving closer to the coast in austral summer than in winter. The maximum EAC-to-coast distance usually occurs during winter, ranging from 30 to 40 km. In contrast, the minimum distance usually occurs during summer, ranging from 15 to 25 km. Further spatial analyses indicate that the EAC undergoes a seasonal shift upstream of 29°40′S and seasonal widening downstream. This is the first time that the seasonality of the EAC intrusion has been confirmed by long-term remote-sensing observation. The findings provide new insights into seasonal upwelling and shelf circulation previously observed off the NSW coast.

The Rate of Coastal Temperature Rise Adjacent to a Warming Western Boundary Current is Nonuniform with Latitude

AbstractWestern 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 little is known of how WBC intensification will affect temperatures in adjacent continental shelf waters where societal impact is greatest. We use the well‐observed East Australian Current (EAC) to investigate WBC warming impacts on shelf waters and show that temperature increases are nonuniform in shelf waters along the latitudinal extent of the EAC. Shelf waters poleward of 32°S are warming more than twice as fast as those equatorward of 32°S. We show that nonuniform shelf temperature trends are driven by an increase in lateral heat advection poleward of the WBC separation, along Australia's most populous coastline. The large‐scale nature of the process indicates that this is applicable to WBCs broadly, with far‐reaching biological implications.

Biogeochemical characteristics of eddies in the East Australian Current depend on eddy type, history and location

Mesoscale eddies are ubiquitous in the East Australian Current (EAC), a nutrient poor western boundary current, and exert considerable impact on the biogeochemical characteristics of the region. However, in-situ biogeochemical studies of these eddies are relatively rare. Here we combine ship-based and satellite measurements to characterize the physical and biogeochemical properties of three EAC eddies – one cyclone and two anticyclones – in the austral fall of 2018. Deep Chlorophyll Maxima (DCM) were observed in the two northernmost eddies, with Chlorophyll-a concentrations ranging from 0.4 to 1.3 mg m -3 and generally decreasing with increasing depth of the DCM. The anticyclone farthest south, which was also the oldest observed eddy, was more productive and contained higher biomass than the northern eddies, including the cyclone. Deeper mixed layers in this anticyclone suggest that surface cooling resulting from poleward travel and seasonal cooling might have led to increased convection and an injection of nutrients. All eddies were nitrogen limited, and nitrogen fixation likely occurred in the surface waters of the two northernmost eddies. Where DCM were present, their depth was well explained as a consequence of an optimisation of light and nitrogen supply by phytoplankton. We observed prominent primary nitrite maxima (PNM) in the eddies. Their depths were highly correlated with, and consistently below, the depths of DCM, indicating that incomplete nitrate reduction by phytoplankton under low-light conditions was likely the main driver for their production. To our knowledge, this is the first description of PNM in eddies of the EAC. • An old anticyclone was more productive than a cyclone. • High productivity in a large, old anticyclonic eddy resulted from convective mixing. • Deep Chlorophyll Maxima (DCM) were biomass maxima. • Depths of DCM were correlated with euphotic zone and nitricline depths. • Depths of Primary Nitrite Maxima were correlated with and found below DCM.

A Study on an Anticyclonic-Cyclonic Eddy Pair Off Fraser Island, Australia

This research examines a cyclonic-anticyclonic eddy (AE) pair off Fraser Island next to the eastern Australian coast in 2009 using the Bluelink Reanalysis data, where the local eddies are poorly understood. This eddy pair formed in July and dissipated in November. We detailed the horizontal and vertical structures of the eddy pair in terms of three-dimensional variations in relative vorticity, hydrographic properties, velocity, and dynamic structures, which presented notable scales of the eddy pair. The AE formed beside the meandering of the East Australian Current (EAC) at 24°S and had a tilting structure in the upper 1,000 m toward the EAC. A cyclonic eddy (CE) formed a month later and interacted with the AE, which had a tilting structure toward the AE in the upper 1,000 m. Heterogeneity in the AE and CE composing this eddy pair was observed in the horizontal and vertical planes. The AE had a stronger and more coherent dynamic structure than the CE. The AE and the EAC interacted in the generation stage when the EAC path shifted eastward, away from the coast. As the EAC subsequently swung back to the coastal area, the AE and the EAC separated. The AE then interacted with the surrounding eddy fields, propagated westward, before finally merging again with the EAC. The energy transfer during this process also indicated the interactions among the eddy pair, the surrounding eddy fields and the EAC. Baroclinic instability (BCI) was a main contributor to the AE in the generation stage. Barotropic instability (BTI) also contributed energy to the AE when it interacted with the EAC but accounted for a much smaller proportion. Both BCI and BTI contributed to the CE for most of its life cycle but to a much less extend than to the AE. The zonal heat and salt mass transported by the AE and CE were calculated based on a Lagrangian framework method, and these amounts were considerable compared with global zonal averaged heat and salt mass transported by other mesoscale eddies.

Assessing the Impact of Nontraditional Ocean Observations for Prediction of the East Australian Current

AbstractAccurate forecasting of ocean currents in dynamic regions remains a critical challenge due to the sparsity of observations in global ocean observing networks and the limited resolution of present‐day regional ocean models. Lately, traditional observing platforms have been complemented by newly available data streams capable of sampling at higher spatial and/or temporal resolutions in dynamically significant regions in near‐real time. However, the relative merits and trade‐offs of incorporating these “nontraditional” observations into ocean state estimates have not been thoroughly investigated. Here, we perform a detailed statistical and dynamical comparison of two high‐resolution reanalysis products assimilating different combinations of traditional and nontraditional observations in the East Australian Current (EAC) system, a vigorous western boundary current. We show that sea‐surface height and temperature are well‐constrained by satellite measurements; however, below the surface, a reanalysis incorporating fully available observations better represents the ocean state. The core of the EAC jet is effectively constrained by subsurface observations from deep water moorings upstream of jet separation, while radar‐derived nearshore surface velocities in the separation zone are found to resolve the submesoscale cyclonic band inshore of the EAC. Cost function sensitivity analysis of both products reveals excessive model adjustment at depth causing the reanalyzes to overestimate alongshore transport relative to a 22‐year freely evolving simulation. Overall, the assimilation of nontraditional observations delivers marked improvement in representing dynamical features of the EAC. However, this improvement is not as pronounced in the model forecast due to the introduction of nonphysical dynamics or forcing, suggesting that other improvements such as increased model resolution are required.

Transport variability over the Hawkesbury Shelf (31.5-34.5°S) driven by the East Australian Current.

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.

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).

Microbial tropicalization driven by a strengthening western ocean boundary current.

Western boundary currents (WBCs) redistribute heat and oligotrophic seawater from the tropics to temperate latitudes, with several displaying substantial climate change-driven intensification over the last century. Strengthening WBCs have been implicated in the poleward range expansion of marine macroflora and fauna, however, the impacts on the structure and function of temperate microbial communities are largely unknown. Here we show that the major subtropical WBC of the South Pacific Ocean, the East Australian Current (EAC), transports microbial assemblages that maintain tropical and oligotrophic (k-strategist) signatures, to seasonally displace more copiotrophic (r-strategist) temperate microbial populations within temperate latitudes of the Tasman Sea. We identified specific characteristics of EAC microbial assemblages compared with non-EAC assemblages, including strain transitions within the SAR11 clade, enrichment of Prochlorococcus, predicted smaller genome sizes and shifts in the importance of several functional genes, including those associated with cyanobacterial photosynthesis, secondary metabolism and fatty acid and lipid transport. At a temperate time-series site in the Tasman Sea, we observed significant reductions in standing stocks of total carbon and chlorophyll a, and a shift towards smaller phytoplankton and carnivorous copepods, associated with the seasonal impact of the EAC microbial assemblage. In light of the substantial shifts in microbial assemblage structure and function associated with the EAC, we conclude that climate-driven expansions of WBCs will expand the range of tropical oligotrophic microbes, and potentially profoundly impact the trophic status of temperate waters.

Regional Versus Remote Atmosphere‐Ocean Drivers of the Rapid Projected Intensification of the East Australian Current

AbstractLike many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5°C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO‐OASIS‐WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present‐day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean‐only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO‐OASIS‐WRF response is closer to linear transport estimates compared to the CMIP5 MMM.

A Water Mass Classification Approach to Tracking Variability in the East Australian Current

The East Australian Current (EAC) is a southward flowing western boundary current that transports relatively warm and nutrient-depleted subtropical water along Australia's east coast. The EAC is a highly variable system that is formed by temporally-varying mixtures of water in the Coral Sea that do not form a linear density gradient or conform to a set range of temperature and salinity values. It can therefore be difficult to track EAC dynamics across both space and time using traditional analytical approaches. In order to more accurately quantify variability and trends in penetration of the EAC we develop a novel machine-learning classification approach to quantify variability in coastal EAC dynamics along a latitudinal gradient within the EAC extension zone in southeastern Australia. Applying our method to data from a 22-year free running regional hydrodynamic model revealed significant decadal-scale changes to EAC dynamics in the region. The annual period (generally in the austral summer) when the EAC is the dominant water mass in the region increased by approximately 2 months over the model time series. The encroachment of the EAC's traditional period of summer dominance into winter may have significant ecological implications through the acceleration of poleward range extensions by vagrant tropical species, facilitation of community phase shifts from temperate to tropical assemblages, and a phenological shift in the timing of major phytoplankton blooms. These results highlight the need to further understand the rapid changes occurring within western boundary current systems, and illustrates how classification approaches may assist in uncovering patterns in these highly variable systems.

Remote Forcing of Tasman Sea Marine Heatwaves

AbstractRecent marine heatwave (MHW) events in the Tasman Sea have had dramatic impacts on the ecosystems, fisheries, and aquaculture off Tasmania’s east coast. However, our understanding of the large-scale drivers (forcing) and potential predictability of MHW events in this region off southeast Australia is still in its infancy. Here, we investigate the role of oceanic Rossby waves forced in the interior South Pacific on observed MHW occurrences off southeast Australia from 1994 to 2016, including the extreme 2015/16 MHW event. First, we used an upper-ocean heat budget analysis to show that 51% of these historical Tasman Sea MHWs were primarily due to increased East Australian Current (EAC) Extension poleward transports through the region. Second, we used lagged correlation analysis to empirically connect the EAC Extension intensification to incoming westward-propagating sea surface height (SSH) anomalies from the interior South Pacific. Third, we dynamically analyzed these SSH anomalies using simple process-based baroclinic and barotropic Rossby wave models forced by wind stress curl changes across the South Pacific. Finally, we show that associated monthly SSH changes around New Zealand may be a useful index of western Tasman Sea MHW predictability, with a lead time of 2–3 years. In conclusion, our findings demonstrate that there is potential predictability of advection-dominated MHW event likelihoods in the EAC Extension region up to several years in advance, due to the deterministic contribution from baroclinic and barotropic Rossby waves in modulating the EAC Extension transports.