East Australian Current Research Articles

Radiocarbon evidence for mid-late Holocene changes in southwest Pacific Ocean circulation

Variability in the southwest (SW) Pacific Ocean circulation is influenced by the changes in the South Pacific subtropical gyre and its western boundary current, the East Australian Current (EAC). The EAC plays a significant role in transporting warm, well-ventilated, nutrient-poor waters to more temperate higher latitudes. Recent climate changes associated with EAC intensification have led to anomalous warming in the South Tasman, with implications for marine ecosystems and environment. A clear understanding of the significance of these changes requires knowledge of past natural variability. Here we have reconstructed a 4500 year record of regional sea surface radiocarbon reservoir ages (R) and local reservoir effects (ΔR). Our results reveal the centennial-scale variability over the last 4500 years, with R ranges as large as 390 14C yr. Older R (~410 14C yr) between 1610 to 1860 A.D. in our record, corresponding to the “Little Ice Age,” suggests a weaker influence of the EAC in the South Tasman. Between 4000 and 1900 cal years B.P., R and ΔR were significantly younger than the modern, with values of ~170 and −130 14C yr, respectively, indicating increased EAC transport of tropical waters into the South Tasman. We propose that the large R variability was influenced by strong and abrupt El Nino events which punctuated the muted El Nino–Southern Oscillation (ENSO) period in the mid-late Holocene and enabled increased westward flow of gyre waters into the SW Pacific. The strengthening of the EAC extension appears to have been a response to the precession-modulated ENSO-Southern Annular Mode interactions.

Invigorating ocean boundary current systems around Australia during 1979–2014: As simulated in a near‐global eddy‐resolving ocean model

AbstractOcean boundary currents, transporting water masses and marine biota along the coastlines, are important for regional climate and marine ecosystem functions. In this study, we review the dominant multi‐decadal trends of ocean boundary currents around Australia. Using an eddy‐resolving global ocean circulation model, this study has revealed that the major ocean boundary current systems around Australia, the East Australian Current (EAC), the Indonesian Throughflow (ITF), the Leeuwin Current, the South Australian Current and the Flinders Current, have strengthened during 1979–2014, consistent with existing observations. Eddy energetics in the EAC, the ITF/South Equatorial Current in the southeast Indian Ocean, and the Leeuwin Current have also enhanced during the same period. The multi‐decadal strengthening of the ocean boundary current systems are primarily driven by large scale wind patterns associated with the dominant modes of climate variability and change – the phase shift of the Inter‐decadal Pacific Oscillation/Pacific Decadal Oscillation strengthens the ITF and the Leeuwin Current/South Australian Current; and the poleward shift and strengthening of surface winds in the subtropical gyres reinforce the EAC and the Flinders Current. The invigorating ocean boundary current systems have induced extreme oceanographic conditions along the Australian coastlines in recent years, including the poleward shift of marine ecosystems off the east coast of Australia and the consecutive Ningaloo Niño – marine heatwave events off the west coast during 2011–2013. Understanding long‐term trends and decadal variations of the ocean boundary currents is crucial to project future changes of the coastal marine systems under the influence of human‐induced greenhouse gas forcing.

Nutrient uplift in a cyclonic eddy increases diversity, primary productivity and iron demand of microbial communities relative to a western boundary current.

The intensification of western boundary currents in the global ocean will potentially influence meso-scale eddy generation, and redistribute microbes and their associated ecological and biogeochemical functions. To understand eddy-induced changes in microbial community composition as well as how they control growth, we targeted the East Australian Current (EAC) region to sample microbes in a cyclonic (cold-core) eddy (CCE) and the adjacent EAC. Phototrophic and diazotrophic microbes were more diverse (2–10 times greater Shannon index) in the CCE relative to the EAC, and the cell size distribution in the CCE was dominated (67%) by larger micro-plankton n}{}(geq 20lrm{mu }mathrm{m}), as opposed to pico- and nano-sized cells in the EAC. Nutrient addition experiments determined that nitrogen was the principal nutrient limiting growth in the EAC, while iron was a secondary limiting nutrient in the CCE. Among the diazotrophic community, heterotrophic NifH gene sequences dominated in the EAC and were attributable to members of the gamma-, beta-, and delta-proteobacteria, while the CCE contained both phototrophic and heterotrophic diazotrophs, including Trichodesmium, UCYN-A and gamma-proteobacteria. Daily sampling of incubation bottles following nutrient amendment captured a cascade of effects at the cellular, population and community level, indicating taxon-specific differences in the speed of response of microbes to nutrient supply. Nitrogen addition to the CCE community increased picoeukaryote chlorophyll a quotas within 24 h, suggesting that nutrient uplift by eddies causes a ‘greening’ effect as well as an increase in phytoplankton biomass. After three days in both the EAC and CCE, diatoms increased in abundance with macronutrient (N, P, Si) and iron amendment, whereas haptophytes and phototrophic dinoflagellates declined. Our results indicate that cyclonic eddies increase delivery of nitrogen to the upper ocean to potentially mitigate the negative consequences of increased stratification due to ocean warming, but also increase the biological demand for iron that is necessary to sustain the growth of large-celled phototrophs and potentially support the diversity of diazotrophs over longer time-scales.

The East Australian Current and Property Transport at 27°S from 2012 to 2013

AbstractThe East Australian Current (EAC) is the complex and highly energetic poleward western boundary current of the South Pacific Ocean. A full-depth current meter and property (temperature and salinity) mooring array was deployed from the continental shelf to the abyssal waters off Brisbane Australia (27°S) for 18 months from April 2012 to August 2013. The EAC mooring array is an essential component of the Australian Integrated Marine Observing System (IMOS). During this period the EAC was coherent with an eddy kinetic to mean kinetic energy ratio of less than 1. The 18-month, mean, poleward-only mass transport above 2000 m is 22.1 ± 7.5 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1). The mean, poleward-only heat transport and flow-weighted temperature above 2000 m are −1.35 ± 0.42 PW and 15.33°C, respectively. A difference in the poleward-only and net poleward mass and heat transports above 2000 m of 6.3 Sv and 0.24 PW reflects the presence of an equatorward EAC retroflection at the eastern (offshore) end of the mooring array. A complex empirical orthogonal function (EOF) analysis of the along-slope velocity anomalies finds that the first two modes explain 72.1% of the velocity variance. Mode 1 is dominant at periods of approximately 60 days, and mode 2 is dominant at periods of 120 days. These dominant periods agree with previous studies in the Tasman Sea south of 27°S and suggest that variability of the EAC in the Tasman Sea may be linked to variability north of 27°S.

The formation of a cold-core eddy in the East Australian Current

Cold-core eddies (CCEs) frequently form in western boundary currents and can affect continental shelf processes. It is not always clear, however, if baroclinic or barotropic instabilities contribute more to their formation. The Regional Ocean Modelling System (ROMS) is used to investigate the ocean state during the formation of a CCE in the East Australian Current (EAC) during October 2009. The observed eddy initially appeared as a small billow (approx. 50km in length) that perturbed the landward edge of the EAC. The billow grew into a mesoscale CCE (approx. 100km in diameter), diverting the EAC around it. A ROMS simulation with a realistic wind field reproduced a similar eddy. This eddy formed from negative vorticity waters found on the continental shelf south of the EAC separation point. A sensitivity analysis is performed whereby the impact of 3 different wind forcing scenarios, upwelling, downwelling, and no winds, are investigated. A CCE formed in all wind scenarios despite the wind induced changes in hydrographic conditions in the continental shelf and slope waters. As such, the source of energy for eddy formation did not come from the interactions of wind with the continental shelf waters. Analysis of strain and energy transformation confirms this by showing that the prevailing source of CCE energy was kinetic energy of the offshore EAC. These results clearly link the formation of the CCE to the swift flowing EAC and barotropic instabilities.

Physical oceanographic processes influence bio-optical properties in the Tasman Sea

Remote sensing observations show optical signatures to conform to the physical oceanographic patterns in the Tasman Sea. To test the link between physical oceanographic processes and bio-optical properties we investigated an in situ bio-optical dataset collected in the Tasman Sea. Analysis of in situ observations showed the presence of four different water masses in the Tasman Sea, formed by the relatively warm and saline East Australia Current (EAC) water, a mesoscale cold core eddy on the continental slope, cooler Tasman Sea water on the shelf and river plume water. The distribution of suspended substances and their inherent optical properties in these water masses were distinctly different. Light absorption and attenuation budgets indicate varying optical complexity between the water masses. Specific inherent optical properties of suspended particulate and dissolved substances in each group were different as they were influenced by physical and biogeochemical processes specific to that water mass. Remote sensing reflectance signature varied in response to changing bio-optical properties between the water masses; thus providing the link between physical oceanographic processes, bio-optical properties and the optical signature. Findings presented here extend our knowledge of the Tasman Sea, its optical environment and the role of physical oceanographic processes in influencing the inherent optical properties and remote sensing signature in this complex oceanographic region.

The separation of the East Australian Current: A Lagrangian approach to potential vorticity and upstream control

AbstractThe East Australian Current (EAC) is the western boundary current flowing along the east coast of Australia separating from the coast at approximately 34°S. After the separation two main pathways can be distinguished, the eastward flowing Tasman Front and the extension of the EAC flowing southward. The area south of the separation latitude is eddy‐rich and the separation latitude of the EAC is variable. Little is known of the properties of the water masses that separate at the bifurcation of the EAC. This paper presents new insights from the Lagrangian perspective, where the water masses that veer east and those that continue south are tracked in an eddy‐permitting numerical model. The transport along the two pathways is computed, and a 1:3 ratio between transport in the EAC extension and transport in the Tasman Front is found. The results show that the “fate” of the particles is to first order already determined by the particle distribution within the EAC current upstream of the separation latitude, where 85% of the particles following the EAC extension originate from below 460 m and 90% of the particles following the Tasman Front originate from the top 460 m depth at 28°S. The separation and pathways are controlled by the structure of the isopycnals in this region. Analysis of anomalies in potential vorticity show that in the region where the two water masses overlap, the fate of the water depends on the presence of anticyclonic eddies that push isopycnals down and therefore enable particles to travel further south.

Do East Australian Current anticyclonic eddies leave the Tasman Sea?

AbstractUsing satellite altimetry and high‐resolution model output we analyze the pathway of large, long‐lived anticyclonic eddies that originate near the East Australian Current (EAC) separation point. We show that 25–30% of these eddies propagate southward, around Tasmania, leave the Tasman Sea, and decay in the Great Australian Bight. This pathway has not been previously documented owing to poor satellite sampling off eastern Tasmania. As eddies propagate southward, they often “stall” for several months at near‐constant latitude. Along the pathway eddies become increasingly barotropic. Eddy intensity is primarily influenced by merging with other eddies and a gradual decay otherwise. Surface temperature anomaly associated with anticyclonic eddies changes as they propagate, while surface salinity anomaly tends to remain relatively unchanged as they propagate.

Seasonal variability in the continental shelf waters off southeastern Australia: Fact or fiction?

Seasonality is an important timescale driving variability in the waters of many continental shelf regions globally. Along the east coast of Australia, it has been recognised that the East Australian Current (EAC), the Western boundary current (WBC) of the South Pacific gyre, warms and strengthens in the Austral summer. Thus it has been hypothesised that shelf currents also warm and strengthen (poleward) annually. However, the EACs highly dynamic nature results in large variations in the latitude of separation from the coast and eddy shedding. Until recently the lack of long term in-situ observations on the shelf has precluded a study into low frequency (seasonal) variability in shelf circulation. Using at least 3 years of moored in situ temperature and velocity observations we investigate low frequency variability in shelf waters at 2 cross-shelf locations (i) upstream and (ii) downstream of the typical EAC separation latitude. The local winds vary bi-modally upstream and tri-modally downstream varying with the passage of fronts, thus do not drive a seasonal response in the circulation. Harmonic analysis of the velocity and temperature fields shows that upstream of the separation zone, only 6% of the velocity variability occurs on the seasonal timescale, compared to 49% of the temperature variability. Cross shelf temperature gradients and vertical velocity shear increase in summer with an increase in poleward heat advection in the EAC. Downstream of the separation point the influence of episodic eddy encroachments precludes seasonality in the vertical structure of the flow despite an annual cycle in the stratification. The seasonal cycle in temperature moves out of phase with increasing depths, with maxima (minima) in March (September) at 30m compared to maxima (minima) in May (November) at the bottom. This is expected to have a large influence on the timing of nutrient injection onto the shelf, and thus phytoplankton species composition and abundance.

Recent freshening of the East Australian Current and its eddies

AbstractThe East Australian Current (EAC) has a relatively weak mean flow and an energetic eddy field that dominates the circulation. The properties of the mean flow have been studied in detail, but the changes in the eddy field have received little attention. We analyze Argo temperature and salinity profiles for 2005–2012 to construct a picture of the time‐mean and time‐varying properties of EAC eddies. We find that eddies and the surrounding waters of the western Tasman Sea are freshening at a rate of 0.017–0.025 practical salinity unit/yr over the top 100 m, with no significant temperature change. Consistent with the observations, fields from an eddy‐resolving ocean model show freshening, with no temperature trend. Moreover, the model results indicate that observed changes are significant in the context of the variability over the last 20 years and may be part of a multiyear (perhaps decadal) cycle. We attribute the freshening of the region to increased precipitation off Eastern Australia.

Projected changes to Tasman Sea eddies in a future climate

AbstractThe Tasman Sea is a hot spot of ocean warming, that is linked to the increased poleward influence of the East Australian Current (EAC) over recent decades. Specifically, the EAC produces mesoscale eddies which have significant impacts on the physical, chemical, and biological properties of the Tasman Sea. To effectively consider and explain potential eddy changes in the next 50 years, we use high‐resolution dynamically downscaled climate change simulations to characterize the projected future marine climate and mesoscale eddies in the Tasman Sea through the 2060s. We assess changes in the marine climate and the eddy field using bulk statistics and by detecting and tracking individual eddies. We find that the eddy kinetic energy is projected to increase along southeast Australia. In addition, we find that eddies in the projected future climate are composed of a higher proportion of anticyclonic eddies in this region and that these eddies are longer lived and more stable. This amounts to nearly a doubling of eddy‐related southward temperature transport in the upper 200 m of the Tasman Sea. These changes are concurrent with increases in baroclinic and barotropic instabilities focused around the EAC separation point. This poleward transport and increase in eddy activity would be expected to also increase the frequency of sudden warming events, including ocean temperature extremes, with potential impacts on marine fisheries, aquaculture, and biodiversity off Tasmania's east coast, through direct warming or competition/predation from invasive migrating species.

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.

Evaluation of ocean forecast performance for Royal Australian Navy exercise areas in the Tasman Sea

The variability of sea surface height (SSH) and sea surface temperature (SST) in eddy-resolving re-analyses and forecasts of the East Australian Current (EAC) separation and eddy-shedding region is investigated. The baseline skill attainable by a naïve forecasting method is established using root mean square errors and correlation coefficients. The forecasting system is assessed against this baseline and found to be skilful throughout the forecast period for SSH, and for four days (of six) for SST. A skilful forecast is much more likely than an unskilful one. However, a ‘failure mode’ of the model, associated with eddy merging, is identified.

A chironomid-inferred summer temperature reconstruction from subtropical Australia during the last glacial maximum (LGM) and the last deglaciation

A chironomid-based mean February temperature reconstruction from Welsby Lagoon, North Stradbroke Island, Australia covering the last glacial maximum (LGM) and deglaciation (between c. ∼23.2 and 15.5 cal ka BP) is presented. Mean February temperature reconstructions show a maximum inferred cooling of c. ∼6.5 °C at c. ∼18.5 cal ka BP followed by rapid warming to near Holocene values immediately after the LGM. The inferred timing, magnitude and trend of maximum cooling and warming display strong similarities to marine records from areas affected by the East Australian current (EAC). The warming trend started at c. ∼18.1 cal ka BP and is consistent with the start of deglaciation from Antarctic records. Near Holocene values are maintained through the deglaciation to 15.5 cal ka BP. These records suggest that changes in the Australian subtropics are linked to southern high latitudes.

Decadal and seasonal changes in temperature, salinity, nitrate, and chlorophyll in inshore and offshore waters along southeast Australia

AbstractSixty years of oceanographic in situ data at Port Hacking (34°S) and Maria Island (42°S) and 15 years of satellite‐derived chlorophyll a (chl a) in inshore and offshore waters of southeast Australia show changes in the seasonality and trend of water properties consistent with long‐term intensification and southerly extensions of East Australian Current (EAC) water. Decadal analyses reveal that the EAC extension water at Maria Island increased gradually from the 1940s to 1980s, followed by a rapid increase since the 1990s. This acceleration coincided with enhanced winter nitrate, implying increased injections of subantarctic water at Maria Island. Satellite‐derived chl a at six coastal sites and offshore companion sites in the western Tasman Sea showed significant inshore‐offshore variations in seasonal cycle and long‐term trend. After 2004–2005, the Maria Island seasonal cycle became increasingly similar to those of Bass Strait and St. Helens, suggesting that the EAC extension water was extending further southward. Comparative analyses of inshore‐offshore sites showed that the presence of EAC extension water declined offshore. Seasonal cycles at Maria Island show a recent shift away from the traditional spring bloom, toward increased winter biomass, and enhanced primary productivity consistent with extensions of warm, energetic EAC extension water and more frequent injections of cooler, fresher nitrate‐replete waters. Overall, we find complex temporal, latitudinal, and inshore‐offshore changes in multiple water masses, particularly at Maria Island, and changes in primary productivity that will profoundly impact fisheries and ecosystems.

Drivers of decadal variability in the Tasman Sea

AbstractIn this study, we compare optimally interpolated monthly time series Tasman Sea XBT data and a comprehensive set of ocean data assimilation models forced by atmospheric reanalysis to investigate the stability of the Tasman Sea thermocline and the transport variability of the East Australian Current (EAC), the Tasman Front, and EAC‐extension. We find that anomalously weaker EAC transport at 25°S corresponds to an anomalously weaker Tasman Front and anomalously stronger EAC‐extension. We further show that, post about 1980 and relative to the previous 30 years, the anomalously weaker EAC transport at 25°S is associated with large‐scale changes in the Tasman Sea; specifically stronger stratification above the thermocline, larger thermocline temperature gradients, and enhanced energy conversion. Significant correlations are found between the Maria Island station Sea Surface Temperature (SST) variability and stratification, thermocline temperature gradient, and baroclinic energy conversion suggesting that nonlinear dynamical responses to variability in the basin‐scale wind stress curl are important drivers of decadal variability in the Tasman Sea. We further show that the stability of the EAC is linked, via the South Caledonian Jet, to the stability of the pan‐basin subtropical South Pacific Ocean “storm track.”

Characterising primary productivity measurements across a dynamic western boundary current region

Determining the magnitude of primary production (PP) in a changing ocean is a major research challenge. Thousands of estimates of marine PP exist globally, but there remain significant gaps in data availability, particularly in the Southern Hemisphere. In situ PP estimates are generally single-point measurements and therefore we rely on satellite models of PP in order to scale up over time and space. To reduce the uncertainty around the model output, these models need to be assessed against in situ measurements before use. This study examined the vertically-integrated productivity in four water-masses associated with the East Australian Current (EAC), the major western boundary current (WBC) of the South Pacific. We calculated vertically integrated PP from shipboard 14C PP estimates and then compared them to estimates from four commonly used satellite models (ESQRT, VGPM, VGPM-Eppley, VGPM-Kameda) to assess their utility for this region. Vertical profiles of the water-column show each water-mass had distinct temperature–salinity signatures. The depth of the fluorescence-maximum (fmax) increased from onshore (river plume) to offshore (EAC) as light penetration increased. Depth integrated PP was highest in river plumes (792±181mgCm−2d−1) followed by the EAC (534±116mgCm−2d−1), continental shelf (140±47mgCm−2d−1) and cyclonic eddy waters (121±4mgCm−2d−1). Surface carbon assimilation efficiency was greatest in the EAC (301±145mgC (mgChl-a)−1d−1) compared to other water masses. All satellite primary production models tested underestimated EAC PP and overestimated continental shelf PP. The ESQRT model had the highest skill and lowest bias of the tested models, providing the best first-order estimates of PP on the continental shelf, including at a coastal time-series station, Port Hacking, which showed considerable inter-annual variability (155–2957mgCm−2d−1). This work provides the first estimates of depth integrated PP associated with the East Australian Current in temperate Australia. The ongoing intensification of all WBCs makes it critical to understand the variability in PP at the regional scale. More accurate predictions in the EAC region will require vertically-resolved in situ productivity and bio-optical measurements across multiple time scales to allow development of other models which simulate dynamic ocean conditions.

Influence of a western boundary current on shelf dynamics and upwelling from repeat glider deployments

The separation zone of a dynamically important western boundary current (WBC) is resolved through a series of sustained glider deployments along the coastal edge of the jet. The comprehensive data set from 23 missions (2008–2014) provides a new high-resolution hydrographic climatology which is exploited to understand the spatial extent of dense water uplift and the depth-averaged momentum balances across the East Australian Current (EAC) separation zone. The predominantly geostrophic shelf circulation and temperature fields are least (most) variable upstream (downstream), where encroachment (separation) dominates. For the first time we resolve the nonlinear advection terms which are considerable in the along-shelf momentum balance. Near bottom water masses indicate dense water uplift, as a result of the EAC encroachment and separation. The data provide both new insight into and a climatology of separation-induced uplift and demonstrate a successful model for repeat glider missions in a dynamic WBC environment.

Seasonal and inter-annual variability of western subtropical mode water in the South Pacific Ocean

The seasonal and inter-annual variability of the western subtropical mode water (hereafter STMW) in the South Pacific Ocean was examined using the Bluelink ReANalysis 2.1 (BRAN2.1) in terms of heat budget. The analysis of heat content change suggested that the seasonal cycle of surface heat flux played a dominant role in the formation of the STMW in the South Pacific Ocean. However, the surface heat flux and the East Australian Current (EAC) heat transport tended to compensate one another during STMW production. Out of phase or different amplitude of the components led to warming or cooling of the mixed layer, and the heat transport by the EAC in the formation of the STMW cannot be ignored. The correlation between volume anomalies of the STMW and net surface heat flux was insignificant, indicating that the inter-annual variability of the STMW was equally influenced by surface thermal forcing and ocean dynamic processes, such as horizontal advection. This study revealed the important role played by the EAC in the inter-annual variability of the STMW, i.e., a weakened heat transport by the EAC led to an increased volume anomaly of the STMW in the South Pacific Ocean. The STMW production can be further enhanced by La Nina, which drives positive anomaly in sea surface salinity in the western South Pacific and creates a favourable preconditioning for surface cooling in austral winter.

Interactions between seasonality and oceanic forcing drive the phytoplankton variability in the tropical-temperate transition zone (~ 30°S) of Eastern Australia

The East Australian Current (EAC) has been shown to be warming rapidly, which is expected to cause latitudinal shifts in phytoplankton abundance, distribution and composition along the east Australian coast. Yet a lack of phytoplankton information exists northward of 34°S. Here, we provide the first detailed taxonomic time-series survey (monthly sampling for about one annual cycle, 2011–2012) in the east Australian tropical-temperate transition zone (~30°S, upstream of the EAC separation point at ~31–32°S). All phytoplankton (categorised depending on their association with specific water-types) show a seasonal signal with abundance maxima (minima) during summer (winter). This seasonal signal is most pronounced in the seasonal/bloom category and least expressed by deep-water taxa, which prefer cold, saline and dense bottom water independent of the season. Different extents of EAC encroachment onto the continental shelf drive the cross-shelf phytoplankton composition and distribution, such that a weak EAC is associated with phytoplankton community being organised along ‘depth’ and ‘distance from the coast’ gradients with high phytoplankton abundances inshore. A strong EAC favours the occurrence of warm-water taxa offshore and an increase in diatom abundance on the mid-shelf (53% shelf width). We conclude that the phytoplankton community in the tropical-temperate transition zone of Eastern Australia is driven by an interaction of intrinsic seasonal cycles and primarily EAC-driven oceanic forcing. Our findings benefit studies located in Western Boundary Current systems worldwide, in which warming and strengthening of these currents are predicted to severely impact phytoplankton dynamics.