East Australian Current Extension Research Articles

Observed multi-decadal trends in subsurface temperature adjacent to the East Australian Current

Abstract. Sea surface temperature observations have shown that western boundary currents, such as the East Australian Current (EAC), are warming faster than the global average. However, we know little about coastal temperature trends inshore of these rapidly warming regions, particularly below the surface. In addition to this, warming rates are typically estimated linearly, making it difficult to know how these rates have changed over time. Here we use long-term in situ temperature observations through the water column at five coastal sites between approximately 27.3–42.6∘ S to estimate warming trends between the ocean surface and the bottom. Using an advanced trend detection method, we find accelerating warming trends at multiple depths in the EAC extension region at 34.1 and 42.6∘ S. We see accelerating trends at the surface and bottom at 34.1∘ S but similar trends in the top 20 m at 42.6∘ S. We compare several methods, estimate uncertainty, and place our results in the context of previously reported trends, highlighting that magnitudes are depth-dependent, vary across latitude, and are sensitive to the data time period chosen. The spatial and temporal variability in the long-term temperature trends highlight the important role of regional dynamics against a background of broad-scale ocean warming. Moreover, considering that recent studies of ocean warming typically focus on surface data only, our results show the necessity of subsurface data for the improved understanding of regional climate change impacts.

Three decades of variability and warming of nearshore waters around Tasmania

The ocean region around Tasmania forms an interface between major ocean basins and represents a significant global ocean hotspot due to a rate of warming higher than the global average. Nearshore Tasmanian waters are of central importance for shallow-water ecosystems and a range of human activities, including recreation, aquaculture and fisheries which have become increasingly impacted by warming in recent decades. However, the fine-scale spatial and temporal variability of sea surface temperature (SST) in these waters is not well understood, limiting opportunities for effective local-scale mitigation and adaptation measures in the face of ongoing warming. We use a high-resolution (0.02 °x 0.02°, 1992–2020) satellite SST received at local stations (validated against in situ temperature) to explore variability over seasonal, interannual, decadal and multi-decadal timescales at spatial scales from the nearshore up to the entire study domain. The SST is used as a proxy for seasonal circulation by removal of the broad-scale radiative forcing (estimated by a region-wide latitudinal gradient of SST). The main circulation drivers of nearshore SST are the East Australian Current Extension (EACx) and Zeehan Current (ZC) acting on the east and west coasts with respective peaks in summer and winter. New results include the seasonality of three summer upwelling systems, the first direct evidence of EACx inflow into eastern Bass Strait, a time series of the endpoint of the ZC/EACx intersection, and a full year description of the ZC, from warm winter inflow transitioning into a summer cooler than ambient intrusion. We find that local SST variations are controlled by coastal geometry with intensified atmospheric forcing in enclosed waters. We have identified two modes of low frequency SST variability (including 1, 2 and 10-year periods) associated with the EACx (63% of the variance) and the ZC (12%). The EACx mode induces a rapid adjustment of the entire ocean region around Tasmania. Multi-decadal warming trends occur in all regions, modulated by coastal boundary flows, changes in upwelling mechanisms and local topography. Trends in the eastern shelf arise from intrusions of EACx waters, with local processes driving trends on the western shelf. Both east and west nearshore SST trends are around + 0.4°C decade−1 with the highest value of + 0.55°C decade−1 on the central west coast, possibly associated with outflow from Macquarie Harbour.

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

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

Why the Mixed Layer Depth Matters When Diagnosing Marine Heatwave Drivers Using a Heat Budget Approach

Marine heatwaves (MHWs) are extreme warming events that can result in significant damage to marine ecosystems and local economies. The primary drivers of these events have been frequently studied using an upper ocean heat budget. However, various surface mixed layer (SML) depths have been used with little attention paid to the impact of the depth chosen on heat budget term estimates. We analyse MHW drivers in two dynamically contrasting regions off the east coast of Australia (East Australian Current extension) and the west coast of New Zealand over a 30-year period (1985–2014, inclusive). We compare the magnitude of the air-sea heatflux and advection terms in a volume-averaged heat budget using three different SML depth estimates. We show that the SML depth over which the heat budget is calculated has direct consequences on the identification of MHW dominant drivers. The air-sea heatflux term is amplified when the SML depth is underestimated and dampened when overestimated. The variation in the magnitude of the advection term is dependent on the barotropic or baroclinic structure of the currents. We, also, show that the impact on MHW driver classification is both temporally and regionally dependent. Generally, a deep SML estimate results in more MHWs being classified as advection and less classified as air-sea heatflux-driven. However, during the cool months, a shallow estimate produces the opposite pattern and to a varying degree of intensity depending on the region's dynamics. Use of daily and spatially variable SML depth in a heat budget calculation allows the comparison between regions with different dynamics influencing the mixed layer depth. These results show that when using a heat budget approach to explore marine heatwaves over extended time and space (e.g., regions and seasons), it is imperative to consider the temporal and spatial variability in the SML depth.

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.

Local Grid Refinement in New Zealand's Earth System Model: Tasman Sea Ocean Circulation Improvements and Super‐Gyre Circulation Implications

AbstractThis paper describes the development of New Zealand's Earth System Model (NZESM) and evaluates its performance against its parent model (United Kingdom Earth System Model, UKESM) and observations. The main difference between the two earth system models is an embedded high‐resolution (1/5°) nested region over the oceans around New Zealand in the NZESM. Due to this finer ocean model mesh, currents such as the East Australian Current, East Australian Current Extension, Tasman Front, and Tasman Leakage, and their volume and heat transports are better simulated in the NZESM. The improved oceanic transports have led to a reduction in upper ocean temperature and salinity biases over the nested region. In addition, net transports through the Tasman Sea of volume, heat and salt in the NZESM agree better with previously reported estimates. A consequence of the increased cross‐Tasman Sea transports in the NZESM is increased temperatures and salinity west of Australia and in the Southern Ocean reducing the meridional sea surface temperature gradient between the subtropics and sub‐Antarctic. This also leads to a weakening of the westerly winds between 60°S and 45°S over large parts of the Southern Ocean, which reduces the northward Ekman transport, reduces the formation of Antarctic Intermediate Water, and allows for a southward expansion of the Super‐Gyre in all ocean basins. Connecting an improved oceanic circulation around New Zealand to a basin‐wide Super‐Gyre response is an important step forward in our current understanding of how local scales can influence global scales in a fully coupled earth system model.

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.

Variability and Long‐Term Trends in the Shelf Circulation Off Eastern Tasmania

AbstractThis study investigates trends and interannual variability of the marine climate across the continental shelf off eastern Tasmania for 1993–2016. This region is a hotspot for global warming and biodiversity. Eastern Tasmania lies at the boundary between two ocean currents (the East Australian Current Extension, or EAC Extension, and the Zeehan Current, ZC) leading to the local marine climate exhibiting trends and variability from both boundary currents. A numerical ocean model is used to provide high‐resolution (∼2 km) estimates of the temperature, salinity, and circulation for the region. Results indicate significant positive trends in temperature, salinity, and southward flow over the shelf, consistent with an increasing EAC Extension. These trends are particularly strong in autumn, indicating a lengthening of the warm season. The interannual variability in the EAC Extension and ZC was quantified by a simple index, based on a modal analysis of surface circulation, indicating the relative dominance of each current. Strong EAC years were related to significantly more summertime marine heat wave days. Large‐scale remote drivers of variability were considered, and we found weak but significant links with El Niño–Southern Oscillation and Tasman Sea Blocking. El Niño–Southern Oscillation was found to modulate the EAC Extension in summer with a El Niño leading to enhanced southward flow and warming over the shelf. Tasman Sea Blocking was found to drive enhanced southward surface flow, particularly in winter. Nonetheless, large‐scale forcing modes explain less than 25% of the total variability in the EAC‐ZC system indicating that most of the variability is internally generated.

Wind Forced Variability in Eddy Formation, Eddy Shedding, and the Separation of the East Australian Current

AbstractThe East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy‐permitting regional ocean model, we present a suite of simulations forced by the same time‐mean fields, but with different atmospheric and remote ocean variability. These eddy‐permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies.

Zooplankton responses to increasing sea surface temperatures in the southeastern Australia global marine hotspot

Southeastern Australia is a ‘hotspot’ for oceanographic change. Here, rapidly increasing sea surface temperatures, rising at more than double the global trend, are largely associated with a southerly extension of the East Australian Current (EAC) and its eddy field. Maria Island, situated at the southern end of the EAC extension at 42°S, 148°E, has been used as a site to study temperature-driven biological trends in this region of accelerated change. Zooplankton have short life cycles (usually < 1 year) and are highly sensitive to environmental change, making them an ideal indicator of the biological effects of an increased southward flow of the EAC. Data from in-situ net drops and the Continuous Plankton Recorder (CPR), collected since 2009, together with historical zooplankton abundance data, have been analysed in this study. Like the North Atlantic, zooplankton communities of southeastern Australia are responding to increased temperatures through relocation, long-term increases in warm-water species and a shift towards a zooplankton community dominated by small copepods. The biological trends present evidence of extended EAC influence at Maria Island into autumn and winter months, which has allowed for the rapid establishment of warm-water species during these seasons, and has increased the similarity between Maria Island and the more northerly Port Hacking zooplankton community. Generalised Linear Models (GLM) suggest the high salinity and low nutrient properties of EAC-water to be the primary drivers of increasing abundances of warm-water species off southeastern Australia. Changes in both the species composition and size distribution of the Maria Island zooplankton community will have effects for pelagic fisheries. This study provides an indication of how zooplankton communities influenced by intensifying Western Boundary currents may respond to rapid environmental change.

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.

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.

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.

Long‐term trends in the East Australian Current separation latitude and eddy driven transport

AbstractAn observed warming of the Tasman Sea in recent decades has been linked to a poleward shift of the maximum wind stress curl, and a strengthening of the poleward flow along the coast of southeastern Australia. However, changes in the East Australian Current (EAC) separation latitude, as well as in the contribution of the EAC, the EAC extension and its eddy field to the total southward transport due to such a strengthening remain unknown. This study uses 30 years (1980–2010) of the Ocean Forecast for the Earth Simulator (OFES) sea surface height and velocity outputs to obtain a three decade long‐time series of (i) the EAC separation latitude, (ii) the southward transport along the coast of southeastern Australia (28°S–39°S), and (iii) the southward transport across the EAC separation latitude. A Lagrangian approach is implemented and the spin parameter Ω is used to provide a quantitative distinction between the transports occurring outside and inside (cyclonic and anticyclonic) eddies. Significant positive trends of the low pass southward transports indicate that the intensification of the poleward flow has occurred both within the EAC and in the EAC extension. In addition, a significant increase in southward transport inside and outside eddies is found. Importantly, the contribution of eddy driven transport has a large temporal variability and shows a sharp increase from 2005 onward. Finally our results show that the EAC has not penetrated further south but it has separated more frequently at the southernmost latitudes within the region where it typically turns eastward.

Extending our understanding of South Pacific gyre “spin-up”: Modeling the East Australian Current in a future climate

The western Tasman Sea represents a global warming marine ‘‘hot spot,’’ where the waters are warming at almost 4 times the global average rate, argued in the literature to be due to a ‘‘spin-up’’ of the South Pacific subtropical gyre and extension of the East Australian Current (EAC). To further investigate and test this paradigm, we analyze climate change simulations of Tasman Sea circulation and metrics on output from the Ocean Forecasting Australia Model for the 20th and 21st centuries, forced by a global climate model simulation under the A1B carbon emissions scenario. First, we show that the 1990s simulation estimates of mean dynamic topography, present-day location of the EAC separation point, and volume transports of the EAC, EAC extension, and flow along the Tasman Front, are consistent with recent observations. We further demonstrate that between the 1990s and 2060s, the volume transport of the EAC extension is projected to increase by 4.3 Sv at the expense of the flow along the Tasman Front (projected to decrease by 2.7 Sv). The transport of the EAC core flow (equatorward of the separation point) is projected to change very little (increase of 0.2 Sv). The model projects a Tasman Sea-wide warming, with mean increases of up to 3C. These results are interpreted using a simple linear, barotropic model which captures both the sign and meridional distribution of the projected changes in mean transport, including negligible change in core EAC transport but enhanced EAC extension. This meridional asymmetry in the transports is consistent with the wind-forced ocean response to changes in the basin-wide wind stress curl.

Nutrient variability in Subantarctic Mode Waters forced by the Southern Annular Mode and ENSO

As the primary source of nutrients to the global thermocline, Subantarctic Mode Waters (SAMWs) play a key role in primary production and climate. Here we use repeat hydrographic World Ocean Circulation Experiment/Climate Variability and Predictability data to quantify interannual SAMW nutrient variability and its forcing. Pacific sector SAMW nutrients were significantly correlated with the Southern Annular Mode (SAM) and wind stress curl anomalies associated with a faster meridional overturning circulation (MOC). A stronger MOC results in greater upwelling of nutrients at high latitudes, increased Ekman transport of nutrients equatorward, and subduction of higher preformed nutrient loads in SAMWs. Australian sector SAMWs were significantly correlated with El Niño–Southern Oscillation (ENSO), likely due to its modulation of transport in the East Australian Current extension. Interannual variability in SAMW nutrients impacts downstream tropical export production by as much as 5–12% of the annual mean.

Marine Downscaling of a Future Climate Scenario for Australian Boundary Currents

Abstract Ocean boundary currents are poorly represented in existing coupled climate models, partly because of their insufficient resolution to resolve narrow jets. Therefore, there is limited confidence in the simulated response of boundary currents to climate change by climate models. To address this issue, the eddy-resolving Ocean Forecasting Australia Model (OFAM) was used, forced with bias-corrected output in the 2060s under the Special Report on Emissions Scenarios (SRES) A1B from the CSIRO Mark version 3.5 (Mk3.5) climate model, to provide downscaled regional ocean projections. CSIRO Mk3.5 captures a number of robust changes that are common to most climate models that are consistent with observed changes, including the weakening of the equatorial Pacific zonal wind stress and the strengthening of the wind stress curl in the Southern Pacific, important for driving the boundary currents around Australia. The 1990s climate is downscaled using air–sea fluxes from the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40). The current speed, seasonality, and volume transports of the Australian boundary currents show much greater fidelity to the observations in the downscaled model. Between the 1990s and the 2060s, the downscaling with the OFAM simulates a 15% reduction in the Leeuwin Current (LC) transport, a 20% decrease in the Indonesian Throughflow (ITF) transport, a 12% increase in the East Australian Current (EAC) core transport, and a 35% increase in the EAC extension. The projected changes by the downscaling model are consistent with observed trends over the past several decades and with changes in wind-driven circulation derived from Sverdrup dynamics. Although the direction of change projected from downscaling is usually in agreement with CSIRO Mk3.5, there are important regional details and differences that will impact the response of ecosystems to climate change.

Ensemble prediction study of the East Australian Current

We examine the role of dynamical instabilities and flow dependent, growing errors in an ensemble prediction study of the East Australian Current. In the region of the Tasman front where the East Australian Current separates from the coast, the circulation is characterised by meandering and significant mesoscale eddy variability, and is dominated by nonlinear dynamics, thereby representing a severe challenge for operational forecasting. We explore the mechanisms for and structures of forecast errors over a month using the Australian operational ocean forecast system. To examine the role of dynamical instabilities, forecast ensemble perturbations are generated as bred vectors allowing the identification of those perturbations to a given initial state that grow most rapidly. We consider a period spanning the Austral autumn/summer corresponding to the seasons of maximum eddy variability. We find that the bred vectors span a low dimensional subspace whose structures correspond to regions of large forecast error. Further, these structures typically occur in regions of instability and in particular in the vicinity of the Tasman front and the East Australian Current extension. Our results suggest that even a very few bred vectors will be useful in adaptive sampling and targeted observations. References G. R. Brassington, P. R. Oke, and T. Pugh, BLUElink Operational ocean prediction in Australia, vol. 5, ch. 11, pp. 87--95, World Scientific Publishing, 2006. M. Corazza, E. Kalnay, D. J. Patil, S.-C Yang, R. Morss, M. Cai, I. Szunyogh, B. R. Hunt, and J. A. Yorke, Use of the breeding technique to estimate the structure of the analysis errors of the day, Nonlin. Proc. in Geophys. 10 (2003), 233--243. http://hal.archives-ouvertes.fr/hal-00302201/ S. M. Griffies, M. J. Harrison, R. C. Pacanowski, and A. Rosati, A technical guide to {MOM4} {GFDL} ocean technical group report no 5, 2003. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.143.8321&rep=rep1&type=pdf H. E. Hurlburt, G. B. Brassington, Y. Drillet, M. Kamachi, M. Benkiran, R. Bourdalle-Badie, E. P. Chassignet, G. A. Jacobbs, O. Le Galloudec, J.-M. Lellouche, E. J. Metzger, P. R. Oke, T. F. Pugh, A. Schiller, O. M. Smedstad, B. Tranchant, H. Tsujino, N. Usui and A. J. Walcraft, High-resolution global and basin-scale ocean analyses and forecasts., Oceanography 22 (2009), no. 3, 80--97. http://handle.dtic.mil/100.2/ADA508067 M. M. Mata, M. Tomczak, S. Wijffels, and J. A. Church, East Australian Current volume transports at 30S: Estimates from the World Ocean Circulation Experiment hydrographic sections PR11/P6 and the PCM3 current meter array., J. Geophys. Res. 105 (2000), 28, 509--28, 526. doi:10.1029/1999JC000121 M. M. Mata, S. E. Wijffels, J. A. Church, and M. Tomczak, Eddy shedding and energy conversion in the East Australian Current, J. Geophys. Res. 111 (2006), C09034. doi:10.1029/2006JC003592 T. J. O'Kane and J. S. Frederiksen, Statistical dynamical subgrid-scale parameterizations for geophysical flows, Physica Scripta T132 (2008), 014033. doi:10.1088/0031-8949/2008/T132/014033 T. J. O'Kane, P. R. Oke and P. A. Sandery, Ocean Modelling, to appear (2011). doi:10.1016/j.ocemod.2011.04.003 P. R. Oke, M. Balmaseda, M. Benkiran, J. A. Cummings, E. Dombrowsky, Y. Fujii, S. Guinehut, G. Larnicol, P.-Y. Le Traon, and M. J. Martin, Observing System Evaluations using GODAE systems, Oceanography (2009), 144--153. http://handle.dtic.mil/100.2/ADA511325. P. R. Oke and D. A. Griffin, The cold-core eddy and strong upwelling off the coast of New South Wales in early 2007, Deep Sea Research 58 (2011), 574--591. doi:10.1016/j.dsr2.2010.06.006 D. Pazo, M. A. Rodr\OT1\i guez, and J. M. Lopez, Spatio-temporal evolution of perturbations in ensembles initialized by bred, Lyapunov and singular vectors, Tellus 62A (2010), 10--23. doi:10.1111/j.1600-0870.2009.00419.x P. A. Sandery, G. B. Brassington, and J. Freeman, Adaptive nonlinear dynamical initialization., J. Geophys. Res. 116 (2011), C01021. doi:10.1029/2010JC006260 A. Schiller, P. R. Oke, G. B. Brassington, M. Entel, R. Fiedler, D. A. Griffin, and J.V. Mansbridge, Eddy-resolving ocean circulation in the Asian-Australian region inferred from an ocean reanalysis effort., Progress in Oceanography 76 (2008), 334--365. doi:10.1016/j.pocean.2008.01.003 Z. Toth and E. Kalnay, Ensemble forecasting at {NMC}, Bull. Amer. Meteor. Soc. 74 (1993), 2317--2330. doi:10.1175/1520-0477(1993)074&lt;2317:EFANTG>2.0.CO;2

Decadal changes in the South Pacific western boundary current system revealed in observations and ocean state estimates

Observations and ocean state estimates are used to investigate the nature and mechanism of decadal variability in the East Australian Current (EAC) system and South Pacific subtropical gyre. A 62 year record on the Tasmanian continental shelf shows decadal variations of temperature and salinity, as well as a long-term trend, which has been related to wind-driven variations in the poleward extension of the EAC. Repeat expendable bathythermograph lines spanning the last 15 years suggest that low-frequency variations in the transport of the EAC extension and Tasman Front are anticorrelated, but the time series are too short to draw firm conclusions. Here we use two ocean state estimates spanning the past 50 years to diagnose the physical mechanisms and spatial structure of the decadal variability of the South Pacific subtropical gyre. The observations and state estimates paint a consistent picture of the decadal variability of the gyre and EAC system. Strengthening of the basin-wide wind stress curl drives a southward expansion of the subtropical gyre. As the gyre shifts south, the EAC extension pathway is favored at the expense of the Tasman Front, resulting in the observed anticorrelation of the these two major currents. The results suggest that the subtropical gyre and western boundary current respond to decadal variability in basin-scale wind stress curl, consistent with Island Rule dynamics; that strong decadal variability of the South Pacific gyre complicates efforts to infer trends from short-term records; and that wind stress curl changes over the South Pacific basin drive changes in the EAC system that are likely to have implications for marine ecosystems and regional climate.