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.

Early to middle Eocene calcareous nannofossils of the SW Pacific: Paleobiogeography and paleoclimate

<p>Earth’s climate underwent a long-term warming trend from the late Paleocene to early Eocene (~58–51 Ma), with global temperature reaching a sustained maximum during the Early Eocene Climatic Optimum (EECO; 53–50 Ma). Geochemical proxies indicate tropical or warm subtropical sea-surface temperature (SST) conditions in middle and high latitudes in the early Eocene, implying a very low latitudinal temperature gradient. This study investigates whether calcareous nannofossil assemblages in the southwest (SW) Pacific provide evidence of these conditions at middle latitudes in the early to middle Eocene, particularly during the EECO. Specifically, this study documents the biogeographic changes of warm- and cold-water nannofossil species along a paleolatitudinal transect through the EECO to track changes in water masses/ocean circulation at that time. Early to middle Eocene calcareous nannofossil assemblages were examined from four sites along a latitudinal transect in the SW Pacific, extending from Lord Howe Rise in the north to Campbell Plateau in the south and spanning a paleolatitude of ~46–54°S. All of the sections studied in this project span nannofossil zones NP10–16 (Martini, 1971). The data indicate up to three regional unconformities through the sections: at mid-Waipara, Deep Sea Drilling Project (DSDP) Site 207 and 277, part or all of Zone NP10 (lower Waipawan) is missing; at Sites 207 and 277 a possible hiatus occurs within NP12 (upper Waipawan–lower Mangaorapan); and at all sites part or all of Zone NP15 (lower Bortonian) is missing. Results of this study indicate that nannofossil assemblages in the SW Pacific are more similar to floras at temperate to polar sites rather than those at tropical/subtropical sites. However, variations in the relative abundance of key species in the SW Pacific are broadly consistent with the trends seen in the geochemical proxy records: an increase in warm-water taxa coincided with the EECO, corroborating geochemical evidence for a temperature maximum in the SW Pacific during this interval. The increase in the abundance and diversity of warm-water taxa and decrease in the abundance of cool-water taxa through the EECO supports previous suggestions that a warm-water mass (northward of the proto-Tasman Front) extended to ~55°S paleolatitude during this interval in response to enhanced poleward heat transport and intensification of the proto-East Australian Current. At the southernmost site, DSDP Site 277, a relatively short-lived influx of warm-water taxa at ~51 Ma suggests that warm waters expanded south at this time. However, greater diversity and abundance of warm-water taxa throughout the EECO at DSDP Site 207, suggests that the proto-East Australian Current exerted greater influence at this latitude for a longer duration than at Site 277. An increase in the abundance of cool-water taxa and decrease in diversity and abundance of warm-water taxa at all sites is recorded following the termination of the EECO. This corresponds with the contraction of the proto-Tasman Front due to weakened proto-East Australian Current flow and associated amplification of the proto-Ross Gyre. Previous estimates of SSTs from geochemical proxies in the SW Pacific during the EECO indicate that there was virtually no latitudinal temperature gradient and temperatures were tropical to subtropical (>20°C). However, nannofossil data from this study indicate warm temperate conditions (~15–20°C) during the EECO, suggesting that a reduced latitudinal gradient was maintained through this interval, which is in agreement with climate models.</p>

Early to middle Eocene calcareous nannofossils of the SW Pacific: Paleobiogeography and paleoclimate

<p>Earth’s climate underwent a long-term warming trend from the late Paleocene to early Eocene (~58–51 Ma), with global temperature reaching a sustained maximum during the Early Eocene Climatic Optimum (EECO; 53–50 Ma). Geochemical proxies indicate tropical or warm subtropical sea-surface temperature (SST) conditions in middle and high latitudes in the early Eocene, implying a very low latitudinal temperature gradient. This study investigates whether calcareous nannofossil assemblages in the southwest (SW) Pacific provide evidence of these conditions at middle latitudes in the early to middle Eocene, particularly during the EECO. Specifically, this study documents the biogeographic changes of warm- and cold-water nannofossil species along a paleolatitudinal transect through the EECO to track changes in water masses/ocean circulation at that time. Early to middle Eocene calcareous nannofossil assemblages were examined from four sites along a latitudinal transect in the SW Pacific, extending from Lord Howe Rise in the north to Campbell Plateau in the south and spanning a paleolatitude of ~46–54°S. All of the sections studied in this project span nannofossil zones NP10–16 (Martini, 1971). The data indicate up to three regional unconformities through the sections: at mid-Waipara, Deep Sea Drilling Project (DSDP) Site 207 and 277, part or all of Zone NP10 (lower Waipawan) is missing; at Sites 207 and 277 a possible hiatus occurs within NP12 (upper Waipawan–lower Mangaorapan); and at all sites part or all of Zone NP15 (lower Bortonian) is missing. Results of this study indicate that nannofossil assemblages in the SW Pacific are more similar to floras at temperate to polar sites rather than those at tropical/subtropical sites. However, variations in the relative abundance of key species in the SW Pacific are broadly consistent with the trends seen in the geochemical proxy records: an increase in warm-water taxa coincided with the EECO, corroborating geochemical evidence for a temperature maximum in the SW Pacific during this interval. The increase in the abundance and diversity of warm-water taxa and decrease in the abundance of cool-water taxa through the EECO supports previous suggestions that a warm-water mass (northward of the proto-Tasman Front) extended to ~55°S paleolatitude during this interval in response to enhanced poleward heat transport and intensification of the proto-East Australian Current. At the southernmost site, DSDP Site 277, a relatively short-lived influx of warm-water taxa at ~51 Ma suggests that warm waters expanded south at this time. However, greater diversity and abundance of warm-water taxa throughout the EECO at DSDP Site 207, suggests that the proto-East Australian Current exerted greater influence at this latitude for a longer duration than at Site 277. An increase in the abundance of cool-water taxa and decrease in diversity and abundance of warm-water taxa at all sites is recorded following the termination of the EECO. This corresponds with the contraction of the proto-Tasman Front due to weakened proto-East Australian Current flow and associated amplification of the proto-Ross Gyre. Previous estimates of SSTs from geochemical proxies in the SW Pacific during the EECO indicate that there was virtually no latitudinal temperature gradient and temperatures were tropical to subtropical (>20°C). However, nannofossil data from this study indicate warm temperate conditions (~15–20°C) during the EECO, suggesting that a reduced latitudinal gradient was maintained through this interval, which is in agreement with climate models.</p>

Variability and meandering of the East Australian Current jet at 27oS

&lt;p&gt;The East Australian Current (EAC) is the complex and highly energetic western boundary current of the South Pacific Ocean gyre. Low frequency (&gt;2 year) variability of the EAC reflects the changes in the wind and buoyancy forcing over the South Pacific. However, local and regional wind and buoyancy forcing drives higher frequency variability (&lt; 1-2 year) of the EAC. Due to the narrow shelf, the EAC-jet&amp;#160; meandering has an immediate impact on the continental shelf circulation. Here we use the IMOS EAC mooring array between May 2015 to September 2019 and satellite observational data to quantify the quantify the EAC variability and assess the potential drives and impact of the on-shelf meandering of the EAC jet on the properties of the Coral and Tasman Seas.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We find that there is considerable variability of Sea Surface Height (SSH) and Sea Surface temperature (SST) that at times co-vary, but at&amp;#160; other times the anomalies are opposed. We compare the surface anomalies with the EAC velocity and transport timeseries. The mean along-slope velocity vectors show poleward velocity dominates from 0-1500 m at the five mooring locations from the 500 m isobath to the deep abyssal basin with the strongest southward flow at the continental shelf. The variance ellipses show that the largest variability in EAC transport is in the along-shore direction. This indicates that the EAC variability is dominated by the movement of the EAC on- and off-shore. The EAC thus maintains its jet structure as it meanders onshore and offshore adjacent to the continental slope. While the mean along-shore velocity vectors provide a picture of the mean EAC, the time-series shows that the EAC has a complex and highly variable structure. Strong southward flow is associated with off-shore flow (positive across-slope velocity). While mostly measuring the EAC core we see times where the flow is northward (positive along-slope velocity). This northward velocity is due to the shelf flow extending from the coast to the shelf, and is generally associated with on-shore flow (negative across-slope velocity). These changes in the direction and strength of the velocity are driven by cyclonic eddies inshore of the jet, and have significant influence on the exchange between the open and shelf ocean.&lt;/p&gt;

Extreme ocean weather induced by upstream meandering of the East Australian Current

&lt;p&gt;We investigate Marine Heat Waves and Marine Cold Spells (MHWs/MCSs) along the east coast of the Australian continent, a western boundary current region with exceedingly complex dynamics. We provide evidence that episodic MWHs/MCSs along the south-east of the Australian continent are driven by upstream variations in the position, but not necessarily the strength, of the East Australian Current, and that these variations are, in turn, controlled by small-scale (100s of kilometers) eddies that propagate into the region from the east. These eddies are able to alternately 'shut-off' and `turn-on' the poleward transport of warm water by the boundary current in a manner analogous to atmospheric blocking. Precursors to these `blocks' are detectable as much as 60 days prior to the onset of an event. We will discuss the implications of our results for the early prediction of MHW/MCS events.&lt;/p&gt;

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.