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

Abstract

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.

Impact of Mesoscale Circulation on the Structure of River Plumes During Large Rainfall Events Inshore of the East Australian Current

“…These three scenarios closely resemble the three modes identified by Ribbat et al (2020b) in a 2-year simulation and defined by Roughan et al (2022) to explore shelf transport pathways. However, as the modes presented here are identified from a multi-decadal (25-year) observation and simulation, we consider them to be more robust because EOF analysis can capture more time-scale variability in a longer time series.…”

“…Previous work in the HSM region by Ribbat et al (2020b) and Roughan et al (2022) showed that there are 3 dominant oceanographic scenarios associated with the separation of the EAC and its eddy field. We use EOF analysis to extract the dominant modes that represent these EAC scenarios from our multi-decadal EAC-ROMS model run, which has been interpolated onto the HSM model grid.…”

“…As the EAC jet and the anticyclonic eddies encroach upon the coast, their shoreward movements accelerate the poleward along-shore current. However, the intrusion of cyclonic eddies result in a northward flow on the shelf Ribbat et al, 2020b;Roughan et al, 2022). Therefore, the variability of the EAC jet and the eddy field play a role in the evolution of the river plumes near the coast.…”

“…The Hawkesbury Shelf region is located downstream of the EAC separation, which has been identified as a region of high productivity (Oke and Middleton, 2001). The shelf circulation in this region is dominated by three prominent circulation patterns, which are associated with the poleward along-shore flow of the EAC jet, the eddies and the eddy dipoles downstream of the typical EAC separation region (33 • S) (Malan et al, 2020;Ribbat et al, 2020b;Roughan et al, 2022). The circulation modes are largely dictated by where the EAC separates and the presence (or absence) of anticyclonic and cyclonic eddies as described in Roughan et al (2022).…”

“…The shelf circulation in this region is dominated by three prominent circulation patterns, which are associated with the poleward along-shore flow of the EAC jet, the eddies and the eddy dipoles downstream of the typical EAC separation region (33 • S) (Malan et al, 2020;Ribbat et al, 2020b;Roughan et al, 2022). The circulation modes are largely dictated by where the EAC separates and the presence (or absence) of anticyclonic and cyclonic eddies as described in Roughan et al (2022). The three modes are shown clearly in the schematic diagram of Ribbat et al (2020b), their Figure 12.…”