Search

Cross-shelf transport plays an important role in the heat, salt, and nutrient budgets of the continental shelf. In this study, we quantify cross-shelf volume transport and explore its dynamics within a high-resolution (2.5–6 km) regional ocean model of the East Australian Current (EAC) System, a western boundary current with a high level of mesoscale eddy activity. We find that the largest time-mean cross-shelf flows (>4 Sv per 100 km; 1 Sv ≡ 106 m3 s−1) occur inshore of the coherent western boundary current, between 26° and 30°S, while the strongest time-varying flows occur in the EAC southern extension, poleward of 32°S, associated with mesoscale eddies. Using a novel diagnostic equation derived from the momentum budget we show that the cross-shelf transport is dominated by the baroclinic and geostrophic component of the velocities, as the EAC jet is relatively free to flow over the variable shelfbreak topography. However, topographic interactions are also important and act through the bottom pressure torque term as a secondary driver of cross-shelf transport. The importance of topographic interaction also increases in shallower water inshore of the coherent jet. Downstream of separation, cross-shelf transport is more time-varying and associated with the interaction of mesoscale eddies with the shelf. The identification of the change in nature and drivers of cross-shelf transport in eddy versus jet dominated regimes may be applicable to understanding cross-shelf transport dynamics in other boundary current systems. Significance Statement Cross-shelf transport, i.e., the movement of water from the open ocean on or off the continental shelf, is not reported often as it is difficult to measure and model. We demonstrate a simple but effective method to do this and, using an ocean model, apply it to the East Australian Current System and show what drives it. The results show two distinct regimes, which differ depending on which part of the current system you are in. Our results help to place observations of cross-shelf transport in better context and provide a framework within which to consider the transport of other things such as heat and carbon from the open ocean to the continental shelf.

Context The spanner crab (Ranina ranina) stock of eastern Australia is distributed across two state jurisdictions and, as a non-migratory species with a pelagic larval phase, connectivity within this stock is likely to occur by larval dispersal, driven by ocean currents. Aims To understand connectivity and patterns of larval supply in the eastern Australian spanner crab stock. Methods Lagrangian particle tracking methods were used to simulate larval transport around the key spanner crab fishing regions in eastern Australia. Key results Spawning off central Queensland (Qld) supplies a large proportion of recruits, supporting both the Qld and New South Wales (NSW) fisheries. Lagged larval settlement showed significant correlations to catch-per-unit-effort and the proportion of total harvest taken within the NSW fishery, providing evidence to suggest that the NSW fishery may be reliant on spawning activity in Queensland. Conclusions The Qld and NSW fisheries are highly connected and the broad-scale patterns identified by the current modelling approach could provide an indicator of potentially good or bad recruitment years, particularly as finer resolution, and refined reproductive biology knowledge on spanner crabs becomes available. Implications The Qld and NSW fisheries are highly connected with a source–sink structure and it is recommended that a co-management strategy be adopted.

Western 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 l...

Recent observations report a poleward shift in several of the world’s western boundary current extensions in response to a warming climate. However, the nature and variability of the Agulhas Return Current (ARC) is still relatively unknown, leading to uncertainty in its response to gyre intensification. The objective of this work is to investigate meridional migration and eddy kinetic energy (EKE) trends in the ARC over the past 27 years using satellite-derived altimetry products. A difference is seen between the western and eastern regions of the ARC, the former exhibiting a predominantly equatorward migration and the latter moving predominantly poleward. The ARC is shown to be a region highly influenced by its bathymetry. Additionally, we show that the regional EKE peaks during austral summer months and that there has been a significant rise in summertime EKE levels between 1994–2011.

Key Points: • A regional ocean model is used to examine multidecadal shelf temperature changes on the Agulhas Bank • There are distinct shelf temperature regime changes in 1966 and 1996 • These regime shifts are caused by changes in coastal upwelling linked to large-scale wind variability The Agulhas Bank is an important area for the spawning of small pelagic fish and other species. Here, within a NEMO ocean model, we investigate changes in temperature over the Bank on multidecadal time scales. In agreement with previous observational studies, a shift to colder temperatures is found in 1997. The model also simulates an earlier shift from colder to warmer temperatures in 1966. These shifts are coastally confined and shown, using a climatologically forced model run as a control, to be driven by a north‐south migration in the large‐scale wind belts, rather than by changes in downward heat fluxes or changes in the Agulhas Current itself. The zonal wind changes on the Agulhas Bank show a significant relationship with the Southern Annular Mode, showing some promise for future predictability of cold and warm regimes on the Agulhas Bank. Thus, while the Agulhas Current has been shown in previous work to have a large impact on intra‐annual and interannual temperature variability, this work shows that multidecadal variability in temperature on the shelf is likely to be wind forced.

Large solitary meanders are arguably the dominant mode of variability in the Agulhas Current. Observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year. Here 10-year time series from two ocean general circulation models are used to create a composite picture of the Agulhas Current and its interactions with the shelf circulation in meandering and nonmeandering modes. Both models show good agreement with the size, propagation speed, and frequency of observed meanders. These composite meanders are then used to examine the response of shelf waters to the onset of large meanders, with the use of model output enabling the dynamics at depth to be explored. Results show a composite mean warming of up to 3°C of depth-averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders. However, this intensification of flow results in cooling of bottom waters, driving cold events at the shelf break of

International audience; South Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co-located Argo floats. A subsurface-eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward-propagating, long-lived features (>3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor'. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150-200 m depth along the 25.4-25.8 kg m-3 potential density layer with anticyclonic subSIDDIES having their cores at 250-300 m along the 26-26.4 kg m-3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy-types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy-flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high-saline water eddy-conveyor toward the western Indian Ocean.