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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.

Funding information: This study was funded by Australian Research Council Linkage Grants (LP110200603 awarded to RH, DS and Iain Field, and LP160100162 awarded to IJ, Martina Doublin, MC, GC, DS, Iain Suthers and RH) with contributions from the Taronga Conservation Society Australia, NSW National Parks and the Australian Antarctic Division. Coastal pelagic ecosystems are highly variable in space and time, with environmental conditions and the distribution of biomass being driven by complex processes operating at multiple scales. The emergent properties of these processes and their interactive effects result in complex and dynamic environmental mosaics referred to as “seascapes”. Mechanisms that link large-scale oceanographic processes and ecological variability in coastal environments remain poorly understood, despite their importance for predicting how ecosystems will respond to climate change. Here we assessed seascape variability along the path of the rapidly intensifying East Australian Current (EAC) Southern Extension in southeast Australia, a hotspot of ocean warming and ecosystem tropicalisation. Using satellite and in situ measures of temperature, salinity and current velocity coupled with contemporaneous measurements of pelagic biomass distribution from nine boat-based active acoustic surveys in five consecutive years, we investigated relationships between the physical environment and the distribution of pelagic biomass (zooplankton and fish) at multiple timescales. Survey periods were characterised by high variability in oceanographic conditions, with variation in coastal conditions influenced by meso-to-large scale processes occurring offshore, including the position and strength of eddies. Intra-annual variability was often of a similar or greater magnitude to inter-annual variability, suggesting highly dynamic conditions with important variation occurring at scales of days to weeks. Two seascape categories were identified being characterised by (A) warmer, less saline water and (B) cooler, more saline water, with the former indicating greater influence of the EAC on coastal processes. Warmer waters were also associated with fewer, deeper and less dense biological aggregations. As the EAC continues to warm and penetrate further south, it is likely that this will have substantial effects on biological activity in coastal pelagic ecosystems, including a potential reduction in the accessibility of prey aggregations to surface-feeding predators and to fisheries. These results highlight the import role of offshore oceanographic processes in driving coastal seascape variability and biological activity in a region undergoing rapid oceanic warming and ecological change. Publisher PDF

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.

[1] Tropical cyclone heat potential (TCHP) has been proposed as being important for hurricane and typhoon intensity. Here, a climatology of TCHP is developed for the Southwest Indian Ocean, a basin that experiences on average 11–12 tropical cyclones per year, many of which impact on Mauritius, Reunion and Madagascar, and Mozambique. SODA data and a regional ocean model forced with the GFDL-CORE v.2b reanalysis winds and heat fluxes are used to derive TCHP values during the 1948–2007 period. The results indicate that TCHP increases through the austral summer, peaking in March. Values of TCHP above 40 kJ cm−2, suggested as the minimum needed for tropical cyclone intensification, are still present in the northern Mozambique Channel in May. A time series of TCHP spatially averaged over the Seychelles-Chagos thermocline ridge (SCTR), an important area for tropical cyclones, is presented. The model time series, which agrees well with XBT-based observations (r = 0.82, p = 0.01), shows considerable interannual variability overlaying an upward tendency that matches with an observed increase in severe tropical cyclone days in the Southwest Indian Ocean. Although an increase in severe storms is seen during 1997–2007, the increasing TCHP tendency time series after 1997 coincides with a decrease in total cyclone numbers, a mismatch that is ascribed to increased atmospheric anticyclonicity over the basin. Seasons of increased (decreased) TCHP over the SCTR appear to be associated with dry (wet) conditions over certain areas of southern and East Africa and are linked with changes in zonal wind and vertical motion in the midtroposphere.