The cross-shelf exchange in the East Australian current region

Cross-shelf exchange is the transport of volume or property (e.g., heat, salt, nutrients) between the open ocean and the continental shelf. The transport of nutrient-rich waters from the deep ocean onto the shelf is vital for the growth of the phytoplankton and hence for the maintenance of the shelf ecosystem. The east coast of Australia is a region of a complex current system, consisting of the East Australian Current (EAC), the western boundary of the South Pacific, and the strongly eddying eastward Tasman Front and southward EAC extension. While significant progress has generally been made in understanding of the cross-shelf exchange in the global ocean, including the role of winds, currents and eddies, our understanding of the cross-shelf exchange off the east coast of Australia remains incomplete. Previous studies of the cross-shelf volume exchange in the EAC region are based on sparse observations and regional models. Currently, the cross-shelf exchange of properties, its drivers, and variability in the EAC region are poorly understood. This thesis aims to address this research gap and investigates the circulation and cross-shelf exchange in the EAC system using a suite of high-resolution regional ocean models with passive tracers.

In the first result chapter (Chapter 2), we develop a high-resolution regional ocean model simulating the dynamics of the EAC system. We validate the model against available sea surface height anomaly (SSHA) and current meter data to assess the model’s skill to simulate the mean current and eddy dynamics of the region. To explore the role of and sensitivity to small-scale ocean dynamics, we conduct two model simulations with horizonal resolution of 1/10° (10 km) and 1/25° (4 km), resolving meso- and submesoscale ocean dynamics, respectively. The model outputs are analysed through a range of diagnostics, including eddy tracking, to quantify the evolution and impacts of the EAC anticyclones on the circulation and temperature distribution in the Tasman Sea. The results show that the 1/25°-resolution simulation has a large area of cooling in the Tasman Front and the EAC extension regions compared with the 1/10°-resolution simulation. A change in the eddy-induced transports and eddy characteristics between the two models suggests that this cooling signal is caused by a reduced meridional heat transport due to a shorter lifetime and smaller southward meridional displacement of the EAC anticyclones in the 1/25°-resolution simulation. We hypothesize that the changes in the eddy characteristics in the 1/25°-resolution simulation are explained by a more efficient eddy energy exchange within the eddy field and eddy energy transfer to smaller scale motions, leading to enhanced dissipation of the EAC anticyclones in the high-resolution simulation. Our study suggests that 1/10°-resolution global ocean models might not represent the eddy dynamics sufficiently well, and hence can overestimate the poleward eddy heat transport in the western boundary current extension regions.

In the second result chapter (Chapter 3), we quantify the cross-shelf exchange in the EAC system using the high-resolution model with a passive tracer simulating the cross-shelf exchange of nutrients from the deep ocean. We focus on the spatial and temporal characteristics of the cross-shelf exchange as well as investigating the tracer sources and pathways for the entire shelf. Consistent with the EAC regimes, we analyse the cross-shelf tracer flux in three subregions: the EAC jet (26-30°S), EAC separation (30-33°S), and EAC extension (33-37.5°S) regions. The results show that the cross-shelf tracer flux is generally weak and highly localized in the EAC jet region, while it is strong and relatively uniform in the EAC separation and extension regions. Experiments with different tracer sources indicate that the deep ocean in the EAC jet region is the main source for the tracer crossing onto the shelf over the entire Australian shelf, including in the separation and extension regions. In the EAC jet region, local upwelling is the main pathway for the tracer onto the shelf, while both southward advection by the EAC and eddies and upwelling play a role in delivering the tracer onto the shelf in the EAC separation and extension regions. The cross-shelf tracer flux sensitivity to the model resolution and tracer consumption rate over the shelf are also discussed.

Finally, in the third result chapter (Chapter 4), we explore various diagnostics used to quantify the cross-shelf exchange. We compare the depth-integrated cross-shelf velocity and tracer flux and find a major discrepancy between the two caused by non-uniform spatial and vertical tracer distribution. This highlights the importance of including tracer information in the estimation of the cross-shelf exchange of properties. We also investigate the drivers of the tracer availability and variability at the shelf boundary. In the EAC jet region, which is the source of the tracer for the entire shelf, the local drivers of the tracer upwelling modulate the tracer concentration over the entire shelf. Specifically, we show that there are two dominant signals of the tracer variability: the annual variability and high-frequency variability on a time scale of 2-3 months. For the annual signal, both the EAC strength and extension of the jet towards the bottom boundary, and wind stress may contribute to the annual variability. In turn, the high-frequency variability, characterised by the southward propagation along the shelf, could be explained by the upwelling induced by the cyclones, which can form locally via the EAC-shelf interaction, or propagate towards the shelf from the interior of the Pacific Ocean. Finally, given that the tracer concentration and its spatial and vertical distribution at the shelf boundary are not readily observed in the ocean, we explore a relationship between the tracer concentration and the ocean circulation and SSHA. The results show that the tracer concentration near the shelf boundary correlates well with SSHA, and hence can potentially be used as an observable proxy for cross-shelf exchange of properties.

Overall, this thesis systematically quantifies the tracer cross-shelf exchange over the entire shelf off the east coast of Australia and improves our understanding of the distribution, sources and pathways of the tracer crossing onto the shelf. We demonstrate the interconnectedness of the EAC system, the dependence of the tracer cross-shelf exchange in the EAC separation and extension regions on the tracer source and ocean dynamics upstream in the EAC jet region. Our results also highlight the importance of having the tracer information for estimating the cross-shelf transport of properties and illustrate the potential of SSHA as a proxy for the cross-shelf tracer exchange. All of these provides a basic framework for describing the magnitude and mechanisms of the cross-shelf exchange in the EAC region and assessing how they may change in the future.