Upwelling dynamics in Southern Australia : numerical modelling and observations

The ocean circulation along the Eastern Great Australian Bight (EGAB) is investigated using models and observations, with a focus on Australia’s strongest upwelling region — the Bonney Coast (BC). A detailed analysis of the shelf circulation in this region has never been fully undertaken. The modelling studies examine how variations in shelf topography and Coastal Trapped Waves (CTWs) influence upwelling. The observations describe one of the most extreme upwelling seasons on record (Austral summer, 2016) with unprecedented coverage. The context for the intense observing program is provided by a detailed analysis of large-scale and local factors, quantifying their impacts. The model used is a regional configuration that includes idealised forcing and initial conditions, and realistic topography and coastal geometry. A key finding from the modelling studies is the demonstration of the influence of alongshore variations in shelf topography - submerged headlands and valleys - on the upwelling circulation. This was the first demonstration of this phenomenon in the BC region. Upwelling is shown to be qualitatively consistent with vorticity dynamics. We show that alongshore baroclinic pressure gradients force a geostrophic onshore flow on the equatorward side of the submerged headlands. A second study also highlights the role of CTWs that act to shut-down the interior upwelling, with the strongest influence nearer the CTW source. The circulation driven by a periodic CTW is also studied as a proxy for wind-driven motions in the western Bight. The results show the mean characteristics of the flow and its adjustment to variable shelf-width and topography, with equatorward (poleward) alongshore velocities of the CTW driving intensified bottom upwelling (downwelling). As the remote forcing frequencies reduce (longer time-scales), local topography rectification and resonance with local winds clearly enhance upwelling in the main EGAB upwelling regions. The observational coverage of the EGAB during Austral summer 2016 was unprecedented, with observations from gliders, moorings, coastal radar, tide gauges, and satellite measurements. The analysis exploited climate indices and reanalysis products to understand the broad scale context of the regional observations. Several factors made the EGAB pre-conditioned to extreme upwelling in 2016. Through the coastal wave guide, the 2015/16 El-Niño uplifted the isotherms over the slope, the South Australian Current was weaker than normal, and the Southern Annular Mode acted to enhance the upwelling favourable winds. Together, these factors meant that the shelf waters responded quickly and efficiently to strong wind-driven events in early 2016. The anomalies associated with this extreme upwelling season included surface temperatures exceeding 2C below average, winds in excess of 0.1Pa, and high Chlorophyll-A. Glider observations show that the upwelled waters were highly skewed (cold and fresh), with origins deeper than 350m (with Flinders Current origin), and with alongshore variations consistent with those implied by the modelling studies. The analysis of climate indices showed significant lagged relationships between large-scale conditions and properties of the shelf circulation in the EGAB. We speculate that these lagged relationships could be used as a predictive tool, to identify when the EGAB is more prone to upwelling. Arguably, the results presented in this thesis represent the most comprehensive description of the shelf circulation in the EGAB. We show, for the first time, the importance of alongshore variations in topography, the impacts of CTWs, and the role of remote forcing on the shelf circulation. This research addresses some important knowledge-gaps of the oceanography of this region, and are particularly important for the management of fisheries and prediction of the EGAB upwelling system as a whole