Upwelling and eddy activity in the Southern Queensland Coastal Marine Zone, Australia

Upwelling and eddy activity in the Southern Queensland Coastal Marine Zone, Australia

Coastal upwelling systems are characterised by high primary productivity caused by the supply of nutrients into the surface layer of the ocean. These areas are key locations in global fish production despite their smaller size when compared with larger oceans. Another coastal mechanism that enhances primary productivity is eddy activity. It has been shown that eddies have three key features in the formation
of the favourable reproductive habitat: enrichment, larval concentration and retention. Additionally, near-coastal eddies enhance primary production upon the neighbouring shelf as well as being mechanisms for the off shore transport. In this thesis, the results from an analysis of upwelling and eddy activity in the Southern Queensland Coastal Marine Zone off eastern Australia are presented. Firstly, the analysis includes the quantification and characterisation of upwelling events using chlorophyll-a data as a proxy. Secondly, a census of near-coastal eddies from a study of sea surface height data, along with the quantification of the potential impact of eddies over the shelf is presented. Finally, the analysis includes an examination of the role of upwelling events and eddies in the general circulation upon the shelf, through the examination of numerical representations.

The analysis of chlorophyll-a data results in the identification of two characteristic patterns. One pattern is associated with upwelling events driven primarily by bottom stress, as well as a less significant wind stress. This pattern is evident during the September to March period each year. From the days related to this particular chlorophyll-a bloom formation, approximately 87% and 60% of these days show upwelling conditions favoured by bottom stress and wind stress, respectively. The periodicity of hese events leads to the identification of the Southeast Fraser Island Upwelling System. The second pattern was associated with upwelling events displaying an offshore transport not associated with wind stress, prompting the examination of the eddy activity in this region. It is suggested that the latter pattern is the result of a secondary upwelling system. It is proposed that this upwelling system is referred to as the Sunshine Coast Upwelling System.

The analysis of the sea surface height data using an eddy detection and tracking tool, results in the identification of three coastal regions. Each region is characterised by ts latitudinal distribution of eddies, as well by the main eddy features. The Southern Queensland Coastal Marine Zone is strongly dominated by cyclonic eddies of short (7-24 days) and long (>24 days) life spans, with approximately 31% and 29% of the total number of eddies, respectively. It is shown by quantifying an event in 2012, that near coastal eddies have the potential to extract offshore a high percent of the shelf water. The 2012 eddy extracted approximately 64% of the shelf water, which was replaced by slope water, resulting in a chlorophyll-a bloom. It is suggested, that the eddy activity has a significant role in the generation of chlorophyll-a blooms that are not associated with the Southeast Fraser Island Upwelling System.

Further insight into the ocean circulation of the Southern Queensland Coastal Marine Zone is gained from the analysis of ocean reanalysis data as well as through the implementation and application of a regional ocean model. The examination of ocean reanalysis data describes the role of eddies and upwelling events in the general circulation upon the shelf. The East Australian Current and wind drive most of the coastal circulation. The high variability of ottom stress resulting from the current, enhances the inshore bottom layer transport. The implementation of a numerical model identified high vorticity associated with sub-mesoscale features as contributing to the existence of the Southeast Fraser Island Upwelling System. The numerical results highlight the key role of the Breaksea Spit shallow waters in defining the source of upwelling water off Fraser Island. The Ekman transport by wind stress is significant in the Southeast Fraser Island Upwelling System (Spring and Summer). The northward transport by wind stress (Winter) could be significant in the generation of mesoscale eddies, potentially creating upwelling events by entraining shelf water. Simultaneously, the high concentration of cyclonic ear-coastal eddies enhances a northward inner shelf current, supporting the reports of a northward transport of sand, ending at the north of Fraser Island.

In summary, a key finding of this research is the identification of the Southeast Fraser Upwelling System. The examination of the Sunshine Coast Upwelling System leads to the characterisation of the Southern Queensland Coastal Marine Zone as being strongly dominated by cyclonic eddies. The analysis of numerical representations suggests that sub-mesoscale eddies have a significant influence on arine dynamics along the shelf break. It is suggested that the findings of this research should guide the future design and implementation of oceanographic surveys examining the Southern Queensland Coastal Marine Zone. The irst objective of these surveys would be to observe upwelling events driving the Southeast Fraser Island Upwelling System as well as surveying the sub-mesoscale activity in this region. Improved knowledge of this region’s physical oceanography would be extremely beneficial in the management of sustainable fishery and would result in clear economic benefits.