Coral reefs are among the most vulnerable ecosystems to climate change, showing strong responses to both ocean warming and acidification as a result of increasing atmospheric greenhouse gas concentrations. The Great Barrier Reef (GBR) is one of Australia's greatest environmental and economic assets, internationally recognized by its listing as a World Heritage Area. While Australia has also established one of the world's largest marine parks to protect the GBR, there are still a number of threats from local and global sources that require effective and timely management responses. Understanding the underlying oceanography of the GBR is central to understanding both the threats and the solutions that managers and policy makers face.
Surface atmospheric and oceanic conditions interact with one other and such interactions have the capacity to control the behaviour of the atmosphere and the ocean, resulting in different climate patterns and variability. The behaviour of the atmosphere and the ocean can be simulated with increased accuracy if such interactions are properly understood. New analytical tools, improved data quality and resolution, longer time-series and new variables provide an opportunity to re-assess existing paradigms. This study focuses on the surface oceanic and atmospheric characteristics of the GBR relating the local circulation to conditions in the Pacific basin, and examines the implications of changes in the physical environment for the GBR ecosystem by exploring the links with mass bleaching events.
The goals of this dissertation are to investigate the dynamics of the key environmental variables impacting the GBR, to characterize the South Equatorial Current (SEC) bifurcation variability and the impacts on the GBR circulation, and to identify how oceanographic patterns and variability are linked to mass coral bleaching events. Emphasis is placed on the influence of El Niño Southern Oscillation (ENSO) events.
A total of nine different parameters derived from different datasets are analysed. The parameters included in this study include, but are not limited to, sea level pressure, sea surface temperature, surface winds and surface oceanic currents. Examination of the relationship between GBR surface climate and the wider tropical Pacific shows that although ENSO events are not the primary driver of inter-annual climate variability on the GBR, their influence is conspicuous. Classical ENSO events have a strong signature in the atmospheric circulation in the northern GBR but no significant relationship with SSTs and the opposite applies for the southern GBR. For the first time, the impact of the El Niño/La Niña Modoki phenomenon on the GBR has been investigated showing that La Niña Modoki is significantly related to summer SSTs on the northern GBR.
An analysis of the SEC, its bifurcation along the GBR and the related strength of its bifurcated arms, the East Australian Current (EAC) and the North Queensland Current (NQC) shows that, on interannual timescales, the meridional migration of the SEC bifurcation and the strength of the NQC are mainly determined by local surface wind stress, the signals of which are not fully represented by ENSO indices. In contrast, the EAC is found to be highly correlated with the strength of the SEC and ENSO, with a lag time of about 6 months from the peak of an ENSO event.
A retrospective analysis of the past documented mass bleaching events that have occurred on the GBR shows that some ocean-atmosphere features are common to bleaching events (e.g. low wind stress, enhanced EAC flow), and the key elements that play an important role in sea surface temperatures (SST) anomaly patterns on the GBR are differences between ENSO events, the Hadley circulation, the influence of the summer monsoon and tropical cyclones, and spatio-temporal variation in the flow of the EAC. Each of these factors are interlinked with each other and it is therefore not yet possible to project SST patterns on a long-term basis due to the amount of variation associated with these features. Current SST forecast of the GBR conditions, need to be improved and the results of this study shows that the use of general circulation models produces more accurate forecasts that solely based on temperatures patterns.
Throughout this study, environmental data has been collated from different sources for four consecutive summers (2008 to 2012), and distributed to a large number of managers, scientists and stakeholders in the form of environmental reports. The results and methodology applied show that scientists, managers and stakeholders will benefit from similar initiatives to the reports presented.
The results and analyses associated with this thesis illustrate the highly complex nature of the GBR ecosystem and make an important contribution to furthering the understanding of the uncertainties of future climate change impacts. Given the central role that basin-scale oceanic and atmospheric interactions play on the GBR, understanding the complexities of these interactions is vital to managers and stakeholders as well as the scientific community, especially given the importance of the GBR Marine Park.