Oceanographic drivers of bleaching in the GBR: from observations to prediction

Craig Steinberg

Led by: Craig Steinberg, AIMS


Project Summary

This project seeks to understand how local, regional and global oceanographic and meteorological processes influence the severity and spatial variability of thermally driven coral bleaching for the Great Barrier Reef (GBR) and Torres Strait.

All available relevant environmental observations of the recent bleaching events will be gathered to be more easily discoverable to researchers and managers via a gateway/summary webpage.

The data will be used to assess how well the eReefs models are modelling the recent warming during bleaching events and then analyse the reasons behind the variations of bleaching response. 3D versions of remotely sensed bleaching products by NOAA and BoM will be produced and a seasonal prediction capability for marine heatwaves will be developed.


Project Publications
Final Report – Volume 1
Final Report – Volume 2
Final Report – Volume 3
Final Report – Volume 4


Media Release
Journal Article
Outcomes Factsheet



Project Webinar – 1st December 2020


Project Description

Mass coral bleaching has occurred on the Great Barrier Reef (GBR) in 2016 and 2017 as part of a continuous global bleaching event that started in late 2014 (NOAA). The combined effect has meant that the majority of the reef has been severely affected.

This project seeks to understand how local, regional and global oceanographic and meteorological processes influence the severity and spatial variability of thermally driven coral bleaching. By doing this a better appreciation of which parts of the reef are more tolerant and therefore more likely to retain their health into the future can be used to better manage the GBR.


The research is structured in three main components:

1 – Summary of oceanographic conditions during the 2015-17 bleaching years.

We will provide a comprehensive report on all available oceanographic observations: IMOS (moorings, sensor networks, and gliders), AIMS temp loggers and GBRMPA MMP arrays, QLD DSITI temperatures from the waverider network, tide gauges, SST and altimetry (for circulation patterns) and collate other major observing programmes. A summary report and website will be created to consolidate access to the relevant oceanographic and meteorological observations. This may include repackaging some of the more complex data types into more readily used products.

An initial effort will seek to locate data sets held by the wide array of groups and agencies that observe the GBR, and bring these data together to provide a collection that can be used as the defining environmental observational record of the recent bleaching events. The data will be quality controlled and flagged as per the national oceanographic and meteorological (e.g. IMOS) standards. Where existing portals are deemed too complex for the majority of users, data will be made available in more readily consumed formats. Tutorials and workshops will be delivered to raise awareness and assist uptake by others by demonstrating existing tools and delivering new products. Workshops will be conducted in Townsville and Brisbane and on the fringe of conferences like Australian Coral Reef Society or Australian Marine Science Association and would include support from IMOS.

The summary data report will include a description of the different conditions experienced between the bleaching years – with some discussion of regional and local drivers. It will also contrast the conditions experienced in non-bleaching periods that preceded these most recent events. A marine heatwave analysis of satellite SST (ReefTemp Next Generation, NOAA Coral Reef Watch) of past major bleaching events will also reveal the thermal histories of reefs making up the GBR.

This element will deliver in the first year of the project and allow projects 4.3 and 4.5 to have a comprehensive high quality data set to provide context for their studies through characterisation of actual thermal stress experienced on the GBR.

2 – Hydrodynamics of bleaching and improved predictions

Regional bleaching patterns and predictions

This component seeks to analyse bleaching response to determine the key hydrodynamic drivers that affect thermal stratification of the water column. By doing so we expect to be able to identify reefs that are more or less prone to heat stress due to their individual characteristics and local oceanographic features resulting in micro-climates. Data provided by the first component of this project will be used to understand the thermal histories of different sections of the reef at scales ranging from regional to sub-reef where bleaching has been assessed from in situ and aerial observations. Through this analysis we will be able to better understand the trigger points for bleaching severity which will enable us to implement improved operational SST based bleaching algorithms such as ReefTemp Next Generation and NOAA’s Coral Reef Watch bleaching products that are currently relied upon by managers and researchers.

We will implement depth-resolved equivalents of the above satellite derived products, by incorporating the vertical temperature structure produced by 3-dimensional hydrodynamic models and from observed data such as the CSIRO Atlas of Regional Seas 2009 and the recent MARVL atlas of shelf seas and possibly a hydrodynamic model generated climatology. This will allow us to understand the depth to which warming occurs for each event.  This is a critical issue as if there is sufficient vertical mixing (due to tidal currents, waves) during bleaching weather periods then the surface warming and thermal stress can be ‘mixed down’ and potentially ameliorated. This approach will also assist in identifying regions where corals on the deeper reef slopes or lagoons may escape bleaching even though those nearer the surface will be subject to bleaching. These models will be validated by hindcasting against actual thermal histories identified by the first component of the project.

A major output from this project will be a bleaching hazard or susceptibility map of the GBR that will show regions that are more or less prone to bleaching and identify the key oceanographic processes that determine that response. We will combine SST and model climatologies with knowledge of topography, circulation, residence times, vertical mixing and intrusive mechanisms to identify the regions that are resistant to warming. The resulting product can then be used to inform reef identification (‘triage’) for preservation activities and identify gaps in the observing system that need filling.

Sub-reef scale thermal response:

This work will build on and extend recent work undertaken under NESP 3.3.1 ‘Quantifying the linkages between water quality and the thermal tolerance of GBR coral reefs’, by looking at groups and individual reefs to determine their propensity for bleaching and why some are unaffected after multiple events e.g. the eastern Torres Strait, northern GBR outer edge reefs and the Pompeys/Swains. From the earlier work we plan to identify reefs that display resilience to bleaching and look at the smaller scale hydrodynamic processes that produce persistent cooling.

3 – Improved seasonal predictions of marine heatwaves in the GBR

Bleaching events coincide with ‘marine heatwaves’ (MHW), which are defined by extreme temperatures exceeding a threshold in intensity and duration. The limited forecast skill of these marine heatwaves, in terms of intensity and spatial variability has made it challenging to pre-emptively design an adequate bleaching observational and response program. One emerging capability for improving our understanding of upcoming summer temperatures is through seasonal predictions of MHW. If such an early warning tool were available, complementing existing seasonal SST-based thermal stress forecast products, then a more proactive and targeted response could be made. This will allow better targeting of response such as where to perform surveys and rehabilitation.

While multi-week to seasonal projections for atmospheric heatwaves are currently available (e.g. http://www.bom.gov.au/australia/heatwave/), an analogous seasonal prediction for marine heatwaves has not been developed yet. Seasonal and multi-week forecasts for ocean temperatures are produced operationally by the Bureau of Meteorology, using the POAMA-2 seasonal coupled ocean-atmosphere prediction system at relatively coarse 250 km resolution. This system is currently being updated to ACCESS-S, with a 25 km ocean grid resolution, providing improved resolution particularly in coastal areas. In order to facilitate planning for marine heatwaves events, a recently established framework for marine heatwaves could be applied to the ACCESS-S forecast data to highlight the areas likely to experience the most extreme thermal stresses in the upcoming summer.

We propose to work with BoM to provide a tailored research product for the GBR utilising their next generation seasonal prediction model ACCESS-S. This will occur in years 2 and 3 of the project.

The proposal is aligned with two new NESP TWQ Hub RPv4 proposals led by Bay (4.4) and Mumby (4.5). The purposeful collaboration among these three new proposals facilitates the comparison of corals on reefs that experienced strong warming to those that did not. Host genetic and symbiont traits (from Bay 4.4) for which strong environmental drivers are identified (delivered by this project) will be spatially extrapolated and fed into Mumby’s (project 4.5) resilience model (Figure 1). In addition the eReefs modelling of thermal and mixing hazard maps will inform the level of stress input to the resilience model

This project seeks to improve the utility of the numerous observing platforms by improving accessibility and analysis of the data and introduce new tools through eReefs modelling to understand the thermal response of the GBR to MHWs.

AIMS leads a number of environmental monitoring programmes such as Queensland’s Integrated Marine Observing System (Q-IMOS) and has a number of active research tasks on analysing the recent bleaching events.

eReefs is a major programme with CSIRO, BoM and AIMS as key developmental partners. This project seeks to build on these efforts.

By providing a reference observational data set that is quality controlled, more easily accessible to researchers and managers and hydrodynamic model products there will be better informed analyses and better targeted management actions.

By collating all the available data an understanding of what observational gaps remain for the GBR and Torres Strait. RIMRep will be informed through participation on the physico-chemical expert working group. Interactions with Beeden (GBRMPA), Schaffelke, Brinkman and Souter (AIMS) will ensure alignment with RIMRep objectives.

The analysis seeks to identify the relative risk of bleaching along the GBR and Torres Strait so that potential refugia can be identified. These refugia will have local oceanographic phenomena that provide persistent cooling mechanisms. This can help inform risk assessment such as the Reef Havens initiative and other reef management activities, including emerging intervention proposals in areas beyond just coral bleaching.

This project does not simply map regions of heat stress. It will provide an understanding of why different regions warm differently and so allow targeting of resources in areas that may be on the fringe of susceptibility that are more able to be respond to remediation and thereby increase the footprint of healthier coral reefs.

Engagement with the Great Barrier Reef Blueprint for Resilience will be sought. The outputs of this project will be underpin models of resilience by Project 4.


NESP 2017 Research Priority Alignment

NESP Priority Theme 2 – Maximise the resilience of vulnerable species to the impacts of climate change and climate variability by reducing other pressures, including poor water quality.

Improve our understanding of the consequences of climate change for the health and resilience of vulnerable freshwater, coastal and marine species, and ecosystems (2.1).


Project Keywords

Coral bleaching; Currents; Temperature; Hydrodynamics; Marine heatwave.


Project Funding

This project is jointly funded through AIMS, CSIRO and the Australian Government’s National Environmental Science Program.