Great Barrier Reef - Coral Bleaching Stress indicator - Degree Heating Week (GBR1) - Experimental

This product shows estimates of Degree Heating Weeks (DHW) for the Great Barrier Reef derived from the eReefs Hydrodynamic model, based on two different reference temperature climatologies. Higher values of DHW indicates areas where coral are more likely to be suffering from thermal heat stress that can lead to coral bleaching.

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Warning: These products are experimental and are under active development and are suspected of having some scaling issues. As a result they are not directly comparable to the Coral Watch NOAA DHW products. Early investigations indicate that the DHW #1 NOAA 4 am snapshot product values are too high and that it falls off too fast with depth, and the DHW #3 SSTAARS 4am snapshot product values are too high by 60 - 70%. Even though these products are under investigation we have decided to leave them online to allow researchers to collaborate better to improve these products.

Coral reefs are under increasing pressure from marine heatwaves as a result of climate change. When water temperature is above the average maximum summer temperature for extended periods of time corals can become thermally stressed, leading to coral bleaching and eventually loss of coral.

The Degree Heating Week (DHW) indicator is an estimate of thermal stress of corals and a strong predictor of coral bleaching. DHW is a widely used indicator and can be estimated from long term temperature logging and remote sensed SST data.

The DHW indicator is calculated by accumulating temperature readings that are more than one degree Celcius over the historical maximum temperature for a given location. Thus if the temperature is 2°C above the summer maximum for 4 weeks then the corresponding DHW indicator is (2°C x 4 weeks) 8 DHW. Over time the thermal stress is accumulated over a 12 week sliding window.

The most widely used and longest operating DHW product is the NOAA coral reef watch product. The NOAA coral reef watch program estimates the DHW from Sea Surface Temperature (SST) satellite readings taken at night. A key challenge with measuring water temperatures from satellites is that clouds obscure the satellite's view, potentially resulting in large holes in the daily data. During cloudy summer wet seasons this can result in extended periods of time, up to several weeks, where there a no new temperature readings resulting in problems in the estimates of the corresponding DHW product.

The key advantages of the eReefs DHW product are the hydrodynamic model provides estimates for temperature even during cloudy conditions and it provides estimates of DHW at multiple depths, which is not possible with remotely sensed DHW products.

A key component of estimating the DHW indicator is determining the spatial historical average maximum summer temperature. The eReefs 1 km hydrodynamic model has not been operating long enough to create a temperature climatology. To resolve this two versions of the DHW products were developed, each based on a different reference historical climatology. The DHW #1 NOAA 4 am snapshot product is based on the same climatology as the NOAA coral reef watch DHW products. The DHW #3 SSTAARS 4 am snapshot uses the SST Atlas of Australian Regional Seas (SSTAARS) climatology, which is a higher resolution regional climatology.

The NOAA and SSTAARS climatologies were transformed into 3D versions (estimates of climatology with depth) by using the available historical eReefs data. The eReefs data was used to estimate a change in temperature with depth, effectively capturing the average level of stratification in the temperature. More details on this can be found in the project report (Volume 2: 3D Bleaching in the GBR: Development and analysis of a 3D climatology and heat accumulation products using eReefs).

Two versions of the DHW products, based on different climatologies and time integration approaches, are provided at this stage because there is insufficient coral bleaching data to determine which of these products provides a better estimate of coral bleaching patterns. It is, however, expected that the use of the higher resolution SSTAARS climatology, along with the DHW integration over the full day (rather than just at night) will produce better estimates of coral bleaching.

Note: The monthly visualisations of DHW show the maximum DHW that occurred during that one month period. This product is updated daily during the month.

Note: The eReefs model appears to show warmer conditions close inshore along the coast than satellite temperature products. This is likely due to a small bias in the model. As a result this leads to higher than expected DHW in those areas.

DHW #1 NOAA 4 am snapshot

This version of the DHW indicator uses the NOAA coral reef watch climatology for estimating the histrical summer maximum temperature and uses a single 4 am daily temperature reading estimate the DHWs. This product was design to closely match the approach taken with the NOAA coral reef watch DHW product. As such the scale of the DHW estimate from this project should be close to the remote sensed NOAA coral reef watch DHW product. These benefit of this product is that it is unaffected by cloud cover.

At depth this product shows a faster roll off of heating than the DHW #3 SSTAARS product. The research team are investigating the cause of this.

DHW #3 SSTAARS 4am snapshot

The DHW is calculated by comparing the eReefs temperature data against the SST Atlas of Australian Regional Seas (SSTAARS) daily temperature climatology. The DHW calculation is performed daily using the temperature at 4 am, so as to closely align with the NOAA approach. This is the product that the research team have the most confidence in. In 2016 and 2020 this product on the surface aligned well with the DHW values generated by the NOAA products. In 2017 and 2022 it showed higher values. The cause of these differences is under investigation.

Temperature

The temperature panel shows the daily average temperature from the GBR 1km hydrodynamic model. This variable can be used to quickly determine the timing of heat events.

Product development

This product was developed as part of the NESP TWQ 4.2 Oceanographic drivers of bleaching in the GBR: from observations to prediction project.