Technical Information


Coral bleaching risk in ReefTemp is estimated by calculating thermal stress indices based on a comparison of daily high-resolution SST data to a long-term climatology.

DailyUpdate     Climatology     Stress Indices

Click here to download a technical report on the development of the ReefTemp product.

Daily Update

The Bureau of Meteorology provides daily updates for the Queensland region of the Australian Mercator Projection SST Mosaic and an age-of-data grid file. The mercator projection is a 15-day composite of SST from NOAA orbits (currently from NOAA 15 and 17) at a resolution of 0.017995° (~2 km) (see:

In cases where SST cannot be calculated due to cloud cover, the most recent temperature calculated for that grid is inserted, unless over 10 days old. Data over 10 days old are not used to estimate bleaching risk. In the great majority of cases, temperatures are from between 1 and 4 days before the provision of the update.

Dense cloud cover is not usually conducive to the further warming of the waters. Therefore, backfilling the data allows for continuity in the estimation of bleaching risk and is conservative because, if temperatures have decreased, the result is a slight overestimate of bleaching risk.


Climatology Dataset

In 2003, the National Oceans Office contracted CSIRO Marine Research to provide a dataset of SST for the Australasian region to cover 1993-2003 (see: CSIRO Ocean Currents Remote Sensing). The dataset was derived from the analysis of NOAA environmental satellite AVHRR thermal imagery and extends from 10N to 65S and 80E to 170W, including composite images formed over 3 day intervals (see: CSIRO Ten Years of SST). The composite images were mapped onto an equal angle grid of 0.042 degrees of latitude (4 km) and 0.036 degrees of longitude (2-4.5 km). The data have been calibrated against ships and drift buoys. The representative SST for each grid cell is determined by taking the SST value at the 65 th percentile of the cumulative frequency distribution within each group of data. The dataset starts on 6 October 1993 and ends at 13 June 2003. This is the best data available for this project due to the capturing of short-term variation, resolution, and the quality control measures on both the SST estimates and image shifting.

Climatology Calculation

Average monthly temperatures were calculated from the 3-day composite for the entire year. The climatology grids were then resampled to the resolution and extent of the daily update grid by taking the weighted average (~20% were not impacted at all by the resampling). Climatologies are now automatically updated by recalculating the long-term average temperature at the end of each month.

To see images of the climatologies for each of the southern summer months, click on the links below.


Stress Indices

Relating temperature characteristics to measures of bleaching response severity on the Great Barrier Reef has revealed that multiple heat stress indices allows for a better estimate of bleaching risk than any single one. This evidence supports the use, in ReefTemp, of more than two stress indices. The indices used have provided consistent and effective estimates of bleaching risk at reefs within the Great Barrier Reef Marine Park during recent bleaching events.

SST: The current SST data being used to calculate the stress indices. The temperature range visualized is from 25 to 35°C.

SST anomaly (+SST): Calculated for each grid cell as the number of °C above the long-term average temperature observed for that month. The temperature anomalies visualized range from +0.1°C to +5°C.

Degree-Heating Days (DHDs): Degree-Heating Days are a measure of the accumulation of heat stress. One DHD is calculated as one degree above the local long-term average temperature for one day. The Degree-Heating Days index within ReefTemp displays the number of DHDs accumulated within the last 120 days, starting December 1st.

Heating Rate: Degree-Heating Days can represent a broad range of heat stress in that 3 weeks at 1°C above the local long-term average results in the same number of DHDs as 1 week at 3°C, the latter representing more severe stress to corals. For this reason, ReefTemp also displays the Heating Rate, calculated as the number of Degree Heating Days divided by the number of days in which temperatures have exceeded the long-term average. This index is initiated after temperatures have been above the long-term average for a minimum of five days.

The value of each index that related to minor, moderate, and severe bleaching responses during past events are shown within ReefTemp as dark to light blue, yellow, and orange to red respectively.