Forecasting coral bleaching events in Western Australia

During the summer 2010/11, the Western Australia coastline experienced a significant warming event, with sea-surface temperature (SSTs) and in situ temperature logger measurements showing that temperatures were > 3°C above the normal summer averages in some regions – with coral bleaching recorded for the first time on mainland Western Australian reefs during the summer 2010/11.

Coral bleaching likelihood is largely determined by sea temperatures, whereby coral bleaching events are usually caused by long periods (usually 4 to 8 weeks) of warmer than average summer SSTs.  Given this knowledge, a number of online SST products are available that provide a seasonal outlook of the likelihood bleaching will occur, and tools that enable near-real time monitoring of temperature stress during the summer months – a summary of which are provided below.

The Predictive Oceanic and Atmospheric Model for Australia (POAMA)

POAMA SST forecasts are produced for up to six months into the future, with significant skill in predictions up to three months ahead.  SST anomalies (SST+) are calculated as the difference between SST values and the monthly long term mean SST.  As such, POAMA provides a seasonal outlook in the months leading up to summer, and based on research from the Great Barrier Reef, probabilistic forecasts of the likelihood of SST+ values exceeding 0.6 degree Celsius result in potential coral bleaching conditions.  Only the Google Earth experimental product is available for Western Australia.

NOAA's Coral Reef Watch Program (CRW)

NOAA's satellites measure SST in near-real-time (~50 km resolution), and CRW uses this information to pinpoint areas around the world where corals are at risk for bleaching.  CRW provides near-real time reef temperature conditions to quickly identify areas at risk for coral bleaching.  NOAA has up to 14 virtual stations along the Western Australia coastline.

CRW’s degree heating weeks (DHW) is an accumulation of any temperature anomalies greater than 1°C over a 12-week window, which shows how stressful conditions have been for corals in the last three months.  DHW is therefore a cumulative measurement of the intensity and duration of thermal stress, and is expressed in the unit °C-weeks.  Based on previous bleaching observations worldwide, evidence suggest that 4°C-weeks results in bleaching and that 8°C-weeks and above result in widespread bleaching.  For example, Scott Reef in Western Australia and the Maldives in the central Indian Ocean suffered significant bleaching mortality in 1998, with 13.3°C-weeks and 10.4°C-weeks respectively.

ReefTemp

ReefTemp provides a higher resolution (~1 km) near-real time system for monitoring temperature stress and subsequent bleaching risk.  Images are available through a Google Earth layer and direct view (to access full Western Australia coastline it is necessary to view the Northern Coast, Western Coast, and Great Australian Bight).

ReefTemp’s SST anomaly (SST+) is calculated for each grid cell as the number of °C above the long-term average temperature observed for that month.

ReefTemp’s degree heating days (DHD) is akin to CRW’s DHW in that it is a measure of the accumulation of heat stress.  One DHD is calculated as 1°C above the local long-term average temperature for one day.  The DHD index within ReefTemp is activated on 1 December and displays the number of DHDs accumulated within the period 1 December to 30 April.  Based on research from the Great Barrier Reef (Maynard et al. 2008):

40 – 65 DHD = mild bleaching (≤25 % coral colonies affected by bleaching)

65 – 100 DHD = moderate bleaching (26 – 50 % coral colonies affected by bleaching)

>100 DHD = severe bleaching (>50 % coral colonies affected by bleaching)

These values have not been tested for Western Australia as yet and therefore should only be used a rough guide until regional specific information becomes available.  The DHD product has a limitation in that it only runs from 1 December to 30 April and therefore potentially underestimates heat accumulation if SST anomalies begin before December (as seen in the summer 2010/11 in Western Australia), or stops registering heat accumulation if the SST anomalies continue into May and June (as seen in the summer 2011/12 in Western Australia).

ReefTemp is being discontinued by CSIRO and migrated across to the Bureau of Meteorology (BoM).  Through eReefs funding, BoM have made significant improvements to the ReefTemp system and the new improved version will be available in December 2012.  Unfortunately, the funding was for refining for the Great Barrier Reef only and the new ReefTemp version will not be available for other parts of Australia.  As such, it is likely that ReefTemp will no longer be available for Western Australia locations for the summer 2012/13.  BoM hopes to secure more funding to provide an Australia wide ReefTemp product in the future.

For more information on the technical information and methodology of the SST products, visit POAMA, CRW, and ReefTemp. 

For more information of how these SST products are integrated into a more formal operational monitoring and response framework, visit the Great Barrier Reef Marine Park Authority's Coral Bleaching Response Plan.