Issue 21, April - May 2024

   Summary

• 2023/24 El Niño has ended. La Niña Watch declared.
• 4th global coral bleaching event declared.
• Pilbara reefs may be the worst affected WA reefs following 2022/23 La Niña conditions at the start of the global event.
• Recent surveys have found some bleaching across the Indian Ocean Territories and little to no bleaching across oceanic reefs during the 2023/24 El Niño. 
• Storm and tropical cyclone activity across the north and northwest has reduced heat accumulation and risk of bleaching.
• Recent sea surface temperatures (SSTs) are close to average across the northwest shelf and the inshore WA coast. Warm anomalies building offshore along the southwest.
• SST outlooks suggests anomalies to build along the northwest coast, warm anomalies around the Indian Ocean Territories and across the southwest.
• Low to moderate warm SST anomalies (<1.5°C) may persist through the winter.
• Outlooks suggest little heat stress remains for Cocos (Keeling) Islands and moderate heat stress along the southwest between Shark Bay and Perth. 

Fourth Global Coral Bleaching Event and Western Australia

There is no denying that these are challenging times for our oceans. The world's coral reefs have perhaps faced no greater threat than that of climate change today.

On Monday, 15th April 2024, the National Oceanic and Atmospheric Administration (NOAA) officially declared the 4th global coral bleaching event with extensive heat stress and significant coral bleaching recorded across the world's coral reefs in both the northern and southern hemispheres since early 2023.

For more than a year, global sea surface temperatures (SSTs) have been the hottest on record for each month, driven by the rising background temperatures of the world's oceans combined with the regional warming patterns linked to ENSO events. 

Climate models indicate an increase in the frequency and magnitude of bleaching impacts and studies have shown that it takes more than a decade for reefs to recover. However, this is the second global bleaching event in less than 10 years, comparable in spatial extent to the 2014-2017 global bleaching event.

Here in Australia, the 5th mass bleaching event since 2016 was unfolding in March on the Great Barrier Reef and aerial and in-water surveys suggest it is the most extensive event on record. In Western Australia, the previous global bleaching event in 2016 saw severe bleaching and mortality across many reefs in the northwest, with some reefs experiencing bleaching for the first time on record.

This current global event began in February 2023, as the third successive La Niña event was drawing to a close, and reefs in the Pilbara region were experiencing persistent heat stress resulting in moderate to severe bleaching at some reefs in the Dampier Archipelago. 

The central tropical Pacific continued to warm, with international climate models indicating a possible El Niño event in 2023.

Low levels of bleaching observed across Ningaloo, Exmouth Gulf and the Abrolhos in October were not likely associated with heat stress and while the cause of the stress remains unclear, it could be the result of exposure from lower than normal sea levels.

In late 2023, a large cold water anomaly developed near the Pilbara, and extended south to the Exmouth Gulf. This unusual, widespread upwelling event persisted over several months and could have been driven by the strong equatorial winds, lower than normal sea levels and climate conditions. 

With the late arrival of the monsoon season, early summer 2023/2024, heat stress was building across the northwest, with some oceanic reefs exceeding 12 DHW by the start of January, comparable to the summer conditions during the significant 2016 El Niño bleaching event.

Outlooks at this time were suggesting a risk for widespread bleaching and mortality across the most northern reefs for summer and autumn 2024 and bleaching was observed across Christmas and Cocos (Keeling) Islands in February. With the passing of several tropical lows and cyclones bringing cooler conditions across the northwest, this risk was greatly reduced.

Recent surveys by AIMS researchers across Ashmore Reef, Scott Reef and the Rowley Shoals have observed little to no evidence of recent bleaching, however observed signs of some damage following the recent cyclone activity. Initial surveys conducted by the Bardi Jawi Rangers in the southern Kimberley have also found little evidence of recent bleaching across their survey sites. 

Recording bleaching observations is critical for understanding and documenting the impacts of heat stress events on coral reefs and provide valuable information that contributes to global summaries and assessments of bleaching events. To facilitate this process, observations can be recorded via the mobile app or by email (n.ryan@aims.gov.au). More information about coral bleaching is available at https://www.aims.gov.au/coral-bleaching

The most recent climate and temperature outlooks provided by the Bureau of Meteorology indicate that the most recent El Niño event has ended, and neutral conditions are expected to remain until at least July. A positive Indian Ocean Dipole (IOD) event may be developing.

The outlook for May suggests cooler conditions expected along the northwest coast and lingering warm anomalies around the Indian Ocean Territories (IOT) while some inshore areas of the southwest may experience temperatures up to 2 degrees warmer than normal. Low to moderate warm SST anomalies may persist through the winter months. Outlooks for the next few months are provided in detail below.

Recent Temperatures

The Bureau of Meteorology (BoM) provides regular updated Sea Temperature Analysis based on either in-water or satellite observations, or a combination of both. 

The monthly analysis for March 2024 shows normal sea surface temperatures (SSTs) across the continental shelf of WA and NT. 

Gulf to the Kimberley, reaching up to 2.5°C warmer than normal in some areas. Temperature anomalies offshore returned close to average for this time of year following the passing of several tropical lows and cyclones.

Over the last two weeks of March, SSTs remained between 0.5 and 2°C warmer than normal along the inshore  Ningaloo, Pilbara and Kimberley coasts. SSTs were close to average for this time of year across most of the continental shelf.

Above:  BoM Sea Surface Temperature daily analysis product for northwest Australia for March 2024 with a scale ranging from -6.0 (dark purple) to +6.0 (dark burgundy) °C. 

extended from southern Ningaloo, south around the Capes, with some areas reaching up to 3°C warmer than usual offshore of Shark Bay and Geraldton. 

Recent Heat Stress

NOAA’s Coral Reef Watch (CRW) daily global 5km satellite Bleaching Alert Area product provides projected heat stress and bleaching updates.

The most recent (25th May 2024) heat stress updates reflect the recent reduction in SST anomalies across the northwest and includes a 'Bleaching Alert Level 1' for Houtman Abrolhos Islands, 'Bleaching Warning Alert' for Shark Bay and Marmion and Rottnest Island, and a 'Bleaching Watch' alert for Cocos (Keeling) and Ningaloo. Christmas Island, Scott to Ashmore, Rowley Shoals, North Western Australia and Central Western Australia have been downgraded to 'No Stress'. There is also 'No Stress' for the Northern Territory and the Gulf of Carpentaria.  

NOAA's CRW daily global 5km satellite Degree Heating Week (DHW) product indicates around 1 DHW across the Indian Ocean Territories, Ashmore Reef, Scott Reef, northern inshore Pilbara and coastal Ningaloo and 2 DHW in the Exmouth Gulf. Around the Houtman Abrolhos Islands is showing up to 7 DHW.

Above: NOAA Coral Reef Watch Daily 5km satellite Coral Bleaching Heat Stress Degree Heating Week product for Indian Ocean 13th May 2024 with a scale ranging from 0 (aqua) to 21 (dark purple) DHW. Source: NOAA. Accessed 25 May, 2024

Climate Update

The El Niño event has ended and ENSO has returned to neutral following recent cooling in the central Pacific. BoM's ENSO Outlook has recently been updated to La Niña Watch. Model forecasts indicating neutral conditions to remain until at least July 2024. 

Three of the seven international models surveyed by BoM indicate that SSTs in the central Pacific could reach La Niña thresholds in August. However, model skill is low for predictions beyond July. Historically, around 50% of El Niño events have been followed by an ENSO-neutral year, and around 50% by a La Niña event. However, given recent unprecedented warming, ENSO predictions based on past events may not be as reliable. Globally, ocean temperatures have been the warmest on record since April 2023, and April 2024 is on track to be warmer than April 2023. 

The IOD is currently neutral. The IOD index has remained within the neutral threshold over the last two weeks following seven weeks of being above the positive threshold. If a positive IOD does develop, it would be earlier in the year than usual. 

The monsoon season has now come to an end. Although cyclones can be damaging to coral reefs, they can also cool oceans during critical times of heat stress.

From mid-February to early April, several consecutive large and slow moving tropical systems crossed the northwest region bringing with them heavy rain, damaging winds and cooler temperatures.

Tropical cyclone (TC) Lincoln crossed the coast in the Gulf of Carpentaria before continuing as ex-tropical cyclone Lincoln and moving across the Kimberley coast on the 20th February. This slow moving system remained as a low north of the Pilbara before crossing the coast near Geraldton on the 25th February.

In early March, two tropical low systems developed in the northwest region. A weak tropical low developed northwest of Cocos and slowly tracked south of Christmas Island on the 12th March and continued towards the Pilbara. Close to WA, the system changed direction and developed into TC Neville. The cyclone remained well of the mainland coast but impact Cocos and Christmas Islands.

In mid-March, a second weak tropical low developed north of the Kimberley and moved across to the Gulf of Carpentaria before developing into TC Megan. TC Megan tracked across central Northern Territory in March bringing heavy rain to the Gulf of Carpentaria. 

A persistent low formed northwest of Cocos Islands in early April. As the slow moving system moved south, it developed into severe TC Olga on the 6th April and tracked south-southwest before strong north westerly winds sent it east towards Geraldton before dissipating on the 18th.

Above:Tropical Cyclone Track Maps issued by BoM for ex-TC Lincoln (top left), TC Neville (top right), TC Megan (bottom left) and TC Olga (bottom right). 

Temperature and Heat Stress Outlooks

BoM’s most recent (25th May 2024) Ocean Temperature Outlooks show low warm SST anomalies offshore of the northwest and cooler SST anomalies across the inshore northern Kimberley for the rest of May.

SSTs up to 0.8°C cooler than normal (1981-2018) are predicted across the inshore northern Kimberley over the next 2 weeks, and warm anomalies (0.2 - 1.5°C) across the oceanic reefs and around the Indian Ocean Territories (IOT). Outlooks moving into June show warm anomalies (0.2 - 1.5°C) increasing across the northwest and further offshore.

Above:  SST anomalies for NW Australia from 23 May - 6 June 2024 (left), and from 6 - 20 June 2024 (right) with a scale ranging from -1.5 (dark blue) to +2.5 (burgundy) °C. Source: BoM

Above:  SST anomalies for Christmas and Cocos (Keeling) Islands from 23 May - 6 June 2024 (left), and from 6 - 20 June 2024 (right) with a scale ranging from -1.5 (dark blue) to +2.5 (burgundy) °C. Source: BoM

Central WA is also showing low warmer than normal SSTs (0.2 - 1.0°C) inshore for the rest May and into June. Further offshore, anomalies are much warmer than normal (up to 2.5°C) and extend from Ningaloo to Shark Bay, and continue offshore along the south west.

The outlook for the rest of May is showing much warmer than normal SSTs along the southwest, with some inshore areas reaching 2.0°C warmer, up to 1.5°C around the Houtman Abrolhos, and some offshore areas exceeding 2.5°C warmer than normal for this time of year.

Above:  SST anomalies for Ningaloo Reef from 23 May - 6 June 2024 (left), and from 6 - 20 June 2024 (right) with a scale ranging from -1.5 (dark blue) to +2.5 (burgundy) °C. Source: BoM

Above:  SST anomalies for South Western Australia from 23 May - 6 June 2024 (left), and from 6 - 20 June 2024 (right) with a scale ranging from -1.5 (dark blue) to +2.5 (burgundy) °C. Source: BoM

The June outlook shows conditions warmer (0.2 - 2.0°C) than average right across Western Australia.

Warm anomalies across the northwest and oceanic islands are predicted to build in June and anomalies between 0.6 - 1.5°C to extend across much of the northwest region by July. Warm SST anomalies (0.2 -1.5°C) are showing to persist across the IOT through winter months to spring. 

In the southwest, the June outlooks show temperatures building up to 1.5°C warmer than average inshore and exceeding 2.5°C offshore, with warm conditions (>0.8°C) predicted to persist through the winter months, both offshore and inshore extending from Shark Bay south around the Capes. 

Bleaching and Heat Stress Outlooks

Bleaching Alert Levels, Degree Heating Week (DHW) and Degree Heating Month (DHM) metrics reflect the predicted accumulation of heat stress, calculated by integrating SST anomalies above expected (i.e., climatological) mean temperature with the duration of heat stress. Risk of reef-wide bleaching is predicted with a Bleaching Alert Level 1, DHW ≥ 4 and DHM ≥ 1. Risk of reef-wide bleaching with mortality is predicted with a Alert Level 2, DHW ≥ 8 and DHM ≥ 2.

BoM’s Degree Heating Month (DHM) Outlook  (25th May 2024) shows no heat stress across the northwest, and low heat stress across Cocos (Keeling) Islands and South Western Australia between June and August 2024.

From June to August, <0.75 DHM is predicted for Cocos (Keeling) Islands and <0.5 DHM for Houtman Abrolhos Islands suggesting little to no risk of coral bleaching. 

Little to no heat stress is predicted for most of the northwest beyond May 2024. The WA Coral Bleaching Group will continue to provide updated assessments of bleaching risk.

BoM Degree Heating Months Outlook product provides an indication of the magnitude and duration of heat stress over a three month period from June to August 2024. Left: Australia. Right: Christmas and Cocos (Keeling) Islands. DHM ≥1 indicates that heat stress and some bleaching is anticipated, while DHM ≥2 predicts extensive bleaching and significant mortality. 

NOAA's Coral Bleaching Outlook predicts the likelihood of coral bleaching heat stress and is based on NOAA's daily SST forecast. The outlook indicates a high probability (>90%) of thermal stress across southwestern Australia between June and September 2024.

The outlook predicts no thermal stress across the northwest. A bleaching Alert Level 2 (risk of reef-wide bleaching and mortality) is predicted for offshore of Shark Bay and Geraldton, and a bleaching Alert Level 1 (risk of reef-wide bleaching) for the Abrolhos Islands in mid May and June. Low heat stress (Bleaching Watch) inshore along the southwest coast is predicted to reduce in June. 

Above: NOAA Coral Reef Watch 90% Probability Bleaching Heat Stress 4-month heat stress outlook for June - September 2024. Alert Level 1 indicates that heat stress and reef-wide bleaching is anticipated, while Alert Level 2 predicts reef-wide bleaching and mortality of heat sensitive corals. Accessed 25 May, 2024

Field Update

A field team from AIMS has surveyed at Ashmore Reef, Scott Reef and Rowley Shoals through April. The most SST and DHM outlooks for April reflected recent cooling across the northwest and little to no coral bleaching was observed across the offshore reefs at this time. Nor was there evidence of recent mortality from heat stress and bleaching during the summer months. 

Bardi Jawi and Oorany rangers have begun conducting bleaching surveys across Bardi Jawi sea country (Dampier Peninsula, southern Kimberley) in April. Initial surveys suggest little to no bleaching at this time.

Research scientists from DBCA will be conducting monitoring activities across the northern Kimberley reefs in May. Recent outlooks suggest a reduced risk for heat stress and coral bleaching.

Research and monitoring activities from multiple field teams across Cocos and Christmas Islands are planned through to May. Recent outlooks suggest reduced heat stress accumulation for Christmas Island, however the risk for reef-wide bleaching remains for Cocos. Observations of coral bleaching across Cocos were noted in February.

Multiple research teams working at Ningaloo Reef and the Exmouth Gulf have reported no evidence of recent coral bleaching.

Going in the field? Our reefs are extensive and diverse but increasing ocean temperatures mean an increasing risk of coral bleaching to WA and NT reefs during most years. The Coral Bleaching App is a useful tool for recording bleaching observations while in the field, or following a field trip, and we strongly encourage our community to record observations from their field trips. Email us at n.ryan@aims.gov.au

OBSERVATIONS OF CORAL BLEACHING

Coral bleaching, Scott Reef 2016. Photo: James Gilmour, AIMS

We encourage the reporting of coral bleaching observations on WA and NT reefs by the wider community, including traditional owners, regional managers, tourist operators and anyone who visits WA’s coral reefs. The Australian Institute of Marine Science (AIMS) has a free mobile app for recording bleaching. Using this app, you can submit observations on the amount of bleaching observed, additional habitat information and photographs. Alternatively, if you would prefer to simply email observations and supporting images, or have questions, please contact n.ryan@aims.gov.au.  More information about coral bleaching is available at https://www.aims.gov.au/coral-bleaching

CORAL BLEACHING APP

The ArcGIS Field Maps App for Android or Apple devices is a simple tool for collecting geo-referenced coral bleaching data. To get started, download the app and register for a username by emailing Ben Radford at b.radford@aims.gov.au.

Once you have received your username and password in an email, you can log in to your app for the first time. To begin using the coral bleaching app, please visit https://www.aims.gov.au/coral-bleaching where you can find the user guide. 

Alternatively, a bleaching observation datasheet has been created which can be printed out for completion by hand. Please visit https://www.aims.gov.au/coral-bleaching to download the datasheet.

We acknowledge the Traditional Owners of Western Australia and Northern Territory and pay our respects to elders past, present and future. We honour their continuing culture, knowledge, beliefs and spiritual relationship and connection to country. We also recognise Aboriginal and Torres Strait Islander peoples as the Traditional Owners of the land and sea country on which the Western Australian Bleaching Group works, and as Australia’s first scientists. Through collaboration and two-way knowledge sharing, we can learn from each other. We will continue to build our relationships with Rangers and Traditional Owners throughout regional WA, to build on our coral bleaching knowledge, confirm predictions and measure the impacts to their sea country.

FURTHER INFORMATION

Sea surface temperature (SST) summaries and coral bleaching predictions are based on the products provided by: 

• Bureau of Meteorology Climate Driver update
• Bureau of Meteorology Ocean Temperature Outlooks
• NOAA Coral Reef Watch
• IMOS Ocean Current

We rely heavily on the SST and heat stress predictions provided by NOAA and particularly Australia’s Bureau of Meteorology (BoM). These organisations' models provide different, but complementary products. Differences in heat stress predictions among the products often reflect uncertainty in future conditions, particularly several months into the future. However, when the different products provide similar predictions of bleaching within 1 or 2 months, then there is a higher degree of confidence in the predictions.

WA Coral Bleaching Group

James Gilmour, Nicole Ryan, Jessica Benthuysen, Claire Ross, Arani Chandrapavan, Rachel Garcia, Scott Heron, Jennifer Hoy, Darren Phillips, William Robbins, Damian Thomson, Claire Spillman

The WA Coral Bleaching Group meets regularly throughout the spring - summer months to discuss coral bleaching risks to Western Australian reefs. The newsletter provides updates based on forecasts, satellite SST monitoring, and in-water and aerial surveys. If you would like to receive our updates click subscribe:

For additional enquiries, contact:

Nicole Ryan, AIMS
n.ryan@aims.gov.au