As summer comes to an end, we start seeing cooler temperatures. But this doesn’t necessarily mean cooler oceans. After months of warmer weather, the end of summer is typically when our oceans are warmest.
Ocean temperatures warm and cool much slower than temperatures for our seasons on land. But research globally suggests our oceans have been warming over the past century. Between 1925 and 2016 marine heatwave days increased by 54%. Much like heatwaves on land, marine heatwaves are prolonged periods of exceptionally warm temperatures. These events are increasing in length and frequency as our ocean temperatures warm in response to our changing climate. A trend that will only increase as our climate continues to warm.
Marine heatwaves can have dire consequences
Marine heatwaves are often associated with coral bleaching. But when ocean temperatures rise to extremes for sustainedperiods many different species can be affected. When waters become too hot, marine species can become stressed and either migrate or perish – greatly impacting ecosystems. These events can also compound with lower oxygen and pH in the water which can be severely detrimental. Other impacts from marine heatwaves can include the proliferation of cyanobacteria, harmful algal blooms, kelp and seagrass dieback, poor performance in farmed salmon, and oyster diseases.
These changing conditions have consequences for ecosystems and industries alike. Aquaculture, tourism and other industries rely on healthy conditions for marine life to flourish. When they migrate or ecosystems become disrupted this can have far reaching effects. Understanding when and why marine heatwaves occur is important to help prepare and plan appropriate responses.
Climate drivers can affect more than our rainfall
Our oceans change our climate – warmer waters influence changes in weather patterns, such as rainfall, and can drive our climate. These shifting patterns can also increase or decrease the likelihood of marine heatwaves around the world. For example, off Western Australia, in the Indian Ocean, marine heatwaves are more often associated with La Niña.
See: https://nespclimate.com.au/wp-content/uploads/2021/04/ESCC_WA-marine-heatwaves_Factsheet.pdf.
Even across Australia’s southern waters, marine heatwaves can have substantial impacts
Marine species become used to a range of temperatures, even within our cooler ocean to the south of Australia. Species used to cooler waters to Australia’s south are also affected by warming oceans and warm ocean temperature extremes, including marine heatwaves. For example, in the Tasman Sea, a positive Southern Annular Mode and La Niña can increase ocean temperatures and the likelihood of marine heatwaves. In some instances, marine heatwaves have caused damage to marine fisheries and aquaculture, for example blacklip abalone and salmon.
Summer 2017–18 sea surface temperatures were some of the warmest ever recorded in the Tasman Sea. Source: Bureau of Meteorology.
Understanding more about marine heatwaves
While the science around predicting marine heatwaves is stronger for open ocean regions where climate drivers are better understood and resolved, coastal regions are where marine heatwaves can have the most impact. These events are harder to predict with modelling, as the interactions of local climate and coastlines are not as well understood. Access to new ultra-high-resolution modelling is enabling hub researchers to better understand how marine heatwaves behave in coastal regions. In estuarine waters, researchers are also working to understand more. Estuaries typically have high biodiversity, and they sustain important economic activities like aquaculture and fisheries. Hub researchers are considering many different variables including changing ocean temperatures and water circulations to understand how ecosystems and species may be at risk.
Have your chance to ask our experts your questions about marine heatwaves at our next webinar. Join us on March 20.
Read the latest in hub research on marine heatwaves and explore some off the work our scientists are contributing to:
Gregory CH, NJ Holbrook, AG Marshall and CM Spillman, 2023: Atmospheric drivers of Tasman Sea marine heatwaves. Journal of Climate, 36, 5197-5214, https://doi.org/10.1175/JCLI-D-22-0538.1.
Hobday AJ, MT Burrows, K Filbee-Dexter, NJ Holbrook, A Sen Gupta, DA Smale, KE Smith, MS Thomsen and T Wernberg, 2023: With the arrival of El Niño, prepare for stronger marine heatwaves. Nature, 621, 38-41, https://doi.org/10.1038/d41586-023-02730-2.
Holbrook NJ, HA Scannell, A Sen Gupta, JA Benthuysen, M Feng, ECJ Oliver, LV Alexander, MT Burrows, MG Donat, AJ Hobday, PJ Moore, SE Perkins-Kirkpatrick, DA Smale, SC Straub and T Wernberg, 2019: A global assessment of marine heatwaves and their drivers. Nature Communications, 10, Article number 2624, https://doi.org/10.1038/s41467-019-10206-z.
Holbrook NJ, A Sen Gupta, ECJ Oliver, AJ Hobday, JA Benthuysen, HA Scannell, DA Smale and T Wernberg, 2020: Keeping pace with marine heatwaves. Nature Reviews Earth and Environment, 1, 482-493, https://doi.org/10.1038/s43017-020-0068-4.
Kajtar JB, NJ Holbrook, A Lyth, AJ Hobday, CN Mundy and SC Ugalde, 2024: A stakeholder-guided marine heatwave hazard index for fisheries and aquaculture. Climatic Change, 177(26), https://doi.org/10.1007/s10584-024-03684-8.
Oliver ECJ, JA Benthuysen, NL Bindoff, AJ Hobday, NJ Holbrook, CN Mundy and SE Perkins-Kirkpatrick, 2017: The unprecedented 2015/16 Tasman Sea marine heatwave. Nature Communications, 8, 16101, DOI:10.1038/ncomms16101.
Oliver ECJ, MG Donat, MT Burrows, PJ Moore, DA Smale, LV Alexander, JA Benthuysen, M Feng, A Sen Gupta, AJ Hobday, NJ Holbrook, SE Perkins-Kirkpatrick, HA Scannell, SC Straub and T Wernberg, 2018: Longer and more frequent marine heatwaves over the past century. Nature Communications, 9, Article number 1324, DOI:10.1038/s41467-018-03732-9.
Watch this lecture providing a global overview of marine heatwave drivers
Learn more about marine heatwaves globally at Marineheatwaves.org