what is a marine heatwave?

We know that heatwaves occur in the atmosphere. We are all familiar with these extended periods of excessively hot weather. However, heatwaves can also occur in the ocean and these are known as marine heatwaves, or MHWs. These marine heatwaves, when ocean temperatures are extremely warm for an extended period of time can have significant impacts on marine ecosystems and industries.​ Marine heatwaves can occur in summer or winter - they are defined based on differences with expected temperatures for the location and time of year.

How do we measure marine heatwaves?

We use a recently developed definition of marine heatwaves (Hobday et al. 2016). A marine heatwave is defined a when seawater temperatures exceed a seasonally-varying threshold (usually the 90th percentile) for at least 5 consecutive days. Successive heatwaves with gaps of 2 days or less are considered part of the same event.
Hobday, A. J. et al. (2016), A hierarchical approach to defining marine heatwaves, Prog. Ocean., 141, pp. 227-238, 10.1016/j.pocean.2015.12.014
Links to ​Python and R implementations of the Marine Heatwave definition of Hobday et al. (2016).

Marine heatwave properties

Once marine heatwaves are defined, we can associate with each event a set of properties including duration (time from the start to end date, in days), intensity (the temperature anomaly above the climatology, in °C, meaning how much warmer than expected for that time of year mean, and this is either a maximum value or an average over the whole event), and cumulative intensity (the integral of intensity over the duration of the event, similar to “degree heating days” or “degree heating weeks”). Over a whole year we can define the annual frequency (count of events in a particular year) and the annual marine heatwave days (the count of days in a year that were part of a marine heatwave).

Summer marine heatwaves vs. winter warm spells

Heatwaves can happen in summer and also in winter, where they are known as “winter warm-spells”. These winter events can have important impacts, such as in the southeast of Australia where the spiny sea urchin can only colonise further south when winter temperatures are above 12 °C. A winter warm spell there can help promote colonisation!

What causes marine heatwaves?

Marine heatwaves can be caused by a whole range of factors, and not all factors are important for each event. The most common drivers of marine heatwaves include ocean currents which can build up areas of warm water and air-sea heat flux, or warming through the ocean surface from the atmosphere. Winds can enhance or suppress the warming in a marine heatwave, and climate modes like El Niño can change the likelihood of events occurring in certain regions.

How have marine heatwaves changed?

The oceans are warming at an unprecedented rate. Sea surface temperatures have increased at a rate of nearly 0.6°C per century since 1880 (IPCC AR5). This warming in turn increases the likelihood of marine heatwaves from occurring. While marine ecosystems have evolved within a certain coping range, and can adapt to conditions slightly outside that range, marine heatwaves manifest as extreme events that lead to ecosystem vulnerabilities. In a warmer climate we are more likely to experience these vulnerability-causing extremes:

what about the future?

The Intergovernmental Panel on Climate Change (IPCC) 5th assessment report projects that the global ocean will continue to warm well into the 21st century. The warming in the upper ocean is projected to be between 0.6°C and 2°C! We can therefore expect a future continuation, and possibly acceleration, of the historic warming-driven changes in marine heatwaves. What sort of impacts will we see to marine biodiversity, and to our fisheries and aquaculture industries?

Impacts and implications of marine heatwaves?
​Marine heatwaves affect ecosystem structure, by supporting certain species and suppressing others. For example, after the 2011 marine heatwave in Western Australia the fish communities had a much more “tropical” nature than previously (Wernberg et al. 2013) and switched from kelp forests to seaweed turfs (Wernberg et al. 2016).
Marine heatwaves can change the habitat ranges of certain species, such as the spiny sea urchin off southeastern Australia which has been expanding southward into Tasmania at the expense of kelp forests which it feeds upon. Rogue animals can also find their way well outside their normal range, following the warm waters of a marine heatwave, such as this tropical fish found off Tasmania during the 2015/16 marine heatwave! 
Marine heatwaves can cause economic losses through impacts on fisheries and aquaculture. In 2011 in Western Australia, the marine heatwave impacted the abalone fishery in the north of the state and in 2015/16 the marine heatwave off southeastern Australia led to high levels of abalone mortality in Tasmania. That event also led to outbreaks of Pacific oyster mortality syndrome (which affected the pacific oyster aquaculture industry) and also poor performance in Atlantic salmon aquaculture.
Biodiversity can be drastically affected by marine heatwaves. In 2016, marine heatwaves across northern Australia led to severe bleaching of the Great Barrier Reef and have been speculated to be linked to mangrove die-offs in the Gulf of Carpentaria.

How can we raise awareness of marine heatwaves?
Marine heatwaves clearly have the potential to devastate marine ecosystems and cause economic losses in fisheries, aquaculture, and ecotourism industries. However, their effects are often hidden from view under the waves until it is too late. By raising general awareness of these phenomena, and by improving our scientific understanding of their physical properties and ecological impacts, we can better predict future conditions and protect vulnerable marine habitats and resources.