Marine heatwaves (MHWs) have been defined by Hobday et al. (Prog Ocean, 2016) as discrete prolonged anomalously warm water events. These events have be devastating for regional ecosystems with economic impacts on fisheries, aquaculture, and tourism. I have been and am currently involved in a number of studies on MHWs including their definition, physical drivers and processes, spatial patterns and long-term trends, as well as regional studies including around Tasmania and South Africa.

A hierarchical approach to defining marine heatwaves (Hobday et al., Progress in Oceanography, 2016)

Abstract: Marine heatwaves (MHWs) have been observed around the world and are expected to increase in intensity and frequency under anthropogenic climate change. A variety of impacts have been associated with these anomalous events, including shifts in species ranges, local extinctions and economic impacts on seafood industries through declines in important fishery species and impacts on aquaculture. Extreme temperatures are increasingly seen as important influences on biological systems, yet a consistent definition of MHWs does not exist. A clear definition will facilitate retrospective comparisons between MHWs, enabling the synthesis and a mechanistic understanding of the role of MHWs in marine ecosystems. Building on research into atmospheric heatwaves, we propose both a general and specific definition for MHWs, based on a hierarchy of metrics that allow for different data sets to be used in identifying MHWs. We generally define a MHW as a prolonged discrete anomalously warm water event that can be described by its duration, intensity, rate of evolution, and spatial extent. Specifically, we consider an anomalously warm event to be a MHW if it lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period. This structure provides flexibility with regard to the description of MHWs and transparency in communicating MHWs to a general audience. The use of these metrics is illustrated for three 21st century MHWs; the northern Mediterranean event in 2003, the Western Australia ‘Ningaloo Niño’ in 2011, and the northwest Atlantic event in 2012. We recommend a specific quantitative definition for MHWs to facilitate global comparisons and to advance our understanding of these phenomena.
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• Marine heatwave definition demonstrated for three recent events: 

Figure: First row: Sea surface temperatures (SST) anomaly on the peak day of three marine heatwaves (MHW) discussed in the text. (a) Western Australia 2011, (b) northern Mediterranean 2003, (c) northwest Atlantic 2012. Dots show the locations from which 1/4° resolution time series of SST were extracted from NOAA OISST for the detection of MHWs in each case study region. Second row: The SST climatology (blue), 90th percentile MHW threshold (green), and SST time series (black) for each MHW at each location. The red filled area indicates the period of time associated with the identified MHW, while shaded orange indicates other MHWs identified over the year. Third row: The duration (D) of each MHW detected in the time series from each location, with every tenth event identified on the upper x-axis. Fourth row: As for the third row, but illustrating maximum intensity (i max ) of each MHW event in each location. Fifth row: As for the third row, but illustrating cumulative intensity (i cum ) of each MHW event from each location. The WA and northwest Atlantic MHWs are the largest by maximum intensity, such that the red and yellow bars are the same. The northwest Atlantic event is not the largest according to duration or cumulative intensity, but the red bar obscures the yellow bar since they are so close in time.

The unprecedented 2015/16 Tasman Sea marine heatwave (Oliver et al., Nature Communications, 2017)

Abstract: The Tasman Sea off southeast Australia exhibited its longest and most intense marine heatwave ever recorded in 2015/16. Here we report on several inter-related aspects of this event: observed characteristics, physical drivers, ecological impacts, and the role of climate change. This marine heatwave lasted for 251 days reaching a maximum intensity of 2.9⁰C above climatology. The anomalous warming is dominated by anomalous convergence of heat linked to the southward flowing East Australian Current. Ecosystem impacts range from new disease outbreaks in farmed shellfish, mortality of wild molluscs, and out-of-range of species observations. Global climate models indicate it is very likely that the occurrence of an extreme warming event of this duration or intensity in this region is respectively 330 times and 6.8 times as likely due to the influence of anthropogenic climate change. Climate projections indicate that event likelihoods will increase in the future, due to increasing anthropogenic influences.

Figure: (a) The mean 2015/16 austral summer (December to February) mean SST anomalies from NOAA OI SST, the box used to define the southeast Austraila (SEAus) region (black lines) and the location of the Maria Island Time Series (open circle). Anomalies are relative to the 1982–2005 climatology. Also shown are time series of (b) SST and (c) SST anomalies averaged over the SEAus region since 2012 from NOAA OI SST (black lines) and HadISST (circles). The red-filled circles in b,c indicate which months during the event were among the top ten on record since 1880. The grey and blue lines in b indicate the climatological mean and 90th percentile threshold, respectively, calculated from NOAA OI SST. The pink-shaded regions in b,c indicate all MHWs detected using the Hobday et al. 2 definition and the red-shaded region is the 2015/16 event. The (d) duration and (e) maximum intensity are shown for this event as red bars along with values for all previous events on record back to 1982.

​Anthropogenic and natural influences on record 2016 marine heatwaves (Oliver et al., Bulletin of the American Meteorological Society, 2017)

Two of the longest and most intense marine heatwaves in 2016 were up to fifty times more likely due to anthropogenic climate change.

Figure: 2016 MHWs in (left) NA and (right) BSGA. (a),(b) SST anomalies (°C) during the peak of each event (date indicated in panel) and unhatched areas indicate regions defined as MHW on that date according to the Hobday et al. (2016) definition. (c),(d) Daily SSTs (°C): NOAA OI SST (black), threshold (green), and 1961–90 climatology (gray) (e),(f) SST anomalies (°C) averaged over NA and BSGA during 2014–16. In (c)–(f) red shading indicates the 2016 MHW; lighter shading indicates other detected marine heatwaves over the period.