A definition of MHWs (Hobday et al. 2016) has been widely adopted by researchers and other users. A MHW is defined as a period when seawater temperatures exceed a seasonally-varying threshold (usually the 90th percentile) for at least 5 consecutive days. Successive events with gaps of 2 days or less are considered part of the same MHW. In a subsequent study (Hobday et al. 2018) we extended the definition to introduce categories of severity, based on multiples of the threshold being exceeded.

Once MHWs are defined, we can define each event by 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), 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 classified as being a MHW).

Heatwaves can happen at any time of year, although the most extreme absolute sea temperatures are generally observed during summertime MHWs, which can become stressful to marine life. MHWs that occur in other seasons can also 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.

MHWs are caused by a wide range of processes, which vary in their importance between regions and events. The most common drivers of MHWs 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 MHW, and climate modes like El Nio can change the likelihood of events occurring in certain regions.

https://www.marineheatwaves.org/mhw-overview.html